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

This paper investigates the fatigue behavior of superelastic NiTi and two metastable beta 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 oc-curred until failure. All metastable beta titanium alloys that were solution-treated before testing underwent a reversible beta-alpha" SIM transformation and plastic deformation prior to failure. Low-cycle strain-controlled fatigue tests were performed in tension-tension strain-controlled mode at 37 degrees C to evaluate large-strain functional fatigue properties. The fatigue life of metastable beta 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 beta titanium alloys. In addition, failure occurred suddenly in NiTi, whereas cyclic softening was observed before failure in metastable beta 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 beta titanium alloys. In fact, while SIM transformation is homogeneously nucleated in metastable beta 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.(c) 2023 Elsevier B.V. All rights reserved.