Annealing behavior of heavily cold deformed, equiatomic, ferromagnetic iron-palladium intermetallics

Reversibly ordering Fe-Pd intermetallics for equiatomic composition exhibit high temperature stable, disordered FCC phase and low temperature (< Tc) stable L10-ordered phase. In this system FCC to L10 ordering phase transformation has been associated with development of lamellar microstructural morphology known as 'polytwin' microstructure. Previously polytwin microstructure has been replaced by equiaxed grained morphology of L10-ordered phase that exhibits enhanced magnetic properties. A combined reaction (CR) of FCC to L10 ordering phase transformation concomitant with annealing of defects introduced in metastable FCC phase has been utilized to attain equiaxed grained L-10-ordered microstructure. Such CR have also been utilized in other systems that undergo an ordering phase transformation. With exception of some studies, most of these prior investigations have utilized framework of conventional annealing (recovery, recrystallization and grain growth) that has been developed for disordered dilute solid solutions that do not undergo an ordering phase transformation (e.g. annealing of Cu), to understand evolution of microstructure during CR. A clear distinction between conventional annealing (cold deformation of ordered alloy followed by recovery, recrystallization and grain growth in ordered deformed matrix) and combined reaction (an ordering phase transformation concomitant with annealing of defects introduced in disordered parent phase) has not been realized. Creation of a clear distinction between both of these modes of transformation in model Fe-Pd system is the main motivation of this study. Disordered FCC parent phase (prior to CR) and L10-ordered phase (prior to conventional annealing) have been deformed by cold rolling to comparable % reduction in thickness. Both of these annealing transformations have been conducted at same temperature. The microstructural mechanisms that lead to generation of equiaxed grained microstructures of L10-ordered phase in both of these transformations have been investigated. Vital differences in these transformations such as prior cold deformed state, texture evolution during transformation, kinetics of transformation etc. have been recognized. Similarities in these transformations such as mechanisms for crystal defects formation, texture evolution during grain growth have also been identified. Existing theories and modifications thereof have been used to rationalize observed similarities and differences between both of these modes of transformation.