Microstructural instability of Ni-Mo-Al unidirectionally solidified eutectics

The phase stability of gamma/gamma-prime-alpha(Mo) DS eutectics during exposure to high temperature is investigated. The as-cast microstructure of the homogeneous eutectic alloy of percent composition 62 Ni, 5.7 Al, and 32.3 Mo is characterized by a regular array of square cross-section fibers of alpha(Mo). These are all embedded in a gamma-prime matrix. The third phase is gamma-fcc, with small precipitates of gamma-prime in it. Following exposure to 1250 C for 90 min and mild water quenching the continuous gamma-prime is replaced by an extensive regular net of cuboidal gamma-prime particles. The massive gamma-prime phase may be partially dissolved by raising the temperature to 1250 C, restoring the disordered gamma solution. Cooling of the composite causes the reappearance of gamma-prime as cuboidal precipitates.