Microstructural evolution in ductile β(B2) + γ ′(L1₂) Ni-Al-Fe alloys

Results of investigations on the microstructural features of β(B2) + γ′ (L1₂) two-phase Ni-Al-Fe alloys fabricated through various thermomechanical processes are reported. It is found that three types of β + γ′structures: blocky, lamellar and widmanstätten structures, are formed when alloys quenched from high temperatures are reannealed at 800°C. The formation of the blocky structure is brought about by the following successive reactions, the β → β′(L1₀) martensitic transformation, the β′ → Ni₅Al₃ (Pt₅Ga₃ type) ordering reaction and the Ni₅Al₃ → β + γ′ transition similar to a discontinuous transition occurring during heating up to 800°C. On the other hand, the lamellar structure results from the Ni₅Al3 → γ′ transition and a heterogeneous precipitation of the β phase on the boundaries between the γ′ variants after the formation of the Ni₅Al₃ from the β′ phase. It is also observed that the mechanical properties are affected by the type of microstructures present. These results suggest that the Ni-Al-Fe dual-phase alloys are amenable to microstructural control through thermomechanical treatments to realise optimum mechanical properties as in the case of Fe base alloys.