Mössbauer study of structural inhomogeneities formed upon the FCC-L10 transformation in equiatomic FePd alloy from different initial states

A comparative Mössbauer study of structural inhomogeneities that arise in the course of A1→ L1₀ phase transformation in nondeformed (as cast and quenched from 950°C and melt-spun from the ingot) and severely deformed samples of equiatomic FePd alloy upon ordering annealing at T = 450°C has been performed. According to the known experimental works, the chosen temperature of annealing is optimal for achieving the highest coercive force H_c in both quenched and deformed samples. It is shown that in the high-coercivity state both quenched and deformed samples FePd possess an inhomogeneous tetragonal structure, which is preserved even after quite a prolonged (40-100 h) annealing. All the samples contain, along with the configurations of the nearest neighbourhood that are assigned to the ordered L1₀ phase, significant volume fractions of configurations typical of nonequiatomic compositions. This conclusion is inconsistent with the commonly accepted concept on single-phase L1₀ type alloys with maximal values of H_c. An inference is made that the structural inhomogeneities detected in the samples under study result from the mismatch of the position of the point of congruent A1→ L1₀ transformation (≈58 at.% Pd) in the phase diagram of the FePd system to equiatomic composition.