Improved functional fatigue resistance of single crystalline NiTi micropillars with uniformly oriented Ti3Ni4 precipitates

Superelasticity is one promising functional property of shape memory alloys. To utilize superelasticity in application, sufficient fatigue life (i.e., for functional fatigue) during the mechanical cyclic phase transformation is one of the most important issues to be solved. Here, we successfully manufactured the NiTi single crystalline micropillars which exhibit different crystalline orientations, with uniformly oriented Ti3Ni4 precipitates and few defects. The mechanical properties of these NiTi single crystalline micropillars were systematically investigated. The NiTi single crystalline micropillar with the [20 2_ 9]B2 (B2: parent phase) crystalline orientation showed quite stable superelasticity during the cyclic compression, which sustained more than 107 phase transformation cycles with only 8% decay (from 5.1% for the 1st cycle to 4.7% for the 107 cycle). Meanwhile, as the cyclic number increased, the stress-strain curves became more stable, and the critical stress for inducing martensitic transformation (from 574 MPa for the 1st cycle to 312 MPa for the 107 cycle) and the stress hysteresis (from 7.2 MJ/m3 for the 1st cycle to 3.0 MJ/m3 for the 107 cycle) during the loading-unloading processes both decreased. By analyzing the dislocation plasticity assisted by cyclic martensitic transformations, we show that specific martensite variants are selected biasedly by the interplay of external load and inhomogeneous stress field caused by uniformly oriented Ti3Ni4 precipitates, which lead to a number of slip systems effectively impeded by the precipitates. This study opens a new avenue to develop fatigue resistant shape memory alloy through tailoring the aligned precipitates and preferred crystal orientation.