Microstructural stability, hardness and compressive behaviour were investigated in directionally solidified Ni-20.2Al-8.2 Cr-2.44at%Fe alloy. The as-grown lamellar γ(Al)/γ′(Ll2)-β(B2) structure of directionally solidified alloy was found to transform to a γ/γ′-α(A2) structure during annealing at 1023 to 1173K. This annealing was connected with precipitation of spherical α-Cr particles within the lamellae and lath-shaped α-Cr particles in the interlamellar γ/γ′-region. The size of the lath-shaped α-Cr precipitates depended upon the annealing temperature. However, these precipitates were found to be relatively stable with respect to their size and morphology during annealing for 100 to 600h at a given temperature. Annealing at temperatures higher than 1210K stabilized β-lamellae in the microstructure. The volume fraction of transformed lamellae was found to obey a tl/n law, with n = 4, and the activation energy for lamellae transformation was determined to be 236kJ/mol. The kinetics of lamellar transformation was proposed to be governed by diffusion along disordered regions bounding growing ordered domains of γ′-phase around the lamellae. The peak hardness and γ/γ′-region microhardness values were reached after 100h annealing at 1023K. The compressive yield stress increased with increasing temperature, reaching a peak value at about 1000K, and rapidly decreased at higher temperatures. The quasi-steady strain-hardening rate decreased with increasing temperature in the range of 773 to 1273K. Both yield stress and strain-hardening rate depended upon the strain rate.

Microstructural Stability, Hardness and Compressive Behaviour of Directionally Solidified Intermetallic Ni3Al-Based Alloy with γ/γ′-β Structure. Lapin, J., Wierzbinski, S., Pelachová, T.: Intermetallics, 1999, 7[6], 705-15