Concurrent design for NiAl-based (β/γ′) two-phase alloys by controlling microstructure and texture

Control of the crystallography of Ni3Al (γ′) precipitates along grain boundaries of NiAl(β) was systematically studied using β bicrystals with controlled orientations. γ′ phase preferentially precipitated along β grain boundaries showing a film-like shape. The variants of γ′ precipitates were uniquely selected, which satisfies the Kurdjumov–Sachs K–S) relation with a neighboring β grain and deviates from the relation with another adjacent β grain. In the course of tensile deformation, fracture occurred preferentially at the (β/γ′-film) interface deviating from the K–S relation and the fracture stress decreased with increasing deviation angle from the K–S relation. For improvement of the coherency at the irrational (β/γ′-film) boundaries, the control of microstructure and crystal orientation distribution in (β/γ′) two-phase polycrystals was next attempted by thermomechanical processing. After hot-compression in β phase region and subsequently annealing in (β/γ′) two-phase region, γ′ phase transformed from β phase with 〈111〉β fiber texture satisfying the K–S relation,resulting in the formation of 〈110〉γ′ fiber texture. In particular, a large number of ′/γ′-film) boundaries became partially coherent. This thermomechanical processing was effective in controlling the crystallography of γ′-film along β grain boundaries and leads to the harmonic design of strength and ductility for (β/γ′) two-phase alloys.