| Abstract Scope |
Fe-Pd nominally exhibits coexistence of two ordered, ferromagnetic phases, L1<sub>0</sub> + L1<sub>2</sub>, near 62 at% Pd; however, the microstructure has never been explored. We verified two-phase coexistence using x-ray diffraction and transmission electron microscopy. At 650C, L1<sub>0</sub> and L1<sub>2</sub> coexist, but at 525C, L1<sub>2</sub> is in equilibrium with the L1’ phase – a related, tetragonal, ordered structure first predicted in 1938, but only discovered in bulk last year. At both temperatures, the tetragonal phase forms a polytwinned microstructure, while the cubic L1<sub>2</sub> coexists in two morphologies: as a coherent wetting phase along all polytwin and antiphase boundaries, and as extended bulk regions amongst L1<sub>0</sub>(L1’) polytwin “bundles”. With aging, the transformation always follows the sequence L1<sub>2</sub> -> L1<sub>2</sub>+L1<sub>0</sub>(L1’). The mesoscale transformation mechanisms are complex, involving highly anisotropic propagation of polytwin plates, with cooperative formation of new plates in the slow-growth direction. National Science Foundation support through grant DMR-1709914 is gratefully acknowledged. |