Super alloy

Super Alloys For High Temperature Applications

Anand MohanM.Tech

IIT Kharagpur

INTRODUCTION• A super alloy is an alloy that exhibits several key

characteristics like excellent mechanical strength, resistance to thermal creep deformation, good surface stability, and resistance to corrosion or oxidation.

• The crystal structure is typically face-centered cubic .• Examples:- Hastelloy , Inconel , Waspaloy, Rene

alloys, Haynes alloys, Incoloy, MP98T, TMS alloys, and CMSX single crystal alloys.

INTRODUCTION• Super alloys develop high temperature strength

through solid solution strengthening• Another important strengthening mechanism

is precipitation strengthening which forms secondary phase precipitates such as gamma prime and carbides.

• Oxidation or corrosion resistance is provided by elements such as aluminium and chromium.

• Alloying increases the strength and temperature capability.

PROPERTIES• Heat resistant and high strength at high

temperature (760-980◦C).• Good corrosion resistance.• Good oxidation resistance.• High toughness and ductility• Good surface stability

CLASSIFICATION• Super alloys are often classified into

generations and until today there were five generations

• The sixth generation is in the form of project at National Institute of Material Science in Japan (NIMS)

• First generation super alloys are characteristic with a relatively huge amount of chromium in comparison with other generations

CLASSIFICATION• The second and third generation contains about

3 wt % and 6 wt % of rhenium respectively• Rhenium is a very expensive addition but leads to

an improvement in creep strength and fatigue resistance

• As an example of fourth generation of super alloys TMS-138 can be characterised.

• It was developed in NIMS with the addition of Mo for increasing the lattice misfit .

CLASSIFICATION• The representation of the fifth generation of the

super alloys is for example TMS-169 alloy developed at NIMS in collaboration with Ishikawajima-Harima Heavy industries co. Ltd (IHI) in japan in 2006

• TMS-169 is an advanced super alloy containing 5 wt % Ru and 4.6 wt % Cr

• TMS 169 with superior high temperature creep and oxidation resistance by incorporating further Ruthenium and Cr content over the composition of fourth generation alloys

• With Ru additions it will enhance the phase stability.

CLASSIFICATION• Super alloys are classified into three based on

the predominant metal present in the alloy. They are

Nickel based Super alloyIron based Super alloyCobalt based Super alloy

Nickel based Super alloy• Nickel based Super alloys can be strengthened by

either Solid solution strengthening or Precipitation hardening.

• Most Ni based alloy contain 10-20% Cr, up to 8% Al and Ti, 5-10% Co, and small amounts of B , Zr and C

• Other common additions are molybdenum, niobium, and tungsten, all of which play dual roles as strengthening solutes and carbide formers.

• Chromium and aluminium improves surface stability through the formation of Cr2O3 and Al2O3

Iron based Super alloy• Iron based Super alloys are characterised by

high temperature as well as room temperature strength.

• Apart from this, it will have good resistance to creep , oxidation, corrosion and wear

• Oxidation resistance increases with chromium content

Cobalt based Super alloy• Cobalt based Super alloys have their origin in

the stellite alloys.• Cobalt alloys have higher melting points than

nickel alloys .• Cobalt alloys show superior thermal fatigue

resistance and weldability over the nickel alloys.

APPLICATIONS• Aerospace – Turbine blades and jet/rocket engines

Intermediate pressure compressor (IPC),

High pressure compressor (HPC),

High pressure turbine (HPT),

Intermediate pressure turbine (IPT),

Low pressure turbine (LPT),

and the pressure and temperature

profiles along the engine.

APPLICATIONS• Gas Turbine for marine propulsion

APPLICATIONS• Pressurized water reactor vessel head

APPLICATIONS• Gas Turbine at thermal power plants

APPLICATIONS• Rocket Motor Engine

APPLICATIONS• Turbine Blades (Jet Engine)

APPLICATIONS• Engine and turbine of Superbikes

CHEMICAL DEVELOPMENT OF Ni-BASED SUPER-ALLOY

• The properties of Ni based super alloys can be tailored to a certain extent through the addition of many other elements.

Effect Alloying ElementsSolid-solution strengtheners Cr, Mo Fcc matrix stabilizers C, W, Ni Carbide formers Ti,Cr,MoForms γ' Ni3 (Al, Ti) Al, Ni, Ti Retards formation of hexagonal η(Ni3Ti) Al,ZrHardening precipitates Al, Ti, Nb Oxidation resistance Cr Improve hot corrosion resistance La, Y Sulfidation resistance Cr Increases rupture ductility B

• Creep resistance is dependent on slowing the speed of dislocation motion within a crystal structure. In Ni based super alloys, the γ’-Ni3(Al,Ti) phase present acts as a barrier to dislocation motion. For this reason, this γ’ intermetallic phase, when present in high volume fractions, drastically increases the strength of these alloys due to its ordered nature and high coherency with the γ matrix.

• In order to improve the oxidation resistance of these alloys, Al, Cr, B, and Y are added. The Al and Cr form oxide layers that passivize the surface and protect the super alloy from further oxidation while B and Y are used to improve the adhesion of this oxide scale to the substrate.

• grain boundary strengthening is achieved through the addition of C and a carbide former, such as Cr, Mo, W, Nb, Ta, Ti, or Hf, which drives precipitation of carbides at grain boundaries and thereby reduces grain boundary sliding.

MICROSTRUCTURE

• Gamma matrix, γ• Gamma prime, γ'• Gamma double prime, γ'' • Carbides • Topologically close-packed (TCP) type phases

fcc Ni-rich matrix– “γ phase”

Ni3Alprecipitates – “ γ’ phase “(cuboid in shape)

Gamma Matrix (γ)

• It is continuous matrix is an FCC nickel-base nonmagnetic phase that usually contains a high percentage of solid-solution elements .

• Alloying elements found in most commercial Ni-based alloys are, C, Cr, Mo, W, Nb, Fe, Ti, Al, V, and Ta

• No phase transformation upto Tm

Gamma prime, γ'• It is an intermetallic phase based on Ni3(Ti,Al)

which have an ordered FCC structure.• In the γ´-phase the nickel atoms are at the

face-centers and the aluminium or titanium atoms at the cube corners.

Gamma prime, γ'

• It precipitated as spheroidal shape or cuboidal shape depending upon the volume fractions of particles. Cuboidal precipitates were noted in alloys with higher aluminium and titanium contents.

• The change in morphology is related to a matrix-precipitate mismatch. It is observed that γ' occurs as spheres for 0 to 0.2% mismatches, becomes cuboidal for mismatches of 0.5 to 1%, and is plate-like at mismatches above about 1.25%.

Deformation properties of γ and γ’ phases

• γ phase: ductile at all temperatures, moderate strength which decreases with temperature

• γ’ phase: brittle except at very high temperatures, very high strength which increases with temperature up to ~ 1100 K

• The hard γ’ phase constrains dislocation motion in the soft γ phase. Consequence: also the γ phase gets stronger

Gamma double prime, γ''

• nickel and niobium combine in the presence of iron to form body centered tetragonal (BCT) Ni3Nb, which is coherent with the gamma matrix, while including large mismatch strains of the order of 2.9%.

• This phase provides very high strength at low to intermediate temperatures, but is unstable at temperatures above about 650 °C (1200 °F).

Crystal structure for γ" (Ni3Nb) (Body Centered Tetragonal)

Carbides• The common nickel-base alloy carbides are

MC, M23C6, and M6C. MC usually exhibits a coarse, random, cubic, or script morphology.

• they are used to stabilize the structure of the material against deformation at high temperatures. Carbides form at the grain boundaries inhibiting grain boundary motion

Carbides formed in super alloy Inconel 718

Topologically close-packed (TCP) type phases

• if composition has not been carefully controlled, undesirable phases can form either during heat treatment or, more commonly, during service. These precipitates are known as TCP phases. Usually harmful, they may appear as long plates or needles, often nucleating on grain-boundary carbides.

Topologically close-packed (TCP) type phases

• Nickel alloys are especially prone to the formation of σ and μ. The formula for σ is (Fe, Mo)x (Ni, Co)y, where x and y can vary from 1 to 7.

• The σ hardness and its plate-like morphology cause premature cracking, leading to low-temperature brittle failure, although yield strength is unaffected .

HEAT TREATMENT• Alloys are, first, solution treated to dissolve nearly all

γ' and carbides other than the very stable MC carbides

• Typical solution treatments (for creep-limited applications) are in the range of 1050 to 1200 °C and may be followed by a second solution treatment at lower temperature

• Some γ' can form upon air cooling from the solution treatment temperature. Aging is then carried out in several steps to coarsen the γ' that is formed upon cooling, as well as to precipitate additional γ'.

• A two-step aging treatment is commonly used, with the first treatment in the range of 850 to 1100 °C over a period of up to 24 h.

• The finer γ' produced in the second aging treatment is advantageous for tensile strength as well as for rupture life.

• Both solution and aging anneals are followed by air cooling

HEAT TREATMENT

• Carbide distribution also is controlled by the heat treatment schedule. Modifications to the γ' heat treatment procedure often are required to avoid problems with carbide films at grain boundaries.

• Therefore, a lower solution treatment temperature (about 1075 °C) is used to preserve the fine-grain as-worked structure with well-dispersed M6C.

HEAT TREATMENT

Research and development of new super alloys

• Sandia National Laboratories is studying a new method for making super alloys, known as radiolysis.

• It introduces an entirely new area of research into creating alloys and super alloys through nanoparticle synthesis.

• Future paradigm in alloy development focus on reduction of weight, improving oxidation and corrosion resistance while maintaining the strength of the alloy.

• another focus is to reduce the cost of super alloys

REFERENCES• Henkel and Pense , Structure and Properties of

Engineering materials, 5thedition.• Prof. Diego Colombo, Nickel-based super alloys

and their application in the aircraft industry.• Hiroshi Harada and Yuefeng GU, High temperature

materials SUPERALLOYSSUPERALLOYS• Reed, Roger C. The Super alloys: Fundamentals

and Applications. Cambridge, UK: Cambridge University Press, 2006.

REFERENCES• Minoru Doi et.al "Gamma/Gamma-Prime

Microstructure Formed by Phase Separation of Gamma-Prime Precipitates in Ni-Al-Ti Alloys

• https://en.wikipedia.org/wiki/Superalloy• http://www.phasetrans.msm.cam.ac.uk//

2003/Superalloys/superalloys.html

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