SUPERALLOYSSUPERALLOYSINTRODUCTION Nickel-based superalloys possess goodcombinations of high-temperature mechanicalproperties and oxidation resistance up toapproximately 550°C. High temperature heat-resistance alloys, which canretain high strengths at elevated temperatures. Alloying increases the strength and temperaturecapability but reduces the processability.
SUPERALLOYSSUPERALLOYSPROPERTIESHeat resistant and high strength at hightemperature (760-980oC).Good corrosion resistance.Good oxidation resistance.High toughness and ductilityExcellent cryogenic temperature properties.
SUPERALLOYSSUPERALLOYSCLASSIFICATIONThere are three types of Ni-base superalloys:• Nickel base,• Nicke-lron base,• Cobalt base.The alloys contain high Cr with Ti, Al to fromprecipitates and additions of Mo, Co, Nb, Zr, B,Fe.
SUPERALLOYSSUPERALLOYSMICROSTRUCTUREThe major phases present in the nickel-basesuperalloys:γ (gamma) phase – the continuous matrix ofFCC austenite.γ’ (gamma prime) phase – the majorprecipitate phase (more cubic shape).Carbides –various types, mainly M23C6 andMC. (M = metal).
Three strengthening mechanismsare used in Ni superalloys:· Solid solution hardening· Coherent precipitatehardening· Carbide phases on grainboundariesSTRENGTHENING MECHANISMSTRENGTHENING MECHANISMSUPERALLOYSSUPERALLOYS
SUPERALLOYSSUPERALLOYSSolid solution strengthening• Cr, Mo, Al, Nb, Ti and othersPrecipitation strengthening• Mostly due to Al and Ti• Ni3(Al,Ti), gamma primeCarbide phases• M23C6, M6C or MCSTRENGTHENING MECHANISMSTRENGTHENING MECHANISM
SUPERALLOYSSUPERALLOYSMICROSTRUCTUREA two-phase equilibrium microstructure isgenerated, consisting of gamma γ and gamma-prime (γ´)Both phases are face-centered-cubic, havealmost identical lattice dimensions and alsosimilar orientation. Consequently the twophases are almost coherent.The lattice sites in the γ-phase are totallyequivalent and the atoms constituting the solidsolution being distributed randomly.
MICROSTRUCTUREMICROSTRUCTUREIn the γ´-phase (Ni3(Al,Ti) ) the nickel atomsare at the face-centers and the aluminium ortitanium atoms at the cube corners.• The close match in matrix/precipitate latticeparameter (~0-1%) combined with thechemical compatibility allows the γ´ toprecipitate homogeneously throughout thematrix and has a long-time stability.SUPERALLOYSSUPERALLOYS
MICROSTRUCTUREMICROSTRUCTURE Carbides The added content of carbon is approximately 0.05-0.2%. In combination with reactive and refractory elementssuch as titanium, tantalum, and hafnium it formscarbides (e.g., TiC, TaC, or HfC). During heat treatment these carbides begin todecompose and forms lower carbides such as M23C6and M6C, which tend to generate on the grainboundaries. The mainly M elements in M23C6 are chromium, iron,tungsten and molybdenum.SUPERALLOYSSUPERALLOYS
EFFECT OF ALLOYING ELEMENTSEFFECT OF ALLOYING ELEMENTSSUPERALLOYSSUPERALLOYS
SUPERALLOYSSUPERALLOYSCHEMICAL COMPOSITIONMost nickel-based alloys contain 10-20% Cr,up to 8% Al and Ti, 5-10% Co, and smallamounts of B, Zr, and C.Chromium and aluminium are necessary foroxidation resistance.The elements such as C, Cr, Mo, W, C, Nb,Ta, Ti and Hf form carbides.
SUPERALLOYSSUPERALLOYSCHEMICAL COMPOSITIONElements which stabilize the grain boundaryare B, C, and Zr. Their atomic diameters differfrom Ni by 21-27%. The resulting reduction ingrain boundary energy is associated withbetter creep strength and ductility.The elements Co, Fe, Cr, Nb, Ta, Mo, W, V, Tiand Al are also solid-solution strengtheners,both in γ and γ phase.
SUPERALLOYSSUPERALLOYSNi-Iron SuperalloysFe is added to replace some of Ni as it haslower cost.Ni-Fe superalloys contains 25-45%Ni and 15-60%Fe.Microstructure consists of austenistic FCCmatrix and can be strengthened by solidsolution strengthening (Mo, Cr), andprecipitation hardening (Ti, Nb, Al) by formingintermetallic phases.Examples: Inconel 707, 718, 901.
Applications: Aerospace Gas Turbine Engines Space vehicles - Rocket engines, Nuclear reactors Power Generation Turbines Submarines. Petrochemical equipment. High-Temperature Fasteners Combustion Engine Exhaust Valves Hot Working Tooling and Dies,SUPERALLOYSSUPERALLOYS
REQUIREMENTS FOR COMBUSTION ZONETURBINE BLADESSUPERALLOYSSUPERALLOYS
NI BASE SUPERALLOY TURBINE BLADENI BASE SUPERALLOY TURBINE BLADE
REFERENCES: Henkel and Pense, Structure and Properties ofEngineering materials, 5thedition. Loren A. Jacobson, Physical Metallurgy_class notes Prof. Diego Colombo, Nickel-based superalloys andtheir application in the aircraft industry. Hiroshi Harada and Yuefeng GU, High temperaturematerialsSUPERALLOYSSUPERALLOYS