Observations on the Residual Martensite Phase of NiTi Shape Memory Alloy Micro-Tubes Under Uniaxial and Multiaxial Fatigue-Loadings

Abstract

NiTi shape memory alloys (SMAs) are widely used in astronautics and bio-medical fields due to their shape memory property, superelasticity and excellent biological compatibility. In such applications, the NiTi SMA devices are often subjected to a kind of strain- or stress-controlled cyclic loading, leading to the fatigue failure of NiTi SMAs is a key issue that should be investigated. Meanwhile, with the ambient temperature higher than austenite transformation finish temperature (Af), the NiTi SMAs can transform into martensite phase during loading process and recover their austenite phase during unloading, the unique martensite transformation and its reverse of NiTi SMAs indicates their fatigue failure characters much different from ordinary metals. Thus, besides cyclic-loading experiments, the micro-structure observations on the morphology of residual martensite on fracture surfaces is also an important component of investigating the damage mechanisms of NiTi SMAs, which should be taken into considered as well. This paper aims at the micro-structure observations on the fractured NiTi micro-tubes after both uniaxial and multiaxial stress-controlled fatigue loadings, in which the morphology of residual martensite affected by different stress levels and loading paths are compared and analyzed. The results show that the bigger-sized residual martensite phase is inclined to appear in the loading cases with higher stress level or multiaxial paths, corresponding to the higher residue deformations in the fatigue-loading tests. Meanwhile, in multiaxial loadings, due to the effect of gradually increased shear stress along the radial direction, it leads to the size of residual martensite near the outer surface of the NiTi micro-tubes much bigger than that near the inner surface, whereas in uniaxial loadings, the distribution of martensite phase are nearly uniform on the cross section of NiTi specimens. It can be also found that the locations near the internal defects tend to produce bigger sized martensite phase as well, which indicates the stress-concentration effect induced therein. Moreover, the morphology of mutually hindered martensite phase in different directions can be also observed in the fractured specimens, it implies the propagation of the micro-cracks which caused by the transformation induced plasticity between the interface of austenite and martensite will be further prevented, leading to the relatively slower crack propagation rate of NiTi SMAs than ordinary metals. The findings can provide experimental support to investigating the damage mechanisms of NiTi SMAs in microscopic field, and it is also the foundation of establishing the reasonable damage evolution model and life prediction model of NiTi SMAs, describing their distinctive fatigue characters in both uniaxial and multiaxial loading conditions.