An Investigation of the Growth of Fatigue Cracks in Single Crystal Superelastic NiTi Under High Strain Level Using Molecular Dynamics Simulations

In realistic applications, shape memory alloys are mostly under cyclic loading and, thus, fatigue failure is the major mode of failure in these components. Fatigue mainly starts from a nano- or micro-defect studying which is not feasible using experiments. Thus, Molecular Dynamics (MD) simulations are useful for obtaining understanding of the underlying mechanisms leading to failure of the part. In this study, MD simulations were performed on single crystal NiTi models containing a middle crack subjected to cyclic tensile loading in different crystallographic orientations (i.e., [100], [110] and [111]) at two austenitic temperatures. The orientation dependence of the fatigue behavior of NiTi was observed to be significant. The crack did not propagate significantly under [100] and [110] loading due to the stress-induced martensitic phase transformation at the crack tip. The formation of the martensite at the crack tip acted as a barrier to crack propagation. On the other hand, the crack grew significantly in the model loaded along [111] crystallographic orientation. The crack growth was accelerated when the crack met the {110}<111> slip system which is favorable for austenite with B2 crystal structure. In addition, the effect of temperature on the fatigue crack growth of NiTi was studied at 500 K and 550 K, both being above the austenite finish temperature. The results indicated a slower crack growth rate in NiTi at a higher temperature.