Effect of ageing on the microstructure and mechanical behaviour of a directionally solidified Ni3Al-based alloy

The effect of ageing in the temperature range from 1023 to 1373 K on the microstructure and mechanical behaviour of a directionally solidified (DS) Ni3Al-based alloy modified with additions of chromium and iron was investigated. The microstructure of the as-grown alloy consisted of well-aligned and equally spaced lamellas composed of beta(B2) intermetallic compound NiAl (Cr, Fe), some beta'(L1(0)) martensite and spherical alpha-Cr precipitates. The matrix consisted of gamma'(L1(2)) intermetallic compound Ni3Al (Cr, Fe), gamma-phase (Ni-based solid solution) and lath-shaped alpha-Cr precipitates. Ageing at 1123 and at 1173 K was found to be the most effective in transforming the unstable lamellae to gamma'-phase and alpha-Cr precipitates. The change of microstructural characteristics such as volume fraction of lamellae, size, morphology and distribution of gamma'-phase, gamma-phase and alpha-Cr precipitates significantly influenced the room-temperature yield strength and elongation of DS alloy after ageing. The strain-hardening exponent varied with the ageing temperature between 0.30 and 0.46 and the quasi-steady work-hardening rate between 2710 and 5340 MPa. In the specimens with the lowest amount of disordered regions, the strain hardening exponent was found to be 0.46 and the quasisteady work hardening rate was determined to be 3340 MPa. (C) 1997 Elsevier Science Limited.