Abstract
In this paper, the strain rate dependence of hardening behavior of polycrystalline pseudoelastic (PE) and shape memory effect (SME) TiNi alloy under impact loading was investigated by experiments. Measurements of stress–strain curves, hardening modulus, hysteresis loop area, and temperature variation are synchronized using in situ infrared detector system at the strain rate range from 300/s to 2000/s. It is shown that with the strain rate increasing, for PE specimens, strain rate hardening is observed, while SME specimens perform a strong nonlinear strain hardening. The results of synchronous temperature measurement show that in stress-temperature space, for PE samples, the dynamic transformation path is strain rate independent, but for the SME samples, the opposite is true. Thermal-mechanical coupling does not seem to explain this difference, and hardening from microstructure variation should be considered for such difference.
Highlights
Due to the first-order martensitic phase transition, TiNi shape memory alloy (SMA) is well known for two excellent properties: pseudoelasticity (PE) and shape memory effect (SME), which have been exploited in a variety of fields, such as smart actuators, damping device, micro-electro-mechanical-systems, biomedical applications [1], and so on.One of the key concerns in such applications is itsrate sensitivity of the material’s performance
The results show that the mechanism of the strain rate effect lies in the temperature dependence of transformation stress
The mechanism lies in the temperature dependence of phase transformation stress, that is, the temperature rise caused by phase transformation deformation makes the further phase transformation need higher stress to drive, which shows a significant strain rate hardening feature on the macro level
Summary
Due to the first-order martensitic phase transition, TiNi shape memory alloy (SMA) is well known for two excellent properties: pseudoelasticity (PE) and shape memory effect (SME), which have been exploited in a variety of fields, such as smart actuators, damping device, micro-electro-mechanical-systems, biomedical applications [1], and so on.One of the key concerns in such applications is itsrate sensitivity of the material’s performance. The results show that the mechanism of the strain rate effect lies in the temperature dependence of transformation stress. As for dynamic conditions, there are some works which have studied the behavior of SMAs at very high strain rates, up to 103 /s [13]. Experimental techniques, such as the split Hopkinson pressure bar technique (SHPB) have been used to achieve these higher strain rates. Their research only focuses on the strain rate dependence of transformation stress and dissipation energy. Chen et al [14] studied the dynamic compressive PE behavior of a TiNi shape memory alloy at strain rates of 81–750/s. The aim of this work is to Metals 2020, 10, 1157; doi:10.3390/met10091157 www.mdpi.com/journal/metals
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