Solid-solution structure and the weakly first-order displacive transformation in Fe-Pd alloys

The solid-solution structure of an Fe-31 at. pct Pd single crystal quenched from 1173 K was measured by combined X-ray diffraction (XRD) and atom-probe microanalysis, with analysis of diffuse X-ray scattering supplemented by computer simulation. Detailed comparison of composition-fluctuation amplitudes on the size scale of 50 atoms shows good agreement between diffraction and atom-probe results, revealing a nonrandom solution structure with a broadened composition distribution, consistent with the existence of a miscibility gap in the system. Also consistent with the L1₀-FePd ordering present in the Fe-Pd system, the diffuse scattering indicates some short-range order in the higher Pd regions. Computer simulation of the diffuse scattering indicates Fe atom displacements in 〈100〉 directions in Fe-rich regions forming microdomains oriented along {110} planes, consistent with previous electron microscopy observations of fine structures above the nominal transformation temperature. Combining the measured composition distribution with a magnetics-based model of the composition dependence of the parent-phase instability temperature predicts a spreading of the weakly first-order cubic-tetragonal phase transformation, whereby tetragonal domains are locally stabilized at temperatures 300 K above the temperature where macroscopic plate products first appear. Composition fluctuations can thus introduce a significant component of continuous character to a weakly first-order displacive transformation.