The Use of DFT-Based ab-initio Technique to Determine the Stability Difference in B2 Ti-PGM Compounds

Abstract

In this chapter, the density functional theory (DFT) based first-principles approach is used to predict the underlying lattice properties associated with the phase transformation and stability of B2 phase in titanium-platinum group metal (Ti-PGM) compounds. This ab- initio technique provides a good platform to accurately explore phase stability variation between the successful Ti-PGM shape memory alloys (SMAs) (Ti50M50, M = Rh, Pd, Ir, Pt) and other B2 Ti-PGM compounds that do not show any shape memory effect (SME), such as Ti50Os50 and Ti50Ru50. The B2 TiFe, TiNi and TiAu have also been considered in this chapter in order to draw similarities and differences. Amongst the predicted results, the heat of formation was calculated to determine the thermodynamic stability, whereas the total densities of states were used to evaluate the electronic stability of these compounds. Insights on the mechanical stability of the B2 crystals were derived from the calculated elastic constants. Mechanical instability was revealed in some compounds, indicative of a possible phase transition responsible for the intrinsic shape memory character. Although an attempt to correlate this mechanical instability with imaginary frequencies established from the phonon dispersion curves is made, the correlation is not yet conclusive due to some discrepancies observed in TiNi.