Dislocation Assisted Phase Transformation Observed in Iron Alloys

Abstract

This study examined the phase transformation of ordered intermetallic precipitates to thermodynamically stable austenite in iron alloys assisted by a dislocation glide or climb. In Fe-Mn-Ni alloys, ordered fct θ-MnNi precipitates formed in the martensite grains and at the lath and grain boundaries during aging after quenching. With further aging, stacking faults and twins formed in the θ-MnNi particles by the glide of Shockley-type 1/6<112> partial dislocations. The crystal structure of the twins was similar to the face centered cubic structure, and these twins transformed to austenite as a result of iron diffusion into the twins. In Fe–Ni–Ti alloys, austenite nucleated first at the interface of the ordered hexagonal η-Ni₃Ti intermetallic precipitates and martensite matrix. However, with further aging, an ordered face centered cubic (fcc) γ′-Ni₃Ti phase formed from the η-Ni₃Ti phase by the dislocation climb of 1/4<0001> type edge dislocations. The final transformation to austenite occurred by the diffusion of iron into the γ′-Ni₃Ti phase.