First- and second-order phase transitions in Fe-(17-19)at.%Ga alloys

The structure of the Fe-(17.5–19.5)at.%Ga alloys in as-cast and nearly equilibrium states is studied using XRD, ND, and EBSD. In disagreement with the existing phase diagrams, the long-term isothermally annealed at 450–500 °C Fe-17.5%Ga alloy has a two-phase structure with two co-existing ferromagnetic phases: bcc-originated A2(D03) as a matrix and a fcc-originated A1(L12) needle-shaped precipitates. The phase transitions during cooling of 17.5 and 19.5%Ga alloys from the A2 range of the phase diagram with different cooling rates were used to plot, at least partly, Time-Temperature-Transformation (TTT) diagrams. The critical cooling rates for starting first- and second-order transitions are determined for the Fe-19.5%Ga alloy. The effect of long-term annealing on magnetostriction is reported.