Two-phase microstructure consisting of the E21 type (Co, Ni)3AlC, phase, and (Co, Ni) primary solid solution, phase, forms having three different characteristic morphologies in the same alloy according to their thermal history. Two of them are observed in the as-cast microstructure; the primary phase dendrite with fine cuboidal precipitates, and the eutectic lamellar consisting of and phases. During heat treatments of the alloys at high temperatures, the nodular type discontinuous precipitation (DCP) lamellar, consisting of and phases, forms locally in the primary dendrite regions as the third morphology. The inter-lamellar spacing and nodule size of the DCP lamellar becomes larger with increasing heat treatment temperature. Micro-Vickers hardness numbers of the primary dendrites having precipitates and the eutectic lamellar are comparable, and higher than those of the DCP lamellar. The alloy annealed at 1173 K for 96 h exhibits high yield strength, the 0.2% proof stress, exceeding about 800 MPa in compression, and the excellent ductility of 13.6 % plastic strain at room temperature in tension. On the other hand, the DCP lamellar formation lowers the strength of the alloys, however high strength about 750 MPa can be maintained in the alloy heat treated at the highest temperature of 1373 K as the lowest yield stress of all. Elevated temperature strength measured at 1123 K for the alloy heat treated at 1173 K is 486 MPa of yield stress in compression. The DCP lamellar formation can be suppressed by the two-step heat treatments and by the unidirectional solidification. The former has more homogeneous distribution of fine DCP lamellar than single-step heat treated alloys. The latter is very effective to improve the homogeneity and thermal stability of and two-phase aligned microstructure and to maintain high temperature strength of 452 MPa at 1123 K.
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