tudy on composition dependent phase stability of Ni50MnxIn50-x shape memory alloys by ab-initio simulation

A series of first-principle calculations to discover the martensitic transition of the Ni-Mn-In alloys were implemented using the pseudopotential method by VASP and the full charge density method by EMTO-CPA within the framework of the Density Functional Theory (DFT). In the stoichiometric Ni50Mn25In25 alloy, the effect of effective Coulomb and exchange parameters (U + J) on the phase stability, structural and magnetic properties was studied. The lattice parameters and the phase stability vary with the U and J couplings in the cubic L21 austenite. However, the bandgap is still showing metallic bonding in the cubic Ni50Mn25In25 alloy. To figure out the effect of excess Mn atoms on the preference of the martensite structure, the tetragonal distortion was employed in the off-stoichiometric Ni50MnxIn50-x alloys with different Mn contents. The calculations were conducted with the optimization of the magnetic structure, where the stoichiometric Ni50Mn25In25 alloy was served as the reference. The ground state-energy-resolved tetragonal distortion reveals that the excess Mn changes the preferred stable structure from the perfect cubic L21 structure to the tetragonally distorted structures. Furthermore, to find out more accurate preference of the martensite structure, the formation energies were calculated. The results show that, the orthorhombic structure is preferred at lower Mn concentrations (between 29 at. % and 40 at. %), whereas the tetragonal L10 structure is more stable at the higher Mn concentrations (above 40 at. %). Moreover, the effect of the Mn concentration on the magnetic properties was also studied. With the increase of the excess Mn content, the magnetic moment increases linearly in the ferromagnetic cubic L21 structure, whereas it varies in the two kinds of martensite. The excess Mn could lead to the coexistence of ferromagnetism and antiferromagnetism. The appearance of the antiferromagnetism could be attributed to the Ni-Mn antiferromagnetic interaction in the tetragonal martensite. Furthermore, the effects of Mn concentration and thermally excited contributions (including the vibrational, electronic excitation and magnetic contributions) on the phase stability of austenite and martensite from 0 K to finite temperatures in Heusler type Ni50MnxIn50-x shape memory alloys were also studied. Results show that at 0 K, the energy differences between the non-modulated (NM) martensite and the austenite become negative when extra Mn is added, indicating that the added Mn stabilizes the martensite and promotes martensitic transformation. 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. And 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. The present work provides fundamental knowledge to understand the martensitic transition of off-stoichiometric Ni50MnxIn50-x alloys, which could be used for the design of ferromagnetic shape memory alloys with improved functional performances.