Effect of Intermittent Overload Cycles on Thermomechanical Fatigue Life of NiTi Shape Memory Alloy Wire
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In recent years, NiTi shape memory alloy (SMA) wires have been used as thermal actuators in a variety of engineering applications.[1] During actuation, SMAs undergo repeated thermal cycling under load between high-temperature austenite (A) and low-temperature martensite (M) phases, generally referred to as thermomechanical cycling (TMC). The literature[2–6] shows that the fatigue life and functional behavior of NiTi wire upon TMC depend on several factors such as alloy composition, thermomechanical processing history, stabilization treatment, stress–strain-temperature regime of TMC, etc. During the last two decades, extensive studies[7–13] have been reported on fatigue behavior of NiTi-base SMAs undergoing TMC. Bertacchini et al.[7] studied two Ni-rich NiTi alloys of compositions Ni60Ti40 and Ni57Ti43 (wt pct) to evaluate the effect of Ni on TMC fatigue life. It was reported that fatigue life of Ni60Ti40 alloy upon TMC at 150 MPa was ~15,000 cycles, two times higher than that of Ni57Ti43 alloy. In contrast, Ni57Ti43 alloy upon TMC at 250 MPa showed a fatigue life of ~4,000 cycles compared to 2,000 cycles for Ni60Ti40 alloy. The observed variation in fatigue life
C.N. SAIKRISHNA, K.V. RAMAIAH, and S.K. BHAUMIK, Scientists, are with Materials Science Division, Council of Scientific and Industrial Research-National Aerospace Laboratories (CSIRNAL), Bangalore 560017, India. Contact e-mail: kvramaiah@nal. res.in B. VIDYASHANKAR, Staff Researcher, formerly with India Science Lab, General Motors Technical Centre India Private Limited, ITPB, Bangalore 560 066, India, is now with GE John F Welch Technology Center, Bangalore 560 066, India. Manuscript submitted August 16, 2012. Article published online December 1, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
was attributed to the type of precipitates and their coherency with NiTi matrix. Saikrishna et al.[4] investigated the effect of thermomechanical processing history on cyclic behavior of NiTi alloy under different constant stresses. It was reported that the wires annealed at relatively high temperatures ‡723 K (450 C) showed relatively large variations in recovery strain (RS) and remnant deformation (RD) compared with the wire annealed at 673 K (400 C). The difference in yield strengths of the wires with heat treatment was stated as the reason for this response. Bhaumik et al.,[8] Lagoudas et al.[3,9] and Bertacchini et al.[6] have shown that the fatigue life of SMA wire depends on the stress/strain/ temperature of TMC, with lower fatigue lives observed for higher values of stress/strain/temperature. It has been shown[8] that the temperature of heating in austenite phase significantly affects the fatigue life of SMA wire. NiTi SMA wires heated to 40 K above the austenite finish temperature (Af) had a fatigue life of 5,000 cycles compared to 18,000 cycles for wires heated to Af + 10 K, the TMC stress being 300 MPa. In another set of studies,[2,6,8] the fatigue lives of NiTi-base SMA wires, under similar stress–strain condition, were found to be significantly higher when the te
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