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MATERIAL TECHNOLOGY

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TOPICS TO BE COVERED

Structure of Metals and Alloys

Properties of Materials* Metallic

* Non-metallic

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STRUCTURE OF METALS AND ALLOYS

Atoms in metals are bonded together in a

bond called metallic bond, which exist

when some electrons in the valence shell

separate from their atom and exist in a

cloud surrounding all the positively charged

atoms.

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SCHEMATIC ILLUSTRATION OF A

METALLIC BOND

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CHARACTERISTICS OF METALS

Metals conduct electricity ( electrons are free to move in an

electric field)

Metals are opaque and lustrous (free electrons absorb andthen radiate back most of the light energy that falls on them)

Metals conduct heat effectively (free electrons transferthermal energy)

Metals are ductile

Metals are dense and heavy.

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Metallic bond

NON-SPECIFIC ( DIFFERENT METALS CAN BEALLOYED OR JOINED ONE TO ANOTHER)

NON-DIRECTIONAL, PULLINGEQUALLY HARD IN ALL DIRECTIONS.

METAL ATOMS ARE BOUND

TIGHTLY, SO THAT THEIR CORES FITCLOSELY AMONG ONE ANOTHER.

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METALS

Crystalline structure

High thermal and electrical conductivity

Ability to be deformed plastically

Metallic luster or high reflectivity of light

Ability to donate electrons and form a positive

ion.

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CRYSTAL STRUCTURE

BODY-CENTERED CUBIC (BCC)

FACE-CENTERED CUBIC (FCC)

HEXAGONAL CLOSE-PACKED (HCP)

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CRYSTAL TYPES

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CRYSTALLINE STRUCTURE

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BODY-CENTERED CUBIC(BCC)

MADE UP OF NINE(9) ATOMS

STRONG AND HARD STRUCTURE

EXAMPLES:

- Chromium, molybdenum, barium,

- tungsten, sodium, vanadium- steel under 723 degrees C (alpha iron or

ferrite)

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Face Centered Cubic (FCC)

Made up of 14 atoms

Ductile metals

Examples: aluminum, copper, gold, lead,nickel, platinum, silver

Iron, which is body-centered cubic at room

temperature is an FCC structure in thetemperature range 9101400 degrees C(gamma iron or austenite)

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HEXAGONAL CLOSE-PACKED (HCP)

Made up of 17 atoms

Metals with this structure are quite

susceptible to work-hardening

Examples: cadmium, cobalt, magnesium,

titanium and zinc.

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GRAINS (CRYSTALS) AND GRAIN

BOUNDARIES

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GRAIN BOUNDARY FORMATION

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STRUCTURE OF ALLOYS

AN ALLOY IS A SUBSTANCE THAT

HAS METALLIC PROPERTIES AND IS

COMPOSED OF TWO OR MORECHEMICAL ELEMENTS, OF WHICH AT

LEAST ONE IS A METAL.

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MODES OF ARRANGEMENT

PURE METAL

SOLID SOLUTION

- substitutional solid solution

- interstitial solid solution

INTERMETALLIC COMPOUND

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PURE METAL

There exist no B-atoms in A-crystal

grains and no A-atoms in B-grains.A and B metals are mutually insoluble

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SOLID SOLUTIONTHERE EXIST B-ATOMS (SOLUTE)

IN A-CRYSTAL GRAINS(SOLVENT).

Substitutional Solid Solution

There exist B-atoms at the latticepoints of A-crystals.

Example: Cu-Ni System

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SOLID SOLUTION

INTERSTITIAL SOLID SOLUTION- B-ATOMS ARE ACCOMMODATED IN THE

INTERSTICES OF THE LATTICE OF A-

CRYSTAL.

- LIGHT ATOMS WITH SMALL RADII SUCH

AS H, N, C AND B TEND TO TAKE UP

INSTERTITIAL POSITION IN ALLOY.

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INTERMETALLIC COMPOUNDS

Formed between chemically dissimilar metals ;

Combined following the rules of chemical valence;

Have strong bond (ionic or covalent)

Their properties are essentially non-metallic;

Ratio of the number of atoms-A to B-atoms is fixed (m:n)

Crystal structure very complicated

Intermetallic compounds are very hard and brittle due to

complicated crystal structure.

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ALLOTROPIC TRANSFORMATION

Many metals exist in more than one crystal

structure;

Allotropic transformation is a process whena metal changes from one crystal

arrangement to another;

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ALLOTROPIC FORMS OR PHASETRANSFORMATION OF IRON:

1. BCC ( below 1330 degrees F or 704 degrees C );2. FCC ( above 1670 degrees F or 911 degrees C );

3. Delta iron ( between 2550degrees F or 1398

degrees C )THE EXACT TEMPERATURE IS DETERMINED BYTHE AMOUNT OF C AND OTHER ALLOYINGELEMENTS.

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THE PROPERTIES OF IRON AND

STEEL ARE GOVERNED BY THE

PHASE TRANSFORMATION THEYUNDERGO DURING PROCESSING.

UNDERSTANDING THESE

TRANSFORMATIONS IS ESSENTIALTO THE SUCCESSFUL WELDING OF

THESE METALS.

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STEEL is an iron alloy containing less than two

per cent carbon.

Presence of carbon alters the temperature at whichfreezing and phase transformation take place.

Addition of other alloying elements also affects

the transformation temperatures.

Various phases of iron are: austenite, ferrite, and

cementite.

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ON COOLING

- DELTA FERRITE TO AUSTENITE, TRANSFORMATION

OCCURS AT 2535 DEGREES F (1390 DEGREES C) INESSENTIALLY PURE IRON;

- IN STEEL, transformation temperature increases withincreasing carbon content to a maximum of 2718 degrees F

(1492 degrees C);

- STEELS with more than 0.5 per cent CARBON FREEZE

directly to austenite at a temperature below 2718 degrees

F (1492 C) ( delta ferrite does not exist in these steels)

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FURTHER COOLING:

Austenite transforms to ferrite plus iron carbide.Transformation occurs in essentially pure iron at 1670

degrees F ( 910C). At both high and low temperature, presence of carbon

promotes the stability of austenite at the expense ofdelta and alpha ferrite.

Austenite can dissolve up to 2.0 per cent of carbon insolid solution, but ferrite can dissolve only 0.025 percent.

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Note, the carbon equilibrium diagram shown above is only for illustration, in reality it

will be heavily distorted because of the rapid heating and cooling rates involved inthe welding process.

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a.) Mixture of ferrite and pearlite grains; temperature below A1, therefore microstructure not

significantly affected.

b.) Pearlite transformed to Austenite, but not sufficient temperature available to exceed theA3 line, therefore not all ferrite grains transform to Austenite. On cooling, only the

transformed grains will be normalised.

c.) Temperature just exceeds A3 line, full Austenite transformation. On cooling all grains

will be normalised

d.) Temperature significantly exceeds A3 line permitting grains to grow. On cooling, ferrite

will form at the grain boundaries, and a course pearlite will form inside the grains. A

course grain structure is more readily hardened than a finer one, therefore if the cooling

rate between 800C to 500C is rapid, a hard microstructure will be formed. This is why

a brittle fracture is most likely to propagate in this region.

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AT EQUILIBRIUM CONDITIONS, THE FOLLOWING

CONSTITUENTS MAY BE PRESENT:

1) FERRITE: A solid solution of carbon in alpha iron;

2) PEARLITE: A mixture of cementite and ferrite that forms in plates

or lamellae;

3) CEMENTITE: Iron carbide, Fe3C, present in pearlite or as massive

carbides in high carbon steels

4) AUSTENITE: A solid mixture of carbon in gamma iron;5) LEBORITE: A eutectic mixture of austenite & cementite.

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NON-METALLIC MATERIALS

CERAMICS

High hardness and resistance to abrasion and

corrosion;

High temperature properties superior to those of any

metals;

Less ductile;

Intrinsically brittle, susceptible to thermal shock; RESISTANCE TO THERMAL SHOCK IS DIRECTLY

DEPENDENT ON A LOW COEFFICIENT OF THERMAL

EXPANSION AND HIGH THERMAL CONDUCTIVITY.

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CERAMICS

TECHNIQUES IN FABRICATION OF CERAMICS

SLIP CASTING;

WET PRESSING;

EXTRUSION;

INJECTION MOULDING;

ISOSTATIC PRESSING;

TAPE CASTING; DRY PRESSING.

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CERAMICS

CLASSIFICATION OF CERAMICS AS

ENGINEERING MATERIALS

Alumina; Beryllia (beryllium oxide) and boron nitride;

Porcelain (aluminum silicates)

Steatite and forsterite (magnesium silicates)

Silicon nitride and silicon carbide

Titanium diboride; and

Vitreous carbon.

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CERMETS

CERMET is a durable, heat-resistant alloy formed

by compacting and sintering a metal and a ceramic substance, used under conditions of high

temperature and stress.

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CERMETS

TYPES OF METAL CERAMIC COMBINATIONS:

A CERAMIC COATING ON THE METAL; OR

A CHEMICAL AND MECHANICAL COMBINATION OF

METALS AND CERAMICS

THE MECHANICAL PROPERTIES OF THESE TWO TYPES

OF MATERIALS REPRESENT EXTREMES.

Metals have high tensile strength and shock resistance, but lose their

properties rapidly with increasing temperatures.

Ceramics of the refractory kind have extremely high melting points

and excellent general stability, but are low in tensile strength and both

mechanical and thermal shock resistance.

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COMPOSITES

A COMPOSITE is a material in which a stronger,sometimes fibrous material is usually combinedwith another to reinforce or strengthen the

resultant mass.

Forms of composites are based on a plastic matrix.The fibrous reinforcing material maybe in sheetform, as in thermoset plastic laminates; filament

form, woven or ran dom, as in glass reinforcedplastics; or short fibre form as in filled orreinforced thermoplastics.

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CONCRETE

CONCRETE is a mixture of stone and sand held

together by a hardened paste of hydraulic cement

and water. Concrete has great durability and has the ability to

carry high loads especially in

compression.

Composition of concrete are: cement, coarse

aggregate, chemical admixtures, fibrous materials.

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PROPERTIES OF MATERIALS

METALLIC MATERIALS

MECHANICAL PROPERTIESare defined

as the properties of a material that reveal its elasticand inelastic (plastic) behaviour when force is

applied, thereby indicating its suitability for

mechanical applications.

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MECHANICAL PROPERTIES

MODULUS OF ELASTICITY

TENSILE STRENGTH

ELONGATION

HARDNESS

FATIGUE LIMIT

YIELD STRENGTH

YIELD POINT IMPACT STRENGTH

REDUCTION OF AREA

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PHYSICAL PROPERTIES

PHYSICAL PROPERTIES ARE PROPERTIES WHICH

RELATE TO THE PHYSICS OF A METAL SUCH AS:

- DENSITY

- ELECTRICAL PROPERTIES

- THERMAL PROPERTIES

- MAGNETIC PROPERTIES

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PROPERTIES OF METALLIC MATERIALS

ELASTICITY AND PLASTICITY

- STRAINChange in shape

- STRESSForce applied

ELASTICITY is the ability of metal to strain under load and

then return to its original size and shape when unloaded.

HOOKES LAW:

Within the elastic range stress is proportional to strain.

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STRESS-STRAIN DIAGRAM

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IMPORTANT FEATURES OF AN S-S DIAGRAM

THE STRAIGHT LINE OR ELASTIC PART OF THE S-S CURVE

OF A GIVEN METAL HAS A CONSTANT SLOPE;

THE SLOPE IS CALLED THE MODULUS OF ELASTICITY( measures the stiffness of the metal in the elastic range)

* THE STIFFNESS OF ANY METAL VARIES INVERSELY WITH

ITS TEMPERATURE.

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STRENGTH

THE STRENGTH OF A METAL IS ITS

ABILITY TO RESIST CHANGE IN

SHAPE OR SIZE WHEN EXTERNALFORCES ARE APPLIED.

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MECHANICAL PROPERTIES OF METALS

1. Strength

-Is defined as the ability of the material to

withstand applied load .

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Mechanical Properties of Some Metals

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HARDNESS

The HARDNESS of a metal is its ability to resist

being permanently deformed.

WAYS OF MEASURING HARDNESS

1. Resistance to Penetration

2. Elastic Hardness3. Resistance to Abration

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HARDNESS

TYPES OF HARDNESS TESTERS

Rockwell

Vicker

* Brinell

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Micro-hardness Indentations

Portable Hardness Tester

(Ultrasonic Contact Impedance Method)

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DUCTILITY

DUCTILITY IS THE PROPERTY THAT

ALLOWS A METAL TO DEFORM

PERMANENTLY WHEN LOADED IN

TENSION.

ANY METAL THAT CAN BE DRWAN INTO

WIRE IS DUCTILE.

EXAMPLES: STEEL, ALUMINUM, GOLD,SILVER, NICKEL

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DUCTILITY

TENSILE TEST IS USED TO MEASUREDUCTILITY.

PER CENT IN ELONGATION ANDREDUCTION IN AREA AREMEASURES OF DUCTILITY.

HIGH PER CENT ELONGATION ANDREDUCTION IN AREA INDICATESHIGH DUCTILITY.

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Ductility

- Is a term which relates to the ability of a material

to deform, or stretch, under load without failing

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CONDUCTIVITY

CONDUCTIVITY is a measure of the ability ofa material to conduct electric current (mhos per

meter. It is the reciprocal of resistivity( ohm).

The conductivity of metallic elements varies

inversely with absolute temperature.

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THANK YOU AND HAVE A

GOOD DAY!

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Materials 022312

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