The evolution of microscopic damage mechanisms during actuation fatigue cycling in high temperature shape memory alloys

Abstract

Shape memory alloys (SMAs) offer an alternative lightweight and compact option to conventional actuators as solid-state actuators due to their high energy density. Before commercial use in the automotive or aerospace industries can be realized, the durability and functionality of high temperature SMAs (HTSMAs), with operating temperatures exceeding 100 °C, need to be characterized by examining the actuation fatigue properties. Macroscopic evaluation has been well-documented, but to further improve the thermomechanical response and actuation fatigue then the governing microscopic damage mechanisms need to be identified. With this motivation, the effects of upper cycle temperature (UCT) levels on actuation fatigue properties of a Ni-rich Ni_50.3Ti_29.7Hf₂₀ HTSMA were investigated in combination with high energy synchrotron radiation X-ray diffraction (SR-XRD) experiments. Fatigue tests were performed to compare three UCT levels above austenite finish (A_f) temperature under a constant load of 300 MPa and SR-XRD was used to determine the residual micro-strains associated with accumulated strain. Overheating much greater than the A_f (in this case, >100 °C) transformation temperature leads to oxidation and slip deformation associated with grain growth. Whereas operating at temperatures slightly above A_f (<100 °C) was shown to extend fatigue life by preferring twinning deformation. Existing defects and high strain rate conditions will cause actuation strain degradation as a consequence of multiple transformation fronts accumulating stress concentrators at these initiation sites. Lower UCT allows damage deformation to propagate slowly across the sample so that deformation is primarily associated with twinning mechanisms before slip for the duration of the fatigue test.