Migration of $\gamma/\alpha$ interphase boundaries – the initial stage of reverse $\alpha\to\gamma$ transformation at slow heating of metastable allow Fe–32 at.% Ni

During quenching of Fe–Ni metastable alloys with the invar composition from the austenite region, precipitating Fe$_3$Ni disperse particles are transformed into the $\alpha$-martensite structure upon cooling of the alloy in liquid nitrogen. Under slow heating, the inverse $\alpha\to\gamma$ transformation is initiated by the diffusion mechanism (and not the martensite mechanism as has been assumed until now) via migration of the $\gamma/\alpha$ phase boundaries, which is accompanied with coalescence of Fe$_3$Ni particles at these boundaries. The periodic distribution of lumped particles at the $\gamma$/$\alpha$ interfaces determines the periodic distribution of the mobility of various regions in phase boundaries. This leads to a change in the morphology of $\gamma/\alpha$ interfaces from the linear to serrate form in the course of diffusion-controlled evolution of the $\alpha\to\gamma$ transformation.