Ductility reduction upon cobalt substitution in B2 NiTi

Abstract

This paper reports the effect of ternary addition of cobalt to B2 NiTi by varying concentrations × = 0.125, 0.25 and 0.375 in Ni0.5−xCoxTi0.5 and the impact on their crystal structure, electronic and mechanical properties using first-principles calculations. The third element Co was substituted on both Ni and Ti sub-lattices using the supercell approach, site preference energy and formation energy suggest the preference of Co substitution on Ni sub-lattice agreeing with literature. The evolution of the electronic and mechanical properties from undoped NiTi to TiCo by substitution of Ni by Co is hereby discussed. Young’s (E) and shear (G) modulus is found to gradually increase as concentration (x) is increased. Importantly, ductility decreases with increasing cobalt concentration. From DOS, it is observed that the delocalized d states are reduced upon increasing Co addition due to strong d-d hybridization which reduces the metallicity and in turn ductility. This increase in Young’s modulus and decrease in ductility upon cobalt addition is indicative of the overall stiffness and resistance to deformation.