Microstructural stability, hardness and compressive behaviour of directionally solidified intermetallic Ni3Al-based alloy with gamma/gamma '-beta structure

Microstructural stability, hardness and compressive behaviour were investig ated in directionally solidified (DS) Ni-20.2Al-8.2 Cr-2.44 Fe (at.%) alloy . The as-grown lamellar gamma(Al)/gamma'(Ll(2))-beta(B2) structure of DS al loy is found to transform to a gamma/gamma'-alpha(A2) structure during anne aling in the temperature range 1023-1173 K. This annealing is connected wit h precipitation of spherical alpha-Cr particles within the lamellae and lat h-shaped alpha-Cr particles in the interlamellar gamma/gamma'-region. The s ize of the lath-shaped alpha-Cr precipitates depends on the annealing tempe rature. However, these precipitates are found to be relatively stable with respect to their size and morphology during annealing for 100-600 h at give n temperature. Annealing at temperatures higher than 1210 K stabilises beta -lamellae in the microstructure. The volume fraction of transformed lamella e is found to follow Delta V-1 proportional to t(1/n) law with n = 4 and th e activation energy for lamellae transformation is determined to be Q = 236 U/mol. The kinetics of lamella transformation is proposed to be governed b y diffusion along disordered regions bounding growing ordered domains of ga mma'-phase around the lamellae. The peak hardness and gamma/gamma'-region m icrohardness values are reached after 100 h annealing at 1023 K. The compre ssive yield stress increases with increasing temperature, reaching a peak v alue at about 1000 K, and rapidly decreases at higher temperatures. The qua si-steady strain-hardening rate decreases with increasing temperature in th e temperature range 773-1273 K. Both yield stress and strain-hardening rate depend on strain rate. (C) 1999 Elsevier Science Ltd. All rights reserved.