Composition-dependent elastic properties in TiNi-Nb from first principle calculations

In this work, site-preference and elastic properties of TiNi-Nb austenite were investigated for various compositions, by using Exact Muffin-Tin Orbitals method in conjunction with Coherent Potential Approximation (EMTO-CPA). Moreover, composition-dependent elastic constants were calculated and used to explain the relation between composition and martensitic transition temperature. Our calculations suggested that Nb atoms prefer to occupy either Ni sites or Ti sites, depending strongly on composition. Variation of TiNi-Nb compositions is achieved through the formation of four sorts of defects: substituting Ti by Nb, substituting Ni by Nb, Ti anti-sites and Ni anti-sites. Our results suggested that c′increases when Ni is substituted by Nb, while c44 increases when Ni is substituted by Nb or Ni anti-sites form. Moreover, the maps of composition-dependent elastic constants were presented. TiNi-Nb with high c′and c44 could be obtained in Ni-deficient area. However, in Ni-excess area, only high c44 could be obtained. Changing of martensitic transition temperature is sensitive to the variation of c44, rather than c′. With the variation of c44, we explained the composition-dependent martensitic transition temperature for TiNi-Nb.