The effects of Ru, Cu, Zr and Hf on mechanical properties in Ti-Pt high temperature shape memory alloys

Abstract

Shape memory alloys (SMAs) have been widely used in the fields of actuators and aerospace industry due to their pseudo-elasticity and shape memory effect which are displayed in phase transformations. The martensitic transformations (MT) of TiPt is much higher, at approximately 1273 K and this is considered to be of potential technological interest for elevated temperature SMA applications. TiPt based alloys exhibit very low shape memory effect due to low critical stress for slip deformation compared to the stress required for martensitic transformation, hence it is necessary to enhance the mechanical properties of the equiatomic alloy. The first principles approach was employed to study the effect of the third element (M = Ru, Cu, Zr, and Hf) on the TiPt shape memory alloy. The supercell approach in VASP was used to substitute Pt with Ru and Cu, Ti with Zr and Hf on the TiPt structure to evaluate their mechanical stability from elastic properties for actuators and higher temperature applications. The Ti50Pt50-xRux and Ti50Pt50-xCux decreases in density with increase in Ru and Cu concentration, whilst the Ti50-xZrxPt50 and Ti50-xHfxPt50 substitution increases with an increase in their concentration, which result in larger lattice parameters. The heats of formation suggest that Ti50Pt50-xRux substitution is more thermodynamically stable than Ti50Pt50-xCux substitution, and Ti50-xHfxPt50 substitution is more stable than Ti50-xZrxPt50. The elastic properties suggest that the ternary structures become mechanically stable with an increase of the third element. The Ti50Pt50-xRux and Ti50Pt50-xCux substitution became more ductile with the increase in concentration. Zr and Hf substitution became more ductile at higher compositions (31.75 – 43.75 at.%). The Ru and Hf substitutions have potential to be used for high-temperature applications.