Nucleation and growth of phase transformation twins via coupled diffusive-displacive mechanism: A case study in Fe-Ga alloy

Unraveling nucleation and growth of twins is essential for controlling their morphology as well as materials’ property, but remains difficult for the phase transformation twins evolving diffusive and displacive coupled components. Herein, the nucleation and growth of ordered face-centered-cubic (FCC) twins were studied in a Fe₇₂Ga₂₈ alloy, for which the complete transformation from body-centered-cubic (BCC) parent phase requires both drastic lattice distortion and atomic diffusion. Detailed transmission electron microscopy (TEM) investigations revealed that the twinned nano-variants nucleate initially in the form of L6₀-type face-centered-tetragonal (FCT) intermediate phase, separated either by the untransformed BCC matrix or by coherent twin boundaries (CTBs). The asynchronous transformation of FCT nano-variants, i.e. the lattice distortion towards FCC equilibrium phase preferably occurs inside an individual nano-variant, results in the formation of incoherent twin boundaries (ITBs). Upon further growth into equilibrium FCC twins, the diffusion-controlled lattice distortion leads to stacking faults (SFs) at CTBs, {111}<112>-type stages (dislocations) and rhombohedral structure (9R) at ITBs. ITBs have locally high solute concentrations, indicating that they are atomic diffusion barriers and the twin growth strongly interacts with solute partitioning. These findings add new insights into phase transformation twins. Further tests showed that the preferable growth of transformation twins enables a hardness/modulus gradient from non-equilibrium to equilibrium transformed regions, suggesting that controlling the stage of phase transformation twins can effectively tailor the mechanical properties.