Large-strain functional fatigue properties of superelastic metastable β titanium and NiTi alloys: A comparative study

Abstract

This paper investigates the fatigue behavior of superelastic NiTi and two metastable β titanium alloys - commercial Beta III (Ti-11.5Mo-6Zr-4.5Sn wt%) and Ti2448 (Ti–24Nb–4Zr–8Sn wt%) alloys. In situ cyclic tensile tests performed under synchrotron X-ray radiation were used to precisely characterize the stress induced martensitic (SIM) transformation occurring in these alloys. For the NiTi alloy, an intermediate B2-R SIM transformation was detected before the B2-B19′ SIM transformation and no plastic deformation occurred until failure. All metastable β titanium alloys that were solution-treated before testing underwent a reversible β-α" SIM transformation and plastic deformation prior to failure. Low-cycle strain-controlled fatigue tests were performed in tension-tension strain-controlled mode at 37 °C to evaluate large-strain functional fatigue properties. The fatigue life of metastable β titanium alloys was found to be much better than that of NiTi alloy at large applied strains. After a rapid evolution during the first cycles, the mechanical response was found to be constant for NiTi alloy while it evolved continuously for metastable β titanium alloys. In addition, failure occurred suddenly in NiTi, whereas cyclic softening was observed before failure in metastable β titanium alloys. Fatigue properties at higher applied strains are mainly hindered by SIM transformation and defects generated at austenite/martensite interfaces during cycling. This explains why the studied NiTi alloy showed lower fatigue properties than metastable β titanium alloys. In fact, while SIM transformation is homogeneously nucleated in metastable β titanium alloys, SIM transformation is highly localized in NiTi, resulting in higher concentration of defects that promote crack nucleation and, in turn, degrade the functional fatigue properties.