Impacts of atomic and magnetic configurations on the phase stability of Fe-Pd shape memory alloys: A first-principles study

Abstract

The effects of local atomic and magnetic configurations on the phase stability and elastic property of the face-centered cubic (fcc) and two body-centered tetragonal [face-centered tetragonal (fctI) and fctII, with 0.9 < 1 and 0.71 < 0.9, respectively, in the fct unit cell] phases of Fe 1 - x Pd x (0.28 ≤ x ≤ 0.34) shape memory alloys are systematically investigated by using the first-principles exact muffin-tin orbital method in combination with the coherent potential approximation. It is shown that, considering four types of atomic configurations in a fcc unit cell, the two with one random sublattice are both preferable in each x below 300 K. When T = 300 K, the one with three random sublattices also changes to be stabilized for x ≤ 0.30, whereas that with four random sublattices becomes stable in most of these alloys until T ≥ 600 K. Upon tetragonal distortions, in these fully disordered alloys, both the fctI and fctII phases are unstable. The fctI phase is found for 0.29 ≤ x ≤ 0.33, having only the configuration with one random sublattice on the same layer with the Pd site in the unit cell, whereas the fctII phase is obtained for x ≤ 0.30, possessing all the configurations with one, two, and three random sublattices. These results representing the phase diagram of these alloys, their determined equilibrium lattice parameters, and elastic constants of the three phases at 0 K are in line with the experimental and theoretical data, and their estimated structural (T M) and magnetic (T C) transition temperatures are also close to the experimental data. Adding 4% magnetic disorder in Fe 0.70 Pd 0.30, the fctII structure is effectively prevented, whereas the thermoelastic martensitic transformation of fcc-fctI can still be retained at 0 K.