Phase Transformation and Recovery Stress of Ni 47 Ti 44 Nb 9 Alloy During Constrained Heating and Cooling
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INTRODUCTION
NITI-BASED shape memory alloys (SMA) are widely adopted in industrial and biomedical fields, because of their excellent shape recovery effects and superelasticity caused by martensitic transformation and its reverse transformation.[1–3] These two transformations appear at the martensitic starting temperature (Ms) and reverse martensitic starting temperature (As), respectively. Among NiTi-based alloys, the As of NiTiNb alloys can be significantly increased after pre-deformation resulting in a wide phase transformation hysteresis (As–Ms). The characteristic leads NiTiNb alloys to be widely used as seals, connectors, and fasteners, all of which can be stored and transported at room temperature after pre-deformation.[4–8] Currently, the most widely used NiTiNb alloy is Ni47Ti44Nb9 (at. pct) with a phase transformation
Q.C. FAN is with the College of Manufacturing Science and Engineering, Sichuan University, Chengdu, 610065, China and also with the Institute of Machinery Manufacturing Technology, China Academy of Engineering Physics, Mianyang, 621900, China. M.Y. SUN, Y.Y. WANG, K.H. SUN, X.D. CAO, H.B. PENG, S.K. HUANG, and Y.H. ZHANG are with the the Institute of Machinery Manufacturing Technology, China Academy of Engineering Physics. Contact e-mail: [email protected] Y.H. WEN is with the College of Manufacturing Science and Engineering, Sichuan University. Contact e-mail: wenyh @scu.edu.cn Manuscript submitted June 10, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
hysteresis that increases from approximately 30 °C to about 160 °C after proper pre-deformation.[9–12] Generation and relaxation of recovery stress are the core effects that influence the reliability of NiTiNb seals, connectors and fasteners. Recovery stress is generated when a pre-deformed SMA is constrained during the transformation from martensite to austenite, which drives the SMA to recover its original shape. Recovery stress first appears during constrained heating to the austenite transformation temperature, and sharply increases with a further increase in temperature. As shown in Figure 1(a), after reaching a maximum, the recovery stress plateaus across a wide heating temperature range.[13,14] During subsequent constrained cooling, the recovery stress again appears to plateau across a wide cooling temperature range.[13,14] The same as NiTiNb alloys, a wide plateau of the recovery stress also appears in binary NiTi and other NiTi-based alloy systems.[15–17] Unfortunately, the mechanism, which results in the plateau, remains unclear for NiTi and NiTi-based alloy system. Previously, the mechanism for recovery stress evolution of FeMnSi SMAs was investigated by the authors who found that the recovery stress is primarily determined by recovery strain, plastic strain, and thermal expansion and contraction strain.[18,19] Interestingly, as shown in Figure 1(b), thermal expansion results in a decreasing stage of recovery stress during constrained heating, while thermal contraction results in an increasing stage during constrained cooling.[18,19
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