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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 c-c 0 alloys of base composition Co-10Al-5Mo-2Nb with or without the addition of Ni and Ti. On prolonged aging at 800°C of the Co-10Al-5Mo-2Nb alloy, needle-shaped DO 19 -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 c-Co with composition different from the original matrix, CoAl with B2 ordering and Co 3 (Mo, Nb) with DO 19 ordering. All the phases exhibit well-defined crystallographic orientation relationships. The decomposition of the alloys depends on the solvus temperature of the c 0 phase. The Nicontaining alloy exhibits no phase decomposition until 100 h of aging at 800°C without any significant effect on c 0 volume fraction (76 %). However, at 950°C, the alloy decomposes leading to the appearance of four different phases including TCP phases: a Cr 3 Si-type cubic phase, a hexagonal Laves phase, rhombohedral l phase, and solid solution of Co phase. The c-c 0 microstructure in the Co-10Al-5Mo-2Nb and Co-30Ni-10Al-5Mo-2Ta alloys is not stable at 800 and 950°C, respectively, on long-term aging. This shows that the measured solvus temperatures (i.e., 866 and 990°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 c 0 precipitates, even after aging at 950°C for 100 h.

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