The effect of atomic ordering on stability of the β-phase in eutectic Ni-Co-Cr-Al β/γ-alloys

The method of electron microscopy was used to examine B2-->Ni2Al and B2-->Ni5Al3 ordering of the beta solid solution in Ni-Co-Cr-Al beta/gamma eutectics during annealing at 350, 450 and 550 degreesC. Formation of the Ni2Al phase led to depletion of nickel in the beta -matrix and stabilization of this matrix with respect to the martensitic transformation. The stabilization effect was less when Ni5Al3 particles precipitated. It was shown that the Ni5Al3 superstructure, whose symmetry was lower than that of the initial beta -matrix, appeared by the mechanism of heterogeneous nucleation and growth of disk-shaped Ni5Al3 particles in the beta -matrix. Those Ni5Al3 particles had a microdomain internal structure. When the chromium concentration of the alloy increased from 14 to 20 mass %, the growth of Ni5Al3 particles decelerated considerably, The cobalt concentration from 0 to 15 mass % had little effect on the B2-->Ni5Al3 transformation kinetics. However, the intensity of the concurrent decomposition of the supersaturated beta solid solution, which was followed by precipitation alpha -Cr particles, decreased.