Effect of Cu Content on Atomic Positions of Ti50Ni50−xCux Shape Memory Alloys Based on Density Functional Theory Calculations

Abstract

Abstract:The study of crystal structures in shape memory alloys is of fundamental importance for understanding the shape memory effect. In order to investigate the mechanism of how Cu content affects martensite crystal structures of TiNiCu alloys, the present research examines the atomic displacement of Ti50Ni50−xCux (x = 0, 5, 12.5, 15, 18.75, 20, 25) shape memory alloys using density functional theory (DFT). By the introduction of Cu atoms into TiNi martensite crystal to replace Ni, the displacements of Ti and Ni/Cu atoms along the x-axis are obvious, but they are minimal along the y-and z-axes. It is found that along the x-axis, the two Ti atoms in the unit cell move in opposite directions, and the same occurred with the two Ni/Cu atoms. With increasing Cu content, the distance between the two Ni/Cu atoms increases while the Ti atoms draw closer along the x-axis, leading to a rotation of the (100) plane, which is responsible for the decrease in the monoclinic angle. It is also found that the displacements of both Ti atoms and Ni/Cu atoms along the x-axis are progressive, which results in a gradual change of monoclinic angle and a transition to B19 martensite crystal structure.

Blocking of the martensitic transition at the nanoscale in a Ti2NiCu wedge

“…Calculated equilibrium lattice parameters for Ti 2 NiCu are a 0 =3.05 Å for B2 phase, and a = 2.726 Å, b = 4.359 Å, c = 4.701 Å for B19 phase correspondingly. And they are in the good agreement with experimental values [50][51][52] and previous theoretical estimations 53,54 .…”

A review on shape memory metallic alloys and their critical stress for twinning

“…%. Gou et al [78] further corroborated that the addition of Cu has the effect of reducing the monoclinic angle of the martensite phase.…”

Microstructural and Thermo-Mechanical Characterization of Cast NiTiCu20 Shape Memory Alloy

“…Generally, the Cu substitutes Ni in the 3-25 at% range. Different studies about the effect of the Cu content on the martensitic transformation path, microstructure, atomic positions, functional properties, and their improvement with respect to the NiTi system were presented [5][6][7][8][9][10]. Attention has been primarily devoted to the structures involved in thermo-elastic martensitic transformation: the austenite B2 parent phase, the monoclinic B19 martensite for Cu content below 7.5 at.%, B2 to B19 with the intermediate orthorhombic B19 martensite for Cu content between 7.5 and 16 at%, and B2 to B19 for a higher Cu content [1,5].…”

“…The principally investigated NiTiCu alloys are the ones with 5 at.% and 10 at.% of Cu [11][12][13][14][15]. For these systems, a synthetic vision of functional properties related to thermoelastic martensitic transformation is summarized in the following aspects: thermal hysteresis is narrower than in NiTi, thermal stability is obtained in two or three cycles, and the stress developed in mechanical strain recovery, or stress-strain measures, are lower than the one achieved in the NiTi alloy [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16].…”