Brandon – Page 17 – Materials Science & Engineering

ElementCompositional range (Weight %) Purpose Ni, Fe, Co 50-70% These elements form the base matrix γ phase of the superalloy. Ni is necessary because it also forms γ' (Ni3Al). Fe and Co have higher melting points than Ni and offer solid solution strengthening. Fe is also much cheaper than Ni or Co. Cr 5-20% Cr is necessary for oxidation and corrosion resistance; it forms a protective oxide Cr2O3. Al 0.5-6% Al is the main γ' former. It also forms a protective oxide Al2O3, which provides oxidation resistance at a higher temperature than Cr2O3. Ti 1-4% Ti forms γ'. C 0.05-0.2% MC and M23C6 carbides are the strengthening phases in the absence of γ'. B, Zr 0-0.1% Boron and zirconium provide strength to grain boundaries. This is not essential in single-crystal turbine blades, because there are no grain boundaries. Nb 0-5% Nb can form γ'', a strengthening phase at lower (below 700 °C) temperatures. Re, W, Hf, Mo, Ta 1-10% Refractory metals, added in small amounts for solid solution strengthening (and carbide formation). They are heavy, but have extremely high melting points. Adding new elements is usually good because of solid solution strengthening, but engineers need to be careful about which phases precipitate. Precipitates can be classified as geometrically close-packed (GCP), Topologically close-packed (TCP), or carbides. GCP phases are usually good for mechanical properties, but TCP phases are often deleterious. Because TCP phases are not truly close-packed, they have few slip systems and are very brittle. They are additionally bad because they "scavenge" elements away from GCP phases. Many elements that are good for forming γ' or have great solid solution strengthening may precipitate TCPs. Engineers need to find a balance that promotes GCPs while avoiding TCPs.An area of the alloy with TCP phase formation will be weak because[1] [2]the TCP phase has inherently poor mechanical propertiesthe TCP phase is incoherent with the γ matrixthe TCP phase is surrounded by a "depletion zone" where there is no γ'the TCP phase usually forms sharp plate or needle-like morphologies which easily nucleate cracksThe main GCP phase is γ'. Almost all superalloys are Ni-based because of this phase. γ' is an ordered L12 (pronounced L-one-two), which means it has a certain atom on the face of the unit cell, and a certain atom on the corners of the unit cell. For Ni-based superalloys, that usually means Ni on the faces and Ti or Al on the corners.Another "good" GCP phase is γ''. It is also coherent with γ, but it dissolves at high temperatures. PhaseClassificationStructureCompositions AppearanceEffect γ matrixdisordered FCC Ni, Co, Fe and other elements in solid solution The background for other precipitates matrix phase provides ductility and a structure for precipitates γ'GCPL12 (ordered FCC) Ni3(Al,Ti) cubes, rounded cubes, spheres, or platelets (depending on lattice mismatch) main strengthening phase. γ' is coherent with γ, which allows for ductility carbidecarbideFCC MC, M23C6, and M6C (M=metal) string-like clumps, like strings of pearls provides dispersion strengthening and grain boundary stabilization γ''GCPD022 (ordered BCT) Ni3Nb very small disks This disk-shaped precipitate is coherent with γ'. It is the main strengthening phase in IN-718, but γ'' dissolves at high temperatures η GCPGCP D024 (ordered HCP) Ni3Ti may form cellular or Widmanstätten patterns The phase is not the worst, but it's not as good as γ'. It can be useful in controlling grain boundaries δ not close-packed orthorhombic Ni3Nb acicular (needle-like) The main issue with this phase is that it's not coherent with γ, but it is not inherently weak. It typically forms from decomposing γ'', but sometimes it's intentionally added for small amounts for grain boundary refinement σ TCPtetrahedral FeCr, FeCrMo, CrCo elongated globules This TCP is usually considered to have the worst mechanical properties. It is never desirable for mechanical properties μ TCPhexagonal Fe2Nb, Co2Ti, Fe2Ti globules or platelets This phase has typical TCP issues. It is never desirable for mechanical properties LavesTCPrhombohedral (Fe,Co)7(Mo,W)6 coarse Widmanstätten platelets This phase has typical TCP issues. It is never desirable for mechanical properties References Belan, Juraj (2016). "GCP and TCP Phases Presented in Nickel-base Superalloys". Materials Today: Proceedings. 3 (4): 936–941. doi:10.1016/j.matpr.2016.03.024 Rae, C.M.F.; Karunaratne, M.S.A; Small, C.J.; Broomfield, R.W.; Jones, C.N.; Reed, R.C. (2000). "TOPOLOGICALLY CLOSE PACKED PHASES IN AN EXPERIMENTAL RHENIUM-CONTAINING SINGLE CRYSTAL" (PDF). Superalloys 2000: 767–776.