Acta Metall Sin  2019, Vol. 55 Issue (2): 267-273    DOI: 10.11900/0412.1961.2018.00299
 Orginal Article Current Issue | Archive | Adv Search |
The Lattice Instability Induced by Ti-Site Ni in B2 Austenite in TiNi Alloy
Jiangang NIU1(), Wei XIAO2
1 Department of Mechanical Engineering, Hebei University, Baoding 071002, China
2 School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
Abstract

The shape memory effect exists in the temperature range between martensitic phase transformation temperature and reverse martensitic phase transformation temperature, thus the control of martensitic phase transformation temperature is a key issue for the application of shape memory alloys. Valence electrons have been thought to dominate phase stability and phase transformation temperature in TiNi alloy. Inconsistent with the valence electron theory, Ti-site Ni could lead to a significant decrease of phase transformation temperature in TiNi alloy. To deeply understand the effect of Ti-site Ni, a point-defect-perturbation strategy was proposed to prove that Ti-site Ni indeed induced a local lattice instability in B2 austenite. It is the structural feature of instability final phase that one-dimensional <100>B2 atomic column compression and <111>B2 column expansion from the perturbation site. The final phase is energetic lower than B2 structure, and the lowest energy of final phase is 20 meV/atom lower than B2 structure, when the perturbing Ti-site Ni content reaches 2%~4%. In contrast to the case in austenite, Ti-site Ni did not induce the lattice instability in B19′ martensite. The difference between austenite and martensite is to some extent the origin of the significant decrease of phase transformation temperature brought by Ti-site Ni in TiNi alloy.

 ZTFLH: TB31
Fund: Supported by Scientific Research Project of Hebei Education Department (No.QN2016155) and Program for Talents in Hebei University (No.801260201071)
 Fig.1  Schematic demonstration of point defect perturbation strategy Fig.2  Simple cubic (a), bcc (b) and fcc (c) Ti-site-Ni superlattices in 2×2×2 B2 supercells and simple hexagonal (d) Ti-site-Ni superlattice in $2×2×3$ B2 supercell Fig.3  The alternation of simple cubic (a), bcc (b) and fcc (c) superlattice Ti-site Ni and its neighbor atoms in 2×2×2 supercells Fig.4  Changes of the energetic difference between final phase and B2 phase (ΔEfinal-B2) with Ti-site Ni content Fig.5  The differences of bond lengths between final phase and B2 phase (ΔLfinal-B2) in 5×5×5 (a) and $22×22×3$(b) cells (Plus and negative mean elongation and compression respectively, P and n represent perturbation site atom and neighbor, respectively) Fig.6  The comparison of the energetic difference between distorted structure and B2 phase (ΔEdistorted-B2) and the energetic difference between instability final phase and B2 phase (ΔEfinal-B2) Fig.7  The bond length difference between distorted structure and final phase (ΔLdistorted-final) Fig.8  The comparison of the energetic difference between distorted B19′ structure and B19′ phase (ΔEdistorted-B19′) and energetic difference between distorted B2 structure and B2 phase (ΔEdistorted-B2) Fig.9  The comparison of the energetic difference between distorted B2 structure and distorted B19′ structure (ΔEdistortedB2-distortedB19′) and energetic difference between B2 phase and B19′ phase (ΔEB2-B19′)