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 micro-structure 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 β(B2) intermetallic compound NiAl (Cr, Fe), some β′(L1₀) martensite and spherical -Cr precipitates. The matrix consisted of γ′(L1₂) intermetallic compound Ni₃Al (Cr, Fe), γ-phase (Ni-based solid solution) and lath-shaped -Cr precipitates. Ageing at 1123 and at 1173 K was found to be the most effective in transforming the unstable lamellae to γ′-phase and -Cr precipitates. The change of microstructural characteristics such as volume fraction of lamellae, size, morphology and distribution of γ′-phase, γ-phase and -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.