Computation of stability, elasticity and thermodynamics in equiatomic AlCrFeNi medium-entropy alloys

We investigated the phase stability, elastic and thermodynamic properties of equimolar medium-entropy alloys (MEAs) AlCrFeNi by performing first-principles calculations in combination with quasi-harmonic Debye–Grüneisen model. Both body-centered cubic (BCC) and face-centered cubic structures in ferromagnetic (FM) and non-magnetic states are described using the special quasirandom structures technique. All the considered MEAs can form single-phase solid solutions and are dynamically stable, and FM BCC AlCrFeNi is the most stable. The elastic moduli including bulk modulus B, shear modulus G and Young’s modulus E of AlCrFeNi are calculated by first-principles and estimated by using the rule of mixtures (ROM) from their pure components. The lattice constants a of first-principles calculations are well reproduced by ROM. The obtained B and G of the two methods are close to equality lines with a minor scatter. The relevant free energies’ contributions including structural, configurational, vibrational and electronic excitations are taken into account to calculate the equilibrium lattice constants a, volumetric thermal expansion coefficient α, Debye temperature Θ_D, constant volume heat capacity C_v, vibrational entropy S_vib, electronic entropy S_elec, vibrational Helmholtz free energies F_vib and electronic Helmholtz free energies F_elec of AlCrFeNi MEAs at finite temperature. The thermodynamic properties strongly depend on crystal structures and magnetic states, and FM BCC AlCrFeNi shows the largest S_vib and α, and the lowest F_vib among the considered MEAs. Finally, electronic density of states is analyzed to clarify the physical origin of AlCrFeNi MEAs with different crystal structures and magnetic states.