Functional fatigue behavior of [101]- and [111]-oriented superelastic NiTi micropillars

Abstract

Cuboidal micropillars of superelastic NiTi shape memory alloy with [101] and [111] orientations were fabricated using focused ion beam milling and then subjected to cyclic compression. The results indicate that [101]-oriented NiTi is more prone to functional degradation than its [111]-oriented counterpart due to pronounced kinematic irreversibility. In the former, the irreversibility is driven by both dislocation activity and retained martensite, whereas in the latter, degradation is governed exclusively by dislocation accumulation. Cyclic compression causes a significant reduction in the forward transformation stress in the [101] orientation, which disappears upon the removal of retained martensite through heating. In contrast, the transformation stress in the [111] orientation remains broadly invariant, due to the absence of retained martensite, highlighting its dominant role in lowering the forward transformation stress. The work reveals the importance of crystallographic orientations in bringing changes in the fatigue performance and mechanisms of the material at the microscale.