A first-principle study on lattice properties and atom positions of TiNiCu shape memory alloys

Abstract

Since its discovery, TiNi-based shape memory alloys (SMAs) have attracted much attention and have spawned various innovative applications in the biomedical, aerospace, robotic and automotive industries. Besides high strength, ductility and biocompatibility, TiNi-based SMAs exhibit unique properties such as shape memory effect (SME) and superelasticity (SE) which are associated with reversible martensitic transformation (MT). However, even as TiNi-based SMAs have been discovered and used for many decades, a refined atomic-level understanding of MT is still lacking. With the rapid development of high performance computing, computational methods are playing an increasingly important role in materials research. By using computational methods, the martensite structures of TiNi-based SMAs at quantum level are explored in detail here. In the present work, the martensite crystal structures, electronic structures and atomic displacements of TiNi and TiNiCu alloys have been studied based on ab initio Density Functional Theory (DFT) calculations. The computational results are compared with previous experimental data and it is found that the equilibrium lattice constants are in good agreement with reported values. It is observed that for TiNiCu alloys with Cu content between 0 at% to 25 at%, and this Cu addition to TiNi, the martensite lattice parameters a and c and the monoclinic angle decrease, whereas the lattice parameter b increases. When Cu content reaches 20 at%, the monoclinic martensite crystal structure becomes unstable and an orthorhombic crystal structure is formed. However, when Cu content exceeds 25 at%, the changes of lattice parameters are insignificant, in which a decreases slightly while b and c increase slightly, and the stable martensite structure remains orthorhombic. Furthermore, as a result of Cu addition to TiNi, the electrons which escape from Ti atom increase linearly. Since each Cu atom attracts more electrons than Ni atom, fewer charge transfer from Ti to Ni has occurred compared to that in binary TiNi alloy. With increasing Cu content, the distance between two neighboring Ni/Cu atoms increases along the x-axis while two neighboring Ti atoms get closer, which is responsible for the rotation of the (100) plane, leading to a decrease in the monoclinic angle. The charge transfer between Ti and Ni/Cu atoms is suggested to be responsible for the observed atomic displacements. Since the displacements of both Ti and Ni/Cu atoms along the x-axis are progressive, there is no dramatic change in TiNiCu martensite crystal structures but the monoclinic angle decreases gradually until the orthorhombic structure is formed.