Live observation of nanoscopic stress concentrations and transformation fronts in nickel-titanium using in-situ synchrotron 200nm X-ray diffraction coupled with indentation

The evolution of multiaxial stress fields and the propagation of transformation fronts in bulk nickel-titanium shape memory alloys have not been accessible experimentally at the sub-micron scale. In this contribution, cross-sectional X-ray nanodiffraction (CSnanoXRD) with a resolution of ~200 nm has been used to follow the development of localized strain concentrations and the formation of martensite phase in-situ (under load) in a 50µm thick lamellae made of superelastic NiTi shape memory alloy during indentation at the ID13 beamline of ESRF in Grenoble. The unique experimental setup featured a dedicated wedge-indenter system, which was used to load the sample in a stepwise mode, while position resolved diffraction data were collected using a two dimensional detector. The in-situ and ex-situ recorded data were used to evaluate magnitudes of in-plane, out-of-plane and shear stress components in austenite and martensite phases as well as the spatial distributions of both phases at the distinct indenter loads and after the indentation. For the first time, the results allow the correlation between the multi-axial strain-stress concentrations and the evolving martensite distribution in the deformation zone under the indenter. Here, especially the role of the individual stress-strain components (and their respective magnitudes) in the formation and distribution of the marteniste phase will be presented. In summary, the pioneering experimental setup based on CSnanoXRD was used to quantitatively assess the nanoscale morphology of the B2-B19’ (cubic-monoclinic) martensitic phase transformation in indented bulk NiTi.