First-principles study on the effect of Ni addition on the stability of B2 Ti50Ru50 – a supercell approach

Abstract

Shape memory alloys (SMA's) are a special kind of metal alloys with unique structural properties, such as shape memory effect (SME) and pseudo-elasticity (SE) that were well-observed in TiNi alloy, which commercially trades as Nitinol. This polymorphic crystal behaviour arose from the reversible phase transition between an ordered high-temperature B2 and low-temperature martensite phase (L10/B19/B19′). Due to the high ductility of Ru, this work employed a supercell approach to track the effect of nickel addition on highly stable B2-Ti50Ru50 with the intent to stimulate SME. Thermodynamic, mechanical, electronic structure and lattice dynamic characteristics were estimated using first-principles calculations. The phonons spectra of B2 Ti50Ru50 consisted of only positive frequencies, but the addition of Ni above 18.75 at. % resulted in positive and negative frequencies, indicative of a possible martensitic transformation, which is a strong character for SME.