Phase evolution and crystallography of precipitates during decomposition of new “tungsten-free” Co(Ni)–Mo–Al–Nb γ–γ′ superalloys at elevated temperatures

This article reports the microstructural stability and consequent phase decomposition including the appearance of topologically close-packed (TCP) phases at high temperature of recently discovered tungsten-free gamma-gamma' alloys of base composition Co-10Al-5Mo-2Nb with or without the addition of Ni and Ti. On prolonged aging at 800 degrees C of the Co-10Al-5Mo-2Nb alloy, needle-shaped DO19-ordered precipitates with stoichiometry of Co-3(Mo, Nb) start appearing in the microstructure. In addition, growth of cellular domains from the grain boundaries featuring a three-phase composite lamellar structure could be observed. These phases are fcc gamma-Co with composition different from the original matrix, CoAl with B2 ordering and Co-3(Mo, Nb) with DO19 ordering. All the phases exhibit well-defined crystallographic orientation relationships. The decomposition of the alloys depends on the solvus temperature of the gamma' phase. The Ni-containing alloy exhibits no phase decomposition until 100 h of aging at 800 degrees C without any significant effect on gamma' volume fraction (76 %). However, at 950 degrees C, the alloy decomposes leading to the appearance of four different phases including TCP phases: a Cr3Si-type cubic phase, a hexagonal Laves phase, rhombohedral mu phase, and solid solution of Co phase. The gamma-gamma' microstructure in the Co-10Al-5Mo-2Nb and Co-30Ni-10Al-5Mo-2Ta alloys is not stable at 800 and 950 degrees C, respectively, on long-term aging. This shows that the measured solvus temperatures (i.e., 866 and 990 degrees C) are metastable solvus temperatures. We also report that the Ti-containing alloy exhibits superior stability with no evidence of either TCP phase formation or any other decomposition of gamma' precipitates, even after aging at 950 degrees C for 100 h.