Investigation on vibrational, electronic excitation entropy and magnetic moment contributions to phase stability of off-stoichiometric Ni50MnxIn50-x alloys at finite temperatures by first-principle calculations

Abstract

The effects of Mn concentration and thermally excited contributions (including the vibrational, electronic excitation and magnetic contributions) on phase stability of austenite and martensite from 0 K to finite temperatures in Heusler typed Ni50MnxIn50-x shape memory alloys were studied by the first-principle calculations using exact muffin-tin orbitals with coherent potential approximation. Based on this, the martensitic transformation tendency was explored. Results show that at 0 K, the energy differences between the non-modulated martensite and the austenite become negative when extra Mn is added, indicating that the added Mn stabilizes the martensite and promotes martensitic transformation. The promoting effect increases with the increase of Mn content. At finite temperatures, the three thermal contributions (the vibrational, electronic excitation and magnetic contributions) were further calculated based on the equilibrium structure at 0 K. It was revealed that the vibrational entropies of the two phases increase with the increase of the temperature for all Mn contents. Under the two effects (temperature and Mn-content), the austenite has a larger vibrational entropy than the martensite, which indicates that the vibrational entropy contributes to promoting the martensitic transition. The Mn content and the temperature show a similar influence on the electronic entropies of the two phases. However, compared with the vibrational entropy, the contribution of the electronic entropy is much smaller. Furthermore, the influences of Mn content and temperature on the magnetic moment of both phases were simulated in their ferromagnetic state. The results show that the magnetic moments increase linearly with the Mn content, however, the influence of temperature is relatively small. Above 100 K, the magnetic moment of the austenite is higher than that of the martensite in ferromagnetic Ni50Mn29.25In20.75 alloy, suggesting that the magnetic entropy makes a similar contribution to promote the martensitic transformation, like the vibrational and electronic excitation entropies.