Atomic scale understanding of Kurdyumov-Sachs path during BCC to FCC phase transformation in iron-gallium alloy

The Kurdyumov and Sachs (K-S) orientation relationship between body-centered-cubic (bcc) and face-centered-cubic (fcc) phases has been observed in numerous systems undergone displacive component-involved phase transformations. However, the atomic level evidence for its theoretical path containing two steps of lattice shear is still rare. In this work, we provided an atomic-scale understanding of the K-S path by capturing the early-stage phase transformation from bcc to fcc via a coupled diffusional-displacive mechanism in an aged Fe72Ga28 alloy. Nano-embryonic product phases are directly observed, formed firstly by one lattice shear of (211¯)-bcc planes along [1¯11¯]-bcc direction and further lattice shear of the (011)-bcc planes along [1¯1¯2]-fcc direction. The thermally-activated atomic diffusions are involved in the formation of equilibrium fcc phase. These results may not only help to understand the diverse phase transformations in Fe-Ga alloys, but also add atomistic insights towards the solid-state transformation mechanism between bcc and fcc phases.