First-principles study of the effects of solutes on long-range order degree, self-diffusions and antiphase boundary energies in ordered FeCo alloy

Abstract

The brittleness of FeCo alloy is generally caused by the B2 ordering behaviors. Alloying elements V, Nb, Ta, Mo and W were experimentally found to be effective in improving the toughness and plasticity of FeCo alloy. In this work, first-principles calculations were carried out to reveal the toughening mechanism of solutes in ordered FeCo. We used the antisite defect energy as the link to establish the relation between long-range order (LRO) degree and temperature, and successfully predicted the temperature dependence of LRO degree. These considered solutes of V, Nb, Ta, Mo, and W can effectively reduce the maximum LRO degree reached at a specific temperature. We computed the self- and solute diffusion coefficients in ordered FeCo alloy, and found these solutes exhibit faster diffusion characteristics than Fe and Co atoms. Nb and Ta were found to prominently reduce the self-diffusion coefficients in the process of atomic ordering, giving a contribution to the retardation in ordering kinetics. Also, these solutes can substantially reduce the antiphase boundary (APB) energies by repairing the metal bonding along 〈111〉 direction broken by APBs, effective in improving the dislocation mobility. In FeCo alloy, the toughening mechanism of these solutes can be summarized as the following two aspects. First, the atomic ordering can be effectively suppressed by reducing the LRO degree and self-diffusion coefficients. Furthermore, dislocation slip is enhanced by the solute-induced low APB energies. These findings provide some reference significance for improving the ductility of intermetallics.