Age hardening (Precipitation Hardening) Precipitation Hardening Stainless Steels – Alloys, Properties, Fabrication Processes. Background Precipitation hardening stainless steels are chromium and nickel containing steels that provide an optimum combination of the properties of martensitic and austenitic grades. Like martensitic grades, they are known for their ability to gain high strength through heat treatment and they also have the corrosion resistance of austenitic stainless steel. The high tensile strengths of precipitation hardening stainless steels come after a heat treatment process that leads to precipitation hardening of a martensitic or austenitic matrix. Hardening is achieved through the addition of one or more of the elements Copper, Aluminium, Titanium, Niobium, and Molybdenum. The most well known precipitation hardening steel is 17-4 PH. The name comes from the additions 17% Chromium and 4% Nickel. It also contains 4% Copper and 0.3% Niobium. 17-4 PH is also known as stainless steel grade 630. The advantage of precipitation hardening steels is that they can be supplied in a “solution treated” condition, which is readily machineable. After machining or another fabrication method, a single, low temperature heat treatment can be applied to increase the strength of the steel. This is known as ageing or age- hardening. As it is carried out at low temperature, the component undergoes no distortion. Characterisation Precipitation hardening steels are characterised into one of three groups based on their final microstructures after heat treatment. The three types are: martensitic (e.g. 17-4 PH), semi-austenitic (e.g. 17- 7 PH) and austenitic (e.g. A-286). Martensitic Alloys Martensitic precipitation hardening stainless steels have a predominantly austenitic structure at annealing temperatures of around 1040 to 1065°C. Upon cooling to room temperature, they undergo a Page | 1
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BackgroundPrecipitation hardening stainless steels are chromium and nickel containing steels that provide an optimum combination of the properties of martensitic and austenitic grades. Like martensitic grades, they are known for their ability to gain high strength through heat treatment and they also have the corrosion resistance of austenitic stainless steel.
The high tensile strengths of precipitation hardening stainless steels come after a heat treatment process that leads to precipitation hardening of a martensitic or austenitic matrix. Hardening is achieved through the addition of one or more of the elements Copper, Aluminium, Titanium, Niobium, and Molybdenum. The most well known precipitation hardening steel is 17-4 PH.
The name comes from the additions 17% Chromium and 4% Nickel. It also contains 4% Copper and 0.3% Niobium. 17-4 PH is also known as stainless steel grade 630. The advantage of precipitation hardening steels is that they can be supplied in a “solution treated” condition, which is readily machineable. After machining or another fabrication method, a single, low temperature heat treatment can be applied to increase the strength of the steel. This is known as ageing or age-hardening. As it is carried out at low temperature, the component undergoes no distortion.
CharacterisationPrecipitation hardening steels are characterised into one of three groups based on their final microstructures after heat treatment. The three types are: martensitic (e.g. 17-4 PH), semi-austenitic (e.g. 17-7 PH) and austenitic (e.g. A-286).
Martensitic AlloysMartensitic precipitation hardening stainless steels have a predominantly austenitic structure at annealing temperatures of around 1040 to 1065°C. Upon cooling to room temperature, they undergo a transformation that changes the austenite to martensite.
Semi-austenitic AlloysUnlike martensitic precipitation hardening steels, annealed semi-austenitic precipitation hardening steels are soft enough to be cold worked. Semi-austenitc steels retain their austenitic structure at room temperature but will form martensite at very low temperatures.
Austenitic AlloysAustenitic precipitation hardening steels retain their austenitic structure after annealing and hardening by ageing. At the annealing temperature of 1095 to 1120°C the precipitation hardening phase is soluble. It remains in solution during rapid cooling. When reheated to 650 to 760°C, precipitation occurs. This increases the hardness and strength of the material. Hardness remains lower than that for martensitic or semi-austenitic precipitation hardening steels. Austenitic alloys remain nonmagnetic.
Properties
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StrengthYield strengths for precipitation-hardening stainless steels are 515 to 1415 MPa. Tensile strengths range from 860 to 1520 MPa. Elongations are 1 to 25%. Cold working before ageing can be used to facilitate even higher strengths.
Heat TreatmentThe key to the properties of precipitation hardening stainless steels lies in heat treatment. After solution treatment or annealing of precipitation hardening stainless steels, a single low temperature “age hardening” stage is employed to achieve the required properties. As this treatment is carried out at a low temperature, no distortion occurs and there is only superficial discolouration. During the hardening process a slight decrease in size takes place.
This shrinking is approximately 0.05% for condition H900 and 0.10% for H1150. Typical mechanical properties achieved for 17-4 PH after solution treating and age hardening are given in the following table. Condition designations are given by the age hardening temperature in °F. Table 1. Mechanical property ranges after solution treating and age hardening
Typical Physical PropertiesTable 4. Typical physical properties for stainless steel alloy 17-4PH Property ValueDensity 7.75 g/cm3Melting Point °CModulus of Elasticity 196 GPaElectrical Resistivity 0.080x10-6 Ω.mThermal Conductivity 18.4 W/m.K at 100°CThermal Expansion 10.8x10-6 /K at 100°C
Alloy DesignationsStainless steel 17-4 PH also corresponds to a number of following standard designations and specifications. Table 5. Alternate designations for stainless steel alloy 17-4PH Euronorm UNS BS En Grade1.4542 S17400 - - 630
Corrosion ResistancePrecipitation hardening stainless steels have moderate to good corrosion resistance in a range of environments. They have a better combination of strength and corrosion resistance than when compared with the heat treatable 400 series martensitic alloys. Corrosion resistance is similar to that found in grade
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304 stainless steel. In warm chloride environments, 17-4 PH is susceptible to pitting and crevice corrosion.
When aged at 550°C or higher, 17-4 PH is highly resistant to stress corrosion cracking. Better stress corrosion cracking resistance comes with higher ageing temperatures. Corrosion resistance is low in the solution treated (annealed) condition and it should not be used before heat treatment.
Heat Resistance17-4 PH has good oxidation resistance. In order to avoid reduction in mechanical properties, it should not be used over its precipitation hardening temperature. Prolonged exposure to 370-480°C should be avoided if ambient temperature toughness is critical.
FabricationFabrication of all stainless steels should be done only with tools dedicated to stainless steel materials or tooling and work surfaces must be thoroughly cleaned before use. These precautions are necessary to avoid cross contamination of stainless steel by easily corroded metals that may discolour the surface of the fabricated product.
Cold WorkingCold forming such as rolling, bending and hydroforming can be performed on 17-4PH but only in the fully annealed condition. After cold working, stress corrosion resistance is improved by re-ageing at the precipitation hardening temperature.
Hot WorkingHot working of 17-4 PH should be performed at 950°-1200°C. After hot working, full heat treatment is required. This involves annealing and cooling to room temperature or lower. Then the component needs to be precipitation hardened to achieve the required mechanical properties.
MachinabilityIn the annealed condition, 17-4 PH has good machinability, similar to that of 304 stainless steel. After hardening heat treatment, machining is difficult but possible. Carbide or high speed steel tools are normally used with standard lubrication. When strict tolerance limits are required, the dimensional changes due to heat treatment must be taken into account
WeldingPrecipitation hardening steels can be readily welded using procedures similar to those used for the 300 series of stainless steels. Grade 17-4 PH can be successfully welded without preheating. Heat treating after welding can be used to give the weld metal the same properties as for the parent metal. The recommended grade of filler rods for welding 17-4 PH is 17-7 PH.
ApplicationsDue to the high strength of precipitation hardening stainless steels, most applications are in aerospace and other high-technology industries. Applications include: · Gears · Valves and other engine components · High strength shafts · Turbine blades · Moulding dies · Nuclear waste casks
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Supplied Forms
17-4 PH is typically supplied by Aalco in the following forms: · Round bar · Hexagonal bar · BilletSource: Aalco For more information on this source please visit Aalco
ALLOYSAn alloy is a homogeneous mixture of two or more elements, at least one of which is a metal, and where the resulting material has metallic properties. The resulting metallic substance usually has different properties (sometimes substantially different) from those of its components.
Contents 1 Properties 2 Classification 3 Terminology 4 See also
Properties:Alloys are usually prepared to improve on the properties of their components. For instance, steel is stronger than iron, its primary component. The physical properties of an alloy, such as density, reactivity and electrical and thermal conductivity may not differ greatly from the alloy's elements, but engineering properties, such as tensile strength, shear strength and Young's modulus, can be substantially different from those of the constituent materials. This is sometimes due to the differing sizes of the atoms in the alloy—larger atoms exert a compressive force on neighboring atoms, and smaller atoms exert a tensile force on their neighbors. This helps the alloy resist deformation, unlike a pure metal where the atoms move more freely. Unlike pure metals, most alloys do not have a single melting point. Instead, they have a melting range in which the material is a mixture of solid and liquid phases. The temperature at which melting begins is called the solidus, and that at which melting is complete is called the liquidus. However, for most pairs of elements, there is a particular ratio which has a single melting point; this is called the eutectic mixture.
ClassificationAlloys can be classified by the number of their constituents. An alloy with two components is called a binary alloy; one with three is a ternary alloy, and so forth. Alloys can be further classified as either substitution alloys or interstitial alloys, depending on their method of formation. In substitution alloys, the atoms of the components are approximately the same size and the various atoms are simply substituted for one another in the crystal structure. An example of a (binary) substitution alloy is brass, made up of copper and zinc. Interstitial alloys occur when the atoms of one component are substantially smaller than the other and the smaller atoms fit into the spaces (interstices) between the larger atoms.
TerminologyIn practice, some alloys are used so predominantly with respect to their base metals that the name of the primary constituent is also used as the name of the alloy. For example, 14 karat gold is an alloy of gold with other elements. Similarly, the silver used in jewelry and the aluminium used as a structural building material are also alloys. The term "alloy" is sometime used in everyday speech as a synonym for a
particular alloy. For example, automobile wheels made of "aluminium alloy" are commonly referred to as simply "alloy wheels". The usage is obviously indefinite, since steels and most other metals in practical use are also alloys.
See alsoLook up alloy in Wiktionary, the free dictionary.
List of alloys Intermetallics Heat treatment
Retrieved from "http://en.wikipedia.org/wiki/Alloy"
List of alloys This is a list of alloys for which an article exists in Wikipedia (or is proposed but not yet written). They are grouped by base metal, in order of increasing atomic number. Within these headings they are in no particular order. Some of the main alloying elements are optionally listed after the alloy names.
Contents 1 Alloys of magnesium 2 Alloys of aluminium 3 Alloys of potassium 4 Alloys of iron 5 Alloys of cobalt 6 Alloys of nickel 7 Alloys of copper 8 Alloys of zinc 9 Alloys of gallium 10 Alloys of zirconium 11 Alloys of silver 12 Alloys of indium 13 Alloys of tin 14 Rare earth alloys 15 Alloys of gold 16 Alloys of mercury 17 Alloys of lead 18 Alloys of bismuth 19 Alloys of uranium
Alloys of magnesium Magnox (aluminium) T-Mg-Al-Zn (Bergman phase) is a complex metallic alloy Elektron
Duralumin (copper) Nambe (aluminium plus seven other undisclosed metals) Silumin (silicon) AA-8000: used for building wire in the U.S. per the National Electrical Code Magnalium (5% magnesium)/used in airplane bodies, ladders,etc. Aluminium also forms complex metallic alloys, like β-Al-Mg, ξ'-Al-Pd-Mn, T-Al3Mn Alnico - alloy of aluminum, nickel, and cobalt used in magnets
Alloys of potassium NaK (sodium)
Alloys of ironSee also: Category:Ferrous alloys
Steel (carbon) (category:steels) o Stainless steel (chromium, nickel)
o Silicon steel (silicon) o Tool steel (tungsten or manganese) o Bulat steel o Chromoly (chromium, molybdenum) o Crucible steel o Damascus steel o HSLA steel o High speed steel o Maraging steel o Reynolds 531 o Wootz steel
Iron o Anthracite iron (carbon) o Cast iron (carbon) o Pig iron (carbon) o Wrought iron (carbon)
o Calamine brass (zinc) o Chinese silver (zinc) o Dutch metal (zinc) o Gilding metal (zinc) o Muntz metal (zinc) o Pinchbeck (zinc) o Prince's metal (zinc) o Tombac (zinc)
Bronze (tin, aluminium or any other element) o Aluminium bronze (aluminium) o Bell metal (tin) o Florentine bronze (aluminium or tin) o Guanín o Gunmetal (tin, zinc) o Glucydur o Phosphor bronze (tin and phosphorus) o Ormolu (Gilt Bronze) (zinc) o Speculum metal (tin)
Bauschinger effectThe Bauschinger effect refers to a property of materials where the material's stress-strain characteristics change as a result of the microscopic stress distribution of the material. For example, an increase in tensile yield strength at the expense of compressive yield strength.
The Bauschinger effect is named after the German engineer Johann Bauschinger (de:Johann Bauschinger).
While more tensile cold working increases the tensile yield strength, the local initial compressive yield strength after tensile cold working is actually reduced. The greater the tensile cold working, the lower the compressive yield strength.The Bauschinger effect is normally associated with conditions where the yield strength of a metal decreases when the direction of strain is changed. It is a general phenomenon found in most polycrystalline metals.
The basic mechanism for the Bauschinger effect is related to the dislocation structure in the cold-worked metal. As deformation occurs, the dislocations will accumulate at barriers and produce dislocation pileups and tangles.
Based on the cold work structure, two types of mechanisms are generally used to explain the Bauschinger effect.
First, local back stresses may be present in the material, which assist the movement of dislocations in the reverse direction. Thus, the dislocations can move easily in the reverse direction and the yield strength of the metal is lower. The pile-up of dislocations at grain boundaries and Orowan loops around strong precipitates are two main sources of these back stresses.
Second, when the strain direction is reversed, dislocations of the opposite sign can be produced from the same source that produced the slip-causing dislocations in the initial direction. Dislocations with opposite signs can attract and annihilate each other. Since strain hardening is related to an increased dislocation density, reducing the number of dislocations reduces strength. The net result is that the yield strength for strain in the opposite direction is less than it would be if the strain had continued in the initial direction.
BRITTLE FRACTURE |Version 4 - view current pagewhat is brittle fracture?
Basically, brittle fracture is a rapid run of cracks through a stressed material. The cracks usually travel so fast that you can't tell when the material is about to break. In other words, there is very little plastic deformation before failure occurs. In most cases, this is the worst type of fracture because you can't repair visible damage in a part or structure before it breaks. In brittle fracture, the cracks run close to perpendicular to the applied stress.
This perpendicular fracture leaves a relatively flat surface at the break. Besides having a nearly flat fracture surface, brittle materials usually contain a pattern on their fracture surfaces. Some brittle materials have lines and ridges beginning at the origin of the crack and spreading out across the crack surface.
Other materials, like some steels have back to back V-shaped markings pointing to the origin of the crack. These V-shaped markings are called chevrons. Very hard or fine grained materials have no special pattern on their fracture surface, and amorphous materials like ceramic glass have shiny smooth fracture surfaces.
Chevron Fracture Surface (Callister p. 185)
Radiating Ridge Fracture Surface (Callister pg. 186, copyright by John Wiley & Sons, inc.) Types of Brittle Fracture
The first type of fracture is transgranular. In transgranular fracture, the fracture travels through the grain of the material. The fracture changes direction from grain to grain due to the different lattice orientation of atoms in each grain. In other words, when the crack reaches a new grain, it may have to find a new path or plane of atoms to travel on because it is easier to change direction for the crack than it is to rip through. Cracks choose the path of least resistance. You can tell
when a crack has changed in direction through the material, because you get a slightly bumpy crack surface.
The second type of fracture is intergranular fracture. Intergranular fracture is the crack traveling along the grain boundaries, and not through the actual grains. Intergranular fracture usually occurs when the phase in the grain boundary is weak and brittle ( i.e. Cementite in Iron's grain boundaries). Think of a metal as one big 3-D puzzle. Transgranular fracture cuts through the puzzle pieces, and intergranular fracture travels along the puzzle pieces pre-cut edges.
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Ductile to Brittle Fracture Transition
In fracture, there are many shades of gray. Brittle fracture and ductile fracture are fairly general terms describing the two opposite extremes of the fracture spectrum. I will explain the factors that make a material lean toward one type of fracture as opposed to the other type of fracture.
The first and foremost factor is temperature. Basically, at higher temperatures the yield strength is lowered and the fracture is more ductile in nature. On the opposite end, at lower temperatures the yield strength is greater and the fracture is more brittle in nature. This relationship with temperature has to do with atom vibrations. As temperature increases, the atoms in the material vibrate with greater frequency and amplitude. This increased vibration allows the atoms under stress to slip to new places in the material ( i.e. break bonds and form new ones with other atoms in the material). This slippage of atoms is seen on the outside of the material as plastic deformation, a common feature of ductile fracture. When temperature decreases however, the exact opposite is true. Atom vibration decreases, and the atoms do not want to slip to new locations in the material. So when the stress on the material becomes high enough, the atoms just break their bonds and do not form new ones. This decrease in slippage causes little plastic deformation before fracture. Thus, we have a brittle type fracture.
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At moderate temperatures (with respect to the material) the material exhibits characteristics of both types of fracture. In conclusion, temperature determines the amount of brittle or ductile fracture that can occur in a material.
Another factor that determines the amount of brittle or ductile fracture that occurs in a material is dislocation density. The higher the dislocation density, the more brittle the fracture will be in the material. The idea behind this theory is that plastic deformation comes from the movement of dislocations.
As dislocations increase in a material due to stresses above the materials yield point, it becomes increasingly difficult for the dislocations to move because they pile into each other. So a material that already has a high dislocation density can only deform but so much before it fractures in a brittle manner.
The last factor is grain size. As grains get smaller in a material, the fracture becomes more brittle. This phenomena is do to the fact that in smaller grains, dislocations have less space to move before they hit a grain boundary. When dislocations can not move very far before fracture, then plastic deformation decreases. Thus, the material's fracture is more brittle. In ending, I would like to say that these are just the basics of brittle fracture. There are whole books written on just brittle fracture. So, if this section interested you at all, go look the books up at your local university library. Good Luck with your studies MSE 2034/44 students.
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CERAMICS - NOTES |Version 3 - view current page
CeramicS
This article is about ceramic materials. For the fine art, see ceramics (art). Fixed Partial Denture, or "Bridge" The
word ceramic is derived from the Greek word κεραμικός (keramikos). The term covers inorganic non-metallic
materials whose formation is due to the action of heat. Up until the 1950s or so, the most important of these
were the traditional clays, made into pottery, bricks, tiles and are like, along with cements and glass. Clay based
ceramics are described in the article on pottery. A composite material of ceramic and metal is known as cermet.
The word ceramic can be an adjective, and can also be used as a noun to refer to a ceramic material, or a
product of ceramic manufacture. Ceramics is a singular noun referring to the art of making things out of ceramic
materials. The technology of manufacturing and usage of ceramic materials is part of the field of ceramic
engineering. Many ceramic materials are hard, porous and brittle. The study and development of ceramics
includes methods to mitigate problems associated with these characteristics, and to accentuate the strengths of
the materials as well as to investigate novel applications. The American Society for Testing and Materials (ASTM)
defines a ceramic article as “an article having a glazed or unglazed body of crystalline or partly crystalline
structure, or of glass, which body is produced from essentially inorganic, non-metallic substances and either is
formed from a molten mass which solidifies on cooling, or is formed and simultaneously or subsequently matured
by the action of the heat.”[1]
Contents 1 Types of ceramic materials 2 Examples of structural ceramics 3 Examples of whiteware ceramics 4 Classification of technical ceramics
o 4.1 Examples of technical ceramics 5 Properties of ceramics
o 5.1 Mechanical properties o 5.2 Electrical properties
Types of ceramic materialsFor convenience ceramic products are usually divided into four sectors, and these are shown below with some
examples:
Structural, including bricks, pipes, floor and roof tiles Refractories , such as kiln linings, gas fire radiants, steel and glass making crucibles Whitewares, including tableware, wall tiles, decorative art objects and sanitary ware Technical, is also known as Engineering, Advanced, Special, and in Japan, Fine Ceramics. Such items
include tiles used in the Space Shuttle program, gas burner nozzles, ballistic protection, nuclear fuel uranium oxide pellets, bio-medical implants, jet engine turbine blades, and missile nose cones. Frequently the raw materials do not include clays.
Examples of structural ceramics Construction bricks. Floor and roof tiles. Sewage pipes
Examples of whiteware ceramics Bone china Earthenware , which is often made from clay, quartz and feldspar. Porcelain , which are often made from kaolin Stoneware
Classification of technical ceramicsTechnical ceramics can also be classified into three distinct material categories:
Oxides : Alumina, zirconia Non-oxides: Carbides, borides, nitrides, silicides Composites : Particulate reinforced, combinations of oxides and non-oxides.
Each one of these classes can develop unique material properties
Examples of technical ceramics
Barium titanate (often mixed with strontium titanate) displays ferroelectricity, meaning that its mechanical, electrical, and thermal responses are coupled to one another and also history-dependent. It is widely used in electromechanical transducers, ceramic capacitors, and data storage elements. Grain boundary conditions can create PTC effects in heating elements.
Bismuth strontium calcium copper oxide , a high-temperature superconductor Boron carbide (B4C), which is used in ceramic plates in some personnel, helicopter and tank armor. Boron nitride is structurally isoelectronic to carbon and takes on similar physical forms: a graphite-like
one used as a lubricant, and a diamond-like one used as an abrasive. Ferrite (Fe3O4), which is ferrimagnetic and is used in the magnetic cores of electrical transformers and
magnetic core memory. Lead zirconate titanate is another ferroelectric material. Magnesium diboride (MgB2), which is an unconventional superconductor.
Silicon carbide (SiC), which is used as a susceptor in microwave furnaces, a commonly used abrasive, and as a refractory material.
Silicon nitride (Si3N4), which is used as an abrasive powder. Steatite is used as an electrical insulator. Uranium oxide (UO2), used as fuel in nuclear reactors. Yttrium barium copper oxide (Y Ba 2Cu3O7-x), another high temperature superconductor. Zinc oxide (ZnO), which is a semiconductor, and used in the construction of varistors. Zirconium dioxide (zirconia), which in pure form undergoes many phase changes between room
temperature and practical sintering temperatures, can be chemically "stabilized" in several different forms. Its high oxygen ion conductivity recommends it for use in fuel cells. In another variant, metastable structures can impart transformation toughening for mechanical applications; most ceramic knife blades are made of this material.
Properties of ceramicsMechanical properties
Ceramic materials are usually ionic or covalently-bonded materials, and can be crystalline or amorphous. A
material held together by either type of bond will tend to fracture before any plastic deformation takes place,
which results in poor toughness in these materials. Additionally, because these materials tend to be porous, the
pores and other microscopic imperfections act as stress concentrators, decreasing the toughness further, and
reducing the tensile strength. These combine to give catastrophic failures, as opposed to the normally much
more gentle failure modes of metals. These materials do show plastic deformation. However, due to the rigid
structure of the crystalline materials, there are very few available slip systems for dislocations to move, and so
they deform very slowly. With the non-crystalline (glassy) materials, viscous flow is the dominant source of
plastic deformation, and is also very slow. It is therefore neglected in many applications of ceramic materials.
Electrical properties
Semiconductors There are a number of ceramics that are semiconductors. Most of these are transition
metal oxides that are II-VI semiconductors, such as zinc oxide. While there is talk of making blue LEDs from zinc
oxide, ceramicists are most interested in the electrical properties that show grain boundary effects. One of the
most widely used of these is the varistor. These are devices that exhibit the property that resistance drops
sharply at a certain threshold voltage. Once the voltage across the device reaches the threshold, there is a
breakdown of the electrical structure in the vicinity of the grain boundaries, which results in its electrical
resistance dropping from several megohms down to a few hundred ohms. The major advantage of these is that
they can dissipate a lot of energy, and they self reset — after the voltage across the device drops below the
threshold, its resistance returns to being high. This makes them ideal for surge-protection applications. As there
is control over the threshold voltage and energy tolerance, they find use in all sorts of applications. The best
demonstration of their ability can be found in electrical substations, where they are employed to protect the
infrastructure from lightning strikes. They have rapid response, are low maintenance, and do not appreciably
degrade from use, making them virtually ideal devices for this application. Semiconducting ceramics are also
employed as gas sensors. When various gases are passed over a polycrystalline ceramic, its electrical resistance
electronics applications, is "Organic Additives and Ceramic Processing," by D. J. Shanefield, Kluwer Publishers
[Boston], 1996. A slurry can be used in place of a powder, and then cast into a desired shape, dried and then
sintered. Indeed, traditional pottery is done with this type of method, using a plastic mixture worked with the
hands. If a mixture of different materials is used together in a ceramic, the sintering temperature is sometimes
above the melting point of one minor component - a liquid phase sintering. This results in shorter sintering times
compared to solid state sintering.
Other applications of ceramics Ceramics are used in the manufacture of knives. The blade of the ceramic knife will stay sharp for much
longer than that of a steel knife, although it is more brittle and can be snapped by dropping it on a hard surface.
Ceramics such as alumina and boron carbide have been used in ballistic armored vests to repel large-caliber rifle fire. Such plates are known commonly as small-arms protective inserts (SAPI). Similar material is used to protect cockpits of some military airplanes, because of the low weight of the material.
Ceramic balls can be used to replace steel in ball bearings. Their higher hardness means that they are much less susceptible to wear and can often more than triple lifetimes. They also deform less under load meaning they have less contact with the bearing retainer walls and can roll faster. In very high speed applications, heat from friction during rolling can cause problems for metal bearings; problems which are reduced by the use of ceramics. Ceramics are also more chemically resistant and can be used in wet environments where steel bearings would rust. The major drawback to using ceramics is a significantly higher cost. In many cases their electrically insulating properties may also be valuable in bearings.
In the early 1980s, Toyota researched production of an adiabatic ceramic engine which can run at a temperature of over 6000 °F (3300 °C). Ceramic engines do not require a cooling system and hence allow a major weight reduction and therefore greater fuel efficiency. Fuel efficiency of the engine is also higher at high temperature, as shown by Carnot's theorem. In a conventional metallic engine, much of the energy released from the fuel must be dissipated as waste heat in order to prevent a meltdown of the metallic parts. Despite all of these desirable properties, such engines are not in production because the manufacturing of ceramic parts in the requisite precision and durability is difficult. Imperfection in the ceramic leads to cracks, which can lead to potentially dangerous equipment failure. Such engines are possible in laboratory settings, but mass-production is unfeasible with current technology.
Work is being done in developing ceramic parts for gas turbine engines. Currently, even blades made of advanced metal alloys used in the engines' hot section require cooling and careful limiting of operating temperatures. Turbine engines made with ceramics could operate more efficiently, giving aircraft greater range and payload for a set amount of fuel.
Recently, there have been advances in ceramics which include bio-ceramics, such as dental implants and synthetic bones. Hydroxyapatite, the natural mineral component of bone, has been made synthetically from a number of biological and chemical sources and can be formed into ceramic materials. Orthopedic implants made from these materials bond readily to bone and other tissues in the body without rejection or inflammatory reactions. Because of this, they are of great interest for gene delivery and tissue engineering scaffolds. Most hydroxy apatite ceramics are very porous and lack mechanical strength and are used to coat metal orthopedic devices to aid in forming a bond to bone or as bone fillers. They are also used as fillers for orthopedic plastic screws to aid in reducing the inflammation and increase absorption of these plastic materials. Work is being done to make strong-fully dense nano crystalline hydroxapatite ceramic materials for orthopedic weight bearing devices, replacing foreign
metal and plastic orthopedic materials with a synthetic natural bone mineral. Ultimately these ceramic materials may be used as bone replacements or with the incorporation of protein collagens, synthetic bones.
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Ceramics
The word ceramic is derived from the Greek word κεραμικός (keramikos). The term covers inorganic non-
metallic materials whose formation is due to the action of heat. Up until the 1950s or so, the most important of
these were the traditional clays, made into pottery, bricks, tiles and are like, along with cements and glass. Clay
based ceramics are described in the article on pottery.
A composite material of ceramic and metal is known as cermet. The word ceramic can be an adjective, and can
also be used as a noun to refer to a ceramic material, or a product of ceramic manufacture. Ceramics is a singular
noun referring to the art of making things out of ceramic materials. The technology of manufacturing and usage
of ceramic materials is part of the field of ceramic engineering.
Many ceramic materials are hard, porous and brittle. The study and development of ceramics includes methods
to mitigate problems associated with these characteristics, and to accentuate the strengths of the materials as
well as to investigate novel applications.
The American Society for Testing and Materials (ASTM) defines a ceramic article as “an article having a glazed or
unglazed body of crystalline or partly crystalline structure, or of glass, which body is produced from essentially
inorganic, non-metallic substances and either is formed from a molten mass which solidifies on cooling, or is
formed and simultaneously or subsequently matured by the action of the heat .”[1]
Types of ceramic materials
For convenience ceramic products are usually divided into four sectors, and these are shown below with some
examples:
Structural, including bricks, pipes, floor and roof tiles Refractories , such as kiln linings, gas fire radiants, steel and glass making crucibles Whitewares, including tableware, wall tiles, decorative art objects and sanitary ware Technical, is also known as Engineering, Advanced, Special, and in Japan, Fine Ceramics. Such items
include tiles used in the Space Shuttle program, gas burner nozzles, ballistic protection, nuclear fuel uranium oxide pellets, bio-medical implants, jet engine turbine blades, and missile nose cones. Frequently the raw materials do not include clays.
Examples of structural ceramics Construction bricks.
Examples of whiteware ceramics Bone china Earthenware , which is often made from clay, quartz and feldspar. Porcelain , which are often made from kaolin Stoneware
Classification of technical ceramicsTechnical ceramics can also be classified into three distinct material categories:
Oxides : Alumina, zirconia Non-oxides: Carbides, borides, nitrides, silicides Composites : Particulate reinforced, combinations of oxides and non-oxides.
Each one of these classes can develop unique material properties
Examples of technical ceramics
Barium titanate (often mixed with strontium titanate) displays ferroelectricity, meaning that its mechanical, electrical, and thermal responses are coupled to one another and also history-dependent. It is widely used in electromechanical transducers, ceramic capacitors, and data storage elements. Grain boundary conditions can create PTC effects in heating elements.
Bismuth strontium calcium copper oxide , a high-temperature superconductor Boron carbide (B4C), which is used in ceramic plates in some personnel, helicopter and tank armor. Boron nitride is structurally isoelectronic to carbon and takes on similar physical forms: a graphite-like
one used as a lubricant, and a diamond-like one used as an abrasive. Ferrite (Fe3O4), which is ferrimagnetic and is used in the magnetic cores of electrical transformers and
magnetic core memory. Lead zirconate titanate is another ferroelectric material. Magnesium diboride (MgB2), which is an unconventional superconductor. Silicon carbide (SiC), which is used as a susceptor in microwave furnaces, a commonly used abrasive, and
as a refractory material. Silicon nitride (Si3N4), which is used as an abrasive powder. Steatite is used as an electrical insulator. Uranium oxide (UO2), used as fuel in nuclear reactors. Yttrium barium copper oxide (Y Ba 2Cu3O7-x), another high temperature superconductor. Zinc oxide (ZnO), which is a semiconductor, and used in the construction of varistors. Zirconium dioxide (zirconia), which in pure form undergoes many phase changes between room
temperature and practical sintering temperatures, can be chemically "stabilized" in several different forms. Its high oxygen ion conductivity recommends it for use in fuel cells. In another variant, metastable structures can impart transformation toughening for mechanical applications; most ceramic knife blades are made of this material.
The principles of sintering-based methods is simple. Once a roughly held together object (called a "green body")
is made, it is baked in a kiln, where diffusion processes cause the green body to shrink. The pores in the object
close up, resulting in a denser, stronger product.
The firing is done at a temperature below the melting point of the ceramic. There is virtually always some
porosity left, but the real advantage of this method is that the green body can be produced in any way
imaginable, and still be sintered. This makes it a very versatile route.
There are thousands of possible refinements of this process. Some of the most common involve pressing the
green body to give the densification a head start and reduce the sintering time needed. Sometimes organic
binders such as polyvinyl alcohol are added to hold the green body together; these burn out during the firing (at
200–350°C).
Sometimes organic lubricants are added during pressing to increase densification. It is not uncommon to
combine these, and add binders and lubricants to a powder, then press. (The formulation of these organic
chemical additives is an art in itself. This is particularly important in the manufacture of high performance
ceramics such as those used by the billions for electronics, in capacitors, inductors, sensors, etc.
The specialized formulations most commonly used in electronics are detailed in the book "Tape Casting," by R.E.
Mistler, et al., Amer. Ceramic Soc. [Westerville, Ohio], 2000.) A comprehensive book on the subject, for
mechanical as well as electronics applications, is "Organic Additives and Ceramic Processing," by D. J. Shanefield,
Kluwer Publishers [Boston], 1996.
A slurry can be used in place of a powder, and then cast into a desired shape, dried and then sintered. Indeed,
traditional pottery is done with this type of method, using a plastic mixture worked with the hands.
If a mixture of different materials is used together in a ceramic, the sintering temperature is sometimes above
the melting point of one minor component - a liquid phase sintering. This results in shorter sintering times
compared to solid state sintering.
Other applications of ceramics Ceramics are used in the manufacture of knives. The blade of the ceramic knife will stay sharp for much
longer than that of a steel knife, although it is more brittle and can be snapped by dropping it on a hard surface.
Ceramics such as alumina and boron carbide have been used in ballistic armored vests to repel large-caliber rifle fire. Such plates are known commonly as small-arms protective inserts (SAPI). Similar material is used to protect cockpits of some military airplanes, because of the low weight of the material.
Ceramic balls can be used to replace steel in ball bearings. Their higher hardness means that they are much less susceptible to wear and can often more than triple lifetimes. They also deform less under load meaning they have less contact with the bearing retainer walls and can roll faster. In very high speed applications, heat from friction during rolling can cause problems for metal bearings; problems which are reduced by the use of ceramics. Ceramics are also more chemically resistant and can be used in wet environments where steel bearings would rust. The major drawback to using ceramics is a significantly higher cost. In many cases their electrically insulating properties may also be valuable in bearings.
In the early 1980s, Toyota researched production of an adiabatic ceramic engine which can run at a temperature of over 6000 °F (3300 °C). Ceramic engines do not require a cooling system and hence allow a major weight reduction and therefore greater fuel efficiency. Fuel efficiency of the engine is also higher at high temperature, as shown by Carnot's theorem. In a conventional metallic engine, much of the energy released from the fuel must be dissipated as waste heat in order to prevent a meltdown of the metallic parts. Despite all of these desirable properties, such engines are not in production because the manufacturing of ceramic parts in the requisite precision and durability is difficult. Imperfection in the ceramic leads to cracks, which can lead to potentially dangerous equipment failure. Such engines are possible in laboratory settings, but mass-production is unfeasible with current technology.
Work is being done in developing ceramic parts for gas turbine engines. Currently, even blades made of advanced metal alloys used in the engines' hot section require cooling and careful limiting of operating temperatures. Turbine engines made with ceramics could operate more efficiently, giving aircraft greater range and payload for a set amount of fuel.
Recently, there have been advances in ceramics which include bio-ceramics, such as dental implants and synthetic bones. Hydroxyapatite, the natural mineral component of bone, has been made synthetically from a number of biological and chemical sources and can be formed into ceramic materials. Orthopedic implants made from these materials bond readily to bone and other tissues in the body without rejection or inflammatory reactions. Because of this, they are of great interest for gene delivery and tissue engineering scaffolds. Most hydroxy apatite ceramics are very porous and lack mechanical strength and are used to coat metal orthopedic devices to aid in forming a bond to bone or as bone fillers. They are also used as fillers for orthopedic plastic screws to aid in reducing the inflammation and increase absorption of these plastic materials. Work is being done to make strong-fully dense nano crystalline hydroxapatite ceramic materials for orthopedic weight bearing devices, replacing foreign metal and plastic orthopedic materials with a synthetic natural bone mineral. Ultimately these ceramic materials may be used as bone replacements or with the incorporation of protein collagens, synthetic bones.
CeramicsA ceramic has traditionally been defined as “an inorganic, nonmetallic solid that is prepared from powdered
materials, is fabricated into products through the application of heat, and displays such characteristic properties
as hardness, strength, low electrical conductivity, and brittleness." The word ceramic comes the from Greek
word "keramikos", which means "pottery." They are typically crystalline in nature and are compounds formed
between metallic and nonmetallic elements such as aluminum and oxygen (alumina-Al2O3), calcium and oxygen
(calcia - CaO), and silicon and nitrogen (silicon nitride-Si3N4).
Depending on their method of formation, ceramics can be dense or lightweight. Typically, they will demonstrate
excellent strength and hardness properties; however, they are often brittle in nature. Ceramics can also be
formed to serve as electrically conductive materials or insulators. Some ceramics, like superconductors, also
display magnetic properties. They are also more resistant to high temperatures and harsh environments than
metals and polymers. Due to ceramic materials wide range of properties, they are used for a multitude of
applications.
The broad categories or segments that make up the ceramic industry can be classified as:
Structural clay products (brick, sewer pipe, roofing and wall tile, flue linings, etc.) Whitewares (dinnerware, floor and wall tile, electrical porcelain, etc.) Refractories (brick and monolithic products used in metal, glass, cements, ceramics, energy conversion,
petroleum, and chemicals industries) Glasses (flat glass (windows), container glass (bottles), pressed and blown glass (dinnerware), glass fibers
(home insulation), and advanced/specialty glass (optical fibers)) Abrasives (natural (garnet, diamond, etc.) and synthetic (silicon carbide, diamond, fused alumina, etc.)
abrasives are used for grinding, cutting, polishing, lapping, or pressure blasting of materials) Cements (for roads, bridges, buildings, dams, and etc.) Advanced ceramics
o Structural (wear parts, bioceramics, cutting tools, and engine components) o Electrical (capacitors, insulators, substrates, integrated circuit packages, piezoelectrics, magnets
and superconductors) o Coatings (engine components, cutting tools, and industrial wear parts) o Chemical and environmental (filters, membranes, catalysts, and catalyst supports)
Hot working is the deformation that is carried out above the recrystallization temperature. Page | 33
In these circumstances, annealing takes place while the metal is worked rather than being a separate process.
The metal can therefore be worked without it becoming work hardened. Hot working is usually carried out with
the metal at a temperature of about 0.6 of its melting point.
Effects of hot working
· At high temperature, scaling and oxidation exist. Scaling and oxidation produce undesirable surface finish. Most
ferrous metals needs to be cold worked after hot working in order to improve the surface finish.
· The amount of force needed to perform hot working is less than that for cold work.
· The mechanical properties of the material remain unchanged during hot working.
· The metal usually experiences a decrease in yield strength when hot worked. Therefore, it is possible to hot
work the metal without causing any fracture.
Quenching is the sudden immersion of a heated metal into cold water or oil. It is used to make the metal very
hard. To reverse the effects of quenching, tempering is used (reheated of the metal for a period of time)
To reverse the process of quenching, tempering is used, which is the reheat of the metal.
Methods used for Cold, Hot working
ROLLING -- FORGING ------
The advantages of hot working are
Lower working forces to produce a given shape, which means the machines involved don't have to be as strong, which means they can be built more cheaply;
The possibility of producing a very dramatic shape change in a single working step, without causing large amounts of internal stress, cracks or cold working;
Sometimes hot working can be combined with a casting process so that metal is cast and then immediately hot worked. This saves money because we don't have to pay for the energy to reheat the metal.
Hot working tends to break up large crystals in the metal and can produce a favourable alignment of elongated crystals (see DeGarmo Fig. 17-4 below).
Hot working can remove some kinds of defects that occur in cast metals. It can close gas pockets (bubbles) or voids in a cast billet; and it may also break up non-metallic slag which can sometimes get caught in the melt (inclusions).
The main problems, however, are
If the recrystallisation temperature of the worked metal is high e.g. if we are talking about steel, specialised methods are needed to protect the machines that work the metal. The working processes are also dangerous to human operators and very unpleasant to work near (see picture below for some idea why).
Page | 34
The surface finish of hot worked steel tends to be pretty crude because (a) the dies or rollers wear quite rapidly; (b) there is a lot of dimensional change as the worked object cools; and (c) there is the constant annoying problem of scale formation on the surface of the hot steel.
Of course smart people have found ways to minimise the problems or work around them - more below - and as a
result hot working is a very common and useful process. We just have to be aware of its limits and follow the hot
working operations with other types of manufacturing process that can fix the problems that occur.
Cold working
As explained above, when we work a metal below the recrystallisation temperature, there is accumulation of a
kind of material damage at the atomic level, through the pile-up of dislocations. However this is not necessarily a
bad thing. Many useful engineering objects are deliberately cold-worked as part of the manufacturing process to
achieve improved properties. One common example is fencing wire. It is cold-drawn in the final stages, before
being galvanised (plated with zinc) and coiled ready for sale. The cold working stages increase the yeild stress of
the wire, meaning we can pull harder on the wire before it deforms plastically (stretches). That's helpful when
you are stringing a fence. However the cold working does not increase the ultimate strength of the material. So
in a sense, cold working uses up some of the safety margin of the material. If a very strongly cold worked
material is overloaded, it could well just break like a brittle material with no warning. So we try to design cold
working as a compromise. A little bit can be good: too much could be dangerous.
The advantages of cold working are
A better surface finish may be achieved; Dimensional accuracy can be excellent because the work is not hot so it doesn't shrink on cooling; also
the low temperatures mean the tools such as dies and rollers can last a long time without wearing out. Usually there is no problem with oxidative effects such as scale formation. In fact, cold rolling (for
example) can make such scale come off the surface of a previously hot-worked object. Controlled amounts of cold work may be introduced. As with hot working, the grain structure of the material is made to follow the deformation direction,
which can be good for the strength of the final product. Strength and hardness are increased, although at the expense of ductility. OH & S problems related to working near hot metal are eliminated.
However
There is a limit to how much cold work can be done on a given piece of metal. See the discussion above about accumulation of damage in the form of piled up dislocations. There are ways to get around this problem, see below.
Higher forces are required to produce a given deformation, which means we need heavily built, strong forming machines (= $$$).
A neat trick: cold work then normalise
Cold working has many advantages and is very much the more common type of metal forming. However if a
large overall deformation is desired, how can we do it using only cold working? The answer is: do some cold
work, then put the object through a heat-treatment cycle to relieve the atomic-scale damage caused by the cold Page | 35
work. This is called annealing or normalising the metal. It is done by heating the metal object above the
recrystallisation temperature, waiting a few minutes, then allowing it to cool. Of course we have to pay for the
energy to do the heating.
This type of cold-work/anneal/cold-work/anneal sequence is used by plumbers who shape copper tube on a
building site. When a piece of tube has to bent sharply, it is done in easy stages with a proper annealing between
each stage (usually done using a hand-held gas flame). This ensures that metal won't crack during the bending
operations.
Think about working with a sheet of lead on a nice warm day in the Australian sun. The lead will likely be above
its recrystallisation temperature, with no special heating required. This can actually be very useful. It means you
can shape your sheet of lead for hours - bend it back and forth, hammer it out, whatever - and it will probably
accept all the deformation with cracking. This is one of the reasons lead sheet was so popular in ancient times as
a roofing/guttering material (for those who could afford it). Any strange shape needed could be hammered out
of a sheet or even a lump, right on site, and with no special furnaces or other technology.
The advantages of hot working are
Lower working forces to produce a given shape, which means the machines involved don't have to be as strong, which means they can be built more cheaply;
The possibility of producing a very dramatic shape change in a single working step, without causing large amounts of internal stress, cracks or cold working;
Sometimes hot working can be combined with a casting process so that metal is cast and then immediately hot worked. This saves money because we don't have to pay for the energy to reheat the metal.
Hot working tends to break up large crystals in the metal and can produce a favourable alignment of elongated crystals (see DeGarmo Fig. 17-4 below).
Hot working can remove some kinds of defects that occur in cast metals. It can close gas pockets (bubbles) or voids in a cast billet; and it may also break up non-metallic slag which can sometimes get caught in the melt (inclusions).
The main problems, however, are
If the recrystallisation temperature of the worked metal is high e.g. if we are talking about steel, specialised methods are needed to protect the machines that work the metal. The working processes are also dangerous to human operators and very unpleasant to work near (see picture below for some idea why).
The surface finish of hot worked steel tends to be pretty crude because (a) the dies or rollers wear quite rapidly; (b) there is a lot of dimensional change as the worked object cools; and (c) there is the constant annoying problem of scale formation on the surface of the hot steel.
Of course smart people have found ways to minimise the problems or work around them - more below - and as a
result hot working is a very common and useful process. We just have to be aware of its limits and follow the hot
working operations with other types of manufacturing process that can fix the problems that occur.
Page | 36
Cold working
As explained above, when we work a metal below the recrystallisation temperature, there is accumulation of a
kind of material damage at the atomic level, through the pile-up of dislocations. However this is not necessarily a
bad thing. Many useful engineering objects are deliberately cold-worked as part of the manufacturing process to
achieve improved properties. One common example is fencing wire. It is cold-drawn in the final stages, before
being galvanised (plated with zinc) and coiled ready for sale. The cold working stages increase the yeild stress of
the wire, meaning we can pull harder on the wire before it deforms plastically (stretches). That's helpful when
you are stringing a fence. However the cold working does not increase the ultimate strength of the material. So
in a sense, cold working uses up some of the safety margin of the material. If a very strongly cold worked
material is overloaded, it could well just break like a brittle material with no warning. So we try to design cold
working as a compromise. A little bit can be good: too much could be dangerous.
The advantages of cold working are
A better surface finish may be achieved; Dimensional accuracy can be excellent because the work is not hot so it doesn't shrink on cooling; also
the low temperatures mean the tools such as dies and rollers can last a long time without wearing out. Usually there is no problem with oxidative effects such as scale formation. In fact, cold rolling (for
example) can make such scale come off the surface of a previously hot-worked object. Controlled amounts of cold work may be introduced. As with hot working, the grain structure of the material is made to follow the deformation direction,
which can be good for the strength of the final product. Strength and hardness are increased, although at the expense of ductility. OH & S problems related to working near hot metal are eliminated.
However
There is a limit to how much cold work can be done on a given piece of metal. See the discussion above about accumulation of damage in the form of piled up dislocations. There are ways to get around this problem, see below.
Higher forces are required to produce a given deformation, which means we need heavily built, strong forming machines (= $$$).
A neat trick: cold work then normalise
Cold working has many advantages and is very much the more common type of metal forming. However if a
large overall deformation is desired, how can we do it using only cold working? The answer is: do some cold
work, then put the object through a heat-treatment cycle to relieve the atomic-scale damage caused by the cold
work. This is called annealing or normalising the metal. It is done by heating the metal object above the
recrystallisation temperature, waiting a few minutes, then allowing it to cool. Of course we have to pay for the
energy to do the heating.
This type of cold-work/anneal/cold-work/anneal sequence is used by plumbers who shape copper tube on a
building site. When a piece of tube has to bent sharply, it is done in easy stages with a proper annealing between
each stage (usually done using a hand-held gas flame). This ensures that metal won't crack during the bending
operations.
Think about working with a sheet of lead on a nice warm day in the Australian sun. The lead will likely be above
Page | 37
its recrystallisation temperature, with no special heating required. This can actually be very useful. It means you
can shape your sheet of lead for hours - bend it back and forth, hammer it out, whatever - and it will probably
accept all the deformation with cracking. This is one of the reasons lead sheet was so popular in ancient times as
a roofing/guttering material (for those who could afford it). Any strange shape needed could be hammered out
of a sheet or even a lump, right on site, and with no special furnaces or other technology.
Page | 38
Composite material
Composite materials (or composites for short) are engineered materials made from two or more constituent
materials with significantly different physical or chemical properties and which remain separate and distinct on a
macroscopic level within the finished structure.
Background
The most primitive composite materials comprised straw and mud in the form of bricks for building construction;
the Biblical book of Exodus speaks of the Israelites being oppressed by Pharaoh, by being forced to make bricks
without straw. The ancient brick-making process can still be seen on Egyptian tomb paintings in the
Metropolitan Museum of Art [1] . The most advanced examples perform routinely on spacecraft in demanding
environments. The most visible applications pave our roadways in the form of either steel and aggregate
reinforced portland cement or asphalt concrete. Those composites closest to our personal hygiene form our
shower stalls and bath tubs made of fiberglass. Solid surface, imitation granite and cultured marble sinks and
counter tops are widely used to enhance our living experiences.
There are two categories of constituent materials: matrix and reinforcement. At least one portion of each type is
required. The matrix material surrounds and supports the reinforcement materials by maintaining their relative
positions. The reinforcements impart their special mechanical and physical properties to enhance the matrix
properties.
A synergism produces material properties unavailable from the individual constituent materials, while the wide
variety of matrix and strengthening materials allows the designer of the product or structure to choose an
optimum combination. Engineered composite materials must be formed to shape. The matrix material can be
introduced to the reinforcement before or after the reinforcement material is placed into the mold cavity or
onto the mold surface.
The matrix material experiences a melding event, after which the part shape is essentially set. Depending upon
the nature of the matrix material, this melding event can occur in various ways such as chemical polymerization
Polypropylene and bicomponent fibres are used in many different composite products:
fibre-reinforced concrete (to reinforce and prevent cracks), insulation material (to avoid the use of chemical binders), multifunctional liquid transport media (acquisition and distribution layers), woven fabrics (as a dimensional stability network) laminated products (lamination between textiles and boards)
Polypropylene and bicomponent (PP/PE) fibres have the ability to add structural performance, and functionality
to the composite material.
Polypropylene and bicomponent (PP/PE) fibres provide the following advantages in fibre reinforced composites:
Enable lightweight constructions (PP fibres have the lowest specific gravity of all fibres) Easy to process and environmentally friendly thermoplastics Mechanical properties, toughness and impact strength Stability in rigid environments (Resistant to deterioration from chemicals, mildew, perspiration, rot and
weather) Ability to add bulkiness and softness to the composite (if needed).
Viscoelastic creep data can be presented in one of two ways. Total strain can be plotted as a function of time for
a given temperature or temperatures. Below a critical value of applied stress, a material may exhibit linear
viscoelasticity. Above this critical stress, the creep rate grows disproportionately faster. The second way of
graphically presenting viscoelastic creep in a material is by plotting the creep modulus (constant applied stress
divided by total strain at a particular time) as a function of time.[1] Below its critical stress, the viscoelastic creep
modulus is independent of stress applied. A family of curves describing strain versus time response to various
applied stress may be represented by a single viscoelastic creep modulus versus time curve if the applied
stresses are below the material's critical stress value.
Additionally, the molecular weight of the polymer of interest is known to affect its creep behavior. The effect of
increasing molecular weight tends to promote secondary bonding between polymer chains and thus make the
polymer more creep resistant. Similarly, aromatic polymers are even more creep resistant due to the added
stiffness from the rings. Both molecular weight and aromatic rings add to polymers' thermal stability, increasing
the creep resistance of a polymer. (Meyers and Chawla, 1999, 573)
Both polymers and metals can creep.[2] Polymers experience significant creep at all temperatures above ~-
200°C, however there are three main differences between polymetric and metallic creep. Metallic creep:[2]
is not linearly viscoelastic in not recoverable only significant at high temperatures
Other examples The Collapse of the World Trade Center was due in part to creep. The creep rate of hot pressure-loaded components in a nuclear reactor at power can be a significant
design-constraint, since the creep rate is enhanced by the flux of energetic particles. Creep was blamed on the Big Dig tunnel ceiling collapse in Boston, Massachusetts that occurred in July
2006 [1]
See also Stress relaxation Hysteresis Viscoelasticity Biomaterial Biomechanics Brittle-ductile transition zone
Strength / Mechanics of Material Basics, General Equations and Definitions
Stress Strain Hookes Law Youngs Modulus Material Strength Yield Strength Ductility Malleability Toughness Hardness Cold and Hot Working Work ( Strain ) Hardening Creep Heat Treatment Von Mises Criterion ( Maximum Distortion Energy Criterion )
Material Specifications and Characteristics - Ferrous and Non-Ferrous
Properties of Metals / Materials Modulus of Elasticity, Ultimate Strength, Thermal Properties, Aluminum Tempers, Brass, Density, more...
Vibration
Acoustics and Mechanical Vibration General Vibration Definitions and Terminology Miles Equation - Vibration Deflection of Uniform Bar with Free Ends Mechanical Vibration
Pinned Columns
Ideal Pinned Column Buckling Equation and Calculation, Euler's Formula
Bolt / Stud Preload Calculator Fastener / Thread Tensile Area of External Thread Formula Fastener / Threaded Pitch Circle Diameter Formula and Calculation Fastener / Threaded Shear Area Formula and Calculation Minimum Thread Engagement Formula and Calculation ISO Minimum Length of Thread Engagement Formula and Calculations Per FED-STD-H28/2B Shear Area Internal and External Thread Formula and Calculation Per FED-STD-H28/2B
Dislocations and Strengthening Mechanisms Dislocations and Strengthening Mechanisms
1. Introduction The key idea of the chapter is that plastic deformation is due to the motion of a large number of dislocations. The motion is called slip. Thus, the strength (resistance to deformation) can be improved by putting obstacles to slip.
2. Basic Concepts Dislocations can be edge dislocations, screw dislocations and exist combination of the two (Ch. 4.4). Their motion (slip) occurs by sequential bond breaking and bond reforming (Fig. 7.1). The number of dislocations per unit volume is the dislocation density, in a plane they are measured per unit area.
3. Characteristics of Dislocations There is strain around a dislocation which influences how they interact with other dislocations, impurities, etc. There is compression near the extra plane (higher atomic density) and tension following the dislocation line (Fig. 7.4)
4. Dislocations interact among themselves (Fig. 7.5). When they are in the same plane, they repel if they have the same sign and annihilate if they have opposite signs (leaving behind a perfect crystal). In general, when dislocations are close and their strain fields add to a larger value, they repel, because being close increases the potential energy (it takes energy to strain a region of the material). The number of dislocations increases dramatically during plastic deformation. Dislocations spawn from existing dislocations, and from defects, grain boundaries and surface irregularities.
5. Slip Systems In single crystals there are preferred planes where dislocations move (slip planes). There they do not move in any direction, but in preferred crystallographic directions (slip direction). The set of slip planes and directions constitute slip systems. The slip planes are those of highest packing density. How do we explain this? Since the distance between atoms is shorter than the average, the distance perpendicular to the plane has to be longer than average. Being relatively far apart, the atoms can move more easily with respect to the atoms of the adjacent plane. (We did not discuss direction and plane nomenclature for slip systems.)BCC and FCC crystals have more slip systems, that is more ways for dislocation to propagate. Thus, those crystals are more ductile than HCP crystals (HCP crystals are more brittle).
6. Slip in Single Crystals
A tensile stress will have components in any plane that is not perpendicular to the stress. These components are resolved shear stresses. Their magnitude depends on orientation (see Fig. 7.7). R = cos cos If the shear stress reaches the critical resolved shear stress CRSS, slip (plastic deformation) can start. The stress needed is: y=CRSS / (cos cos )max at the angles at which CRSS is a maximum. The minimum stress needed for yielding is when = = 45 degrees: y=CRSS. Thus, dislocations will occur first at slip planes oriented close to this angle with respect to the applied stress (Figs. 7.8 and 7.9).
6. Plastic Deformation of Polycrystalline Materials Slip directions vary from crystal to crystal. When plastic deformation occurs in a grain, it will be constrained by its neighbors which may be less favorably oriented. As a result, polycrystalline metals
Page | 64
are stronger than single crystals (the exception is the perfect single crystal, as in whiskers.)
7. Deformation by Twinning This topic is not included.
Mechanisms of Strengthening in Metals
General principles. Ability to deform plastically depends on ability of dislocations to move. Strengthening consists in hindering dislocation motion. We discuss the methods of grain-size reduction, solid-solution alloying and strain hardening. These are for single-phase metals. We discuss others when treating alloys. Ordinarily, strengthening reduces ductility.
8. Strengthening by Grain Size Reduction This is based on the fact that it is difficult for a dislocation to pass into another grain, especially if it is very misaligned. Atomic disorder at the boundary causes discontinuity in slip planes. For high-angle grain boundaries, stress at end of slip plane may trigger new dislocations in adjacent grains. Small angle grain boundaries are not effective in blocking dislocations. The finer the grains, the larger the area of grain boundaries that impedes dislocation motion. Grain-size reduction usually improves toughness as well. Usually, the yield strength varies with grain size d according to:
y = 0 + ky / d1/2
Grain size can be controlled by the rate of solidification and by plastic deformation.
9. Solid-Solution Strengthening Adding another element that goes into interstitial or substitutional positions in a solution increases strength. The impurity atoms cause lattice strain (Figs. 7.17 and 7.18) which can "anchor" dislocations. This occurs when the strain caused by the alloying element compensates that of the dislocation, thus achieving a state of low potential energy. It costs strain energy for the dislocation to move away from this state (which is like a potential well). The scarcity of energy at low temperatures is why slip is hindered. Pure metals are almost always softer than their alloys.
10. Strain Hardening Ductile metals become stronger when they are deformed plastically at temperatures well below the melting point (cold working). (This is different from hot working is the shaping of materials at high temperatures where large deformation is possible.) Strain hardening (work hardening) is the reason for the elastic recovery discussed in Ch. 6.8. The reason for strain hardening is that the dislocation density increases with plastic deformation (cold work) due to multiplication. The average distance between dislocations then decreases and dislocations start blocking the motion of each one. The measure of strain hardening is the percent cold work (%CW), given by the relative reduction of the original area, A0 to the final value Ad :
%CW = 100 (A0–Ad)/A
11. Recovery, recrystallization and Grain Growth12. Plastic deformation causes 1) change in grain size, 2) strain hardening, 3) increase in
the dislocation density. Restoration to the state before cold-work is done by heating through two processes: recovery and recrystallization. These may be followed by grain growth.
Page | 65
13. Recovery Heating increased diffusion enhanced dislocation motion relieves internal strain energy and reduces the number of dislocation. The electrical and thermal conductivity are restored to the values existing before cold working.
14. Recrystallization Strained grains of cold-worked metal are replaced, upon heating, by more regularly-spaced grains. This occurs through short-range diffusion enabled by the high temperature. Since recrystallization occurs by diffusion, the important parameters are both temperature and time. The material becomes softer, weaker, but more ductile (Fig. 7.22). Recrystallization temperature: is that at which the process is complete in one hour. It is typically 1/3 to 1/2 of the melting temperature. It falls as the %CW is increased. Below a "critical deformation", recrystallization does not occur.
15. Grain Growth
The growth of grain size with temperature can occur in all polycrystalline materials. It occurs by migration of atoms at grain boundaries by diffusion, thus grain growth is faster at higher temperatures. The "driving force" is the reduction of energy, which is proportional to the total area. Big grains grow at the expense of the small ones. Important Terms:
Cold working
Critical resolved
Shear stress
Dislocation density
Grain growth
Lattice strain
Recovery
Recrystallization
Recrystallization temperature
Resolved shear stress
Slip
Slip system
Strain hardening
Solid-solution strengthening
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Ductile FractureDuctile Fracture One of the most important and key concepts in the entire field of Materials Science
and Engineering is fracture. In its simplest form, fracture can be described as a single body being separated into
pieces by an imposed stress. For engineering materials there are only two possible modes of fracture, ductile
and brittle. In general, the main difference between brittle and ductile fracture can be attributed to the amount
of plastic deformation that the material undergoes before fracture occurs. Ductile materials demonstrate large
amounts of plastic deformation while brittle materials show little or no plastic deformation before fracture.
Figure 1 (below), a tensile stress-strain curve, represents the degree of plastic deformation exhibited by both
brittle and ductile materials before fracture.
Crack initiation and propagation are essential
to fracture. The manner through which the crack propagates through the material gives great insight into the
mode of fracture. In ductile materials (ductile fracture), the crack moves slowly and is accompanied by a large
amount of plastic deformation. The crack will usually not extend unless an increased stress is applied. On the
other hand, in dealing with brittle fracture, cracks spread very rapidly with little or no plastic deformation. The
cracks that propagate in a brittle material will continue to grow and increase in magnitude once they are
initiated. Another important mannerism of crack propagation is the way in which the advancing crack travels
through the material. A crack that passes through the grains within the material is undergoing transgranular
fracture. However, a crack that propagates along the grain boundaries is termed an intergranular fracture. Figure
2 (below) shows a scanning electron fractograph of ductile cast iron, examining a transgranular fracture surface.