First-Principles Calculations of Structural, Mechanical, and Electronic Properties of the B2-Phase NiTi Shape-Memory Alloy Under High Pressure

Fang Yu,Yu Liu

doi:10.3390/computation7040057

Abstract

A first-principles calculation program is used for investigating the structural, mechanical, and electronic properties of the cubic NiTi shape-memory alloy (SMA) with the B2 phase under high pressure. Physical parameters including dimensionless ratio, elastic constants, Young’s modulus, bulk modulus, shear modulus, ductile-brittle transition, elastic anisotropy, and Poisson’s ratio are computed under different pressures. Results indicate that high pressure enhances the ability to resist volume deformation along with the ductility and metallic bonds, but the biggest resistances to elastic and shear deformation occur at P = 35 GPa for the B2-phase NiTi SMA. Meanwhile, the strong anisotropy produced by the high pressure will motivate the cross-slip process of screw dislocations, thereby improving the plasticity of the B2-phase NiTi SMA. Additionally, the results of the density of states (DOS) reveal that the B2-phase NiTi SMA is essentially characterized by the metallicity, and it is hard to induce the structural phase transition for the B2-phase NiTi SMA under high pressure, which provides valuable guidance for designing and applying the NiTi SMA under high pressure.

Highlights

As the representative shape-memory alloys (SMAs), NiTi alloys have been extensively applied in medicine, aerospace, automotive, electronics, machinery, and other industries [1–8], which ascribes to their excellent properties of shape memory effects (SMEs), superelasticity (SE), high-strength, corrosion resistance, and biocompatibility [9–13]
This work aims to investigate the structural, mechanical, and electronic properties of the B2-phase NiTi SMA under high pressure by density-functional theory (DFT) calculations, and the relevant conclusions are summarized as follows: (1) High pressure enhances the ability to resist volume deformation, but the strongest resistances to elastic and shear deformations occur at P = 35 GPa for the B2-phase NiTi SMA
(2) Pugh’s ratio B/G reveals that the B2-phase NiTi SMA presents good ductility in nature, and high pressure can improve the ductility of the B2 phase

Summary

Introduction

As the representative shape-memory alloys (SMAs), NiTi alloys have been extensively applied in medicine, aerospace, automotive, electronics, machinery, and other industries [1–8], which ascribes to their excellent properties of shape memory effects (SMEs), superelasticity (SE), high-strength, corrosion resistance, and biocompatibility [9–13]. In terms of experimental researches, Ataei et al [22] applied the warm deformation method (an effective thermomechanical processing, TMP) to enhance the shape memory and mechanical properties of the equiatomic NiTi alloy. They found that the property of shape memory recovery can be well improved due to the fully martensitic structure of the 500 ◦ C processed sample materials, where the shape recovery strain was increased from 1.5% to 3.6%. Many of the above researches have been conducted, they are still unknown for the effects of the applied pressure on the structural, mechanical, and electronic properties of the B2-phase NiTi SMAs due to lack of in-depth researches, thereby preventing the widespread applications of NiTi SMAs under high pressure

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