Transformation of DO24 η-Ni3Ti phase to face-centered cubic austenite during isothermal aging of an Fe–Ni–Ti alloy

The transformation of the grain boundary hexagonal close-packed η-Ni3Ti (DO24) precipitates to face-centered cubic austenite (γ) was investigated in an Fe–Ni–Ti alloy using transmission electron microscopy. The heterogeneous nucleation of austenite on the (0001) basal plane of the η-Ni3Ti precipitate was identified. During the growth of the interface austenite, ordered face-centered cubic (fcc) (L12) γ′-Ni3Ti phase was formed in the η-Ni3Ti by the climbing of the b→=14[0001] edge dislocations. The diffusion of iron from austenite to the γ′-Ni3Ti phase was detected by electron energy loss spectroscopy analysis, resulting in the disordering of the γ′-Ni3Ti phase. The transformation sequence is schematically suggested and the transformation mechanism was discussed based on the misfit strain in the η/γ interface.