Deformation mechanism and load transfer in an in-situ NiTi–Nb composite

In this study, the cyclic deformation behaviours of an in-situ Nb nanowire reinforced NiTi shape memory alloy composite were studied by in-situ synchrotron X-ray diffraction measurements. Localized hardening in Nb nanowire and the corresponding localized relaxation in NiTi matrix were detected in the Lüders band front upon stress-induced martensitic transformation, indicating a phase load transfer from the transformed NiTi to Nb nanowire. Based on the load transfer, a self-propagating transformation process performing under an average phase stress lower than the critical transformation stress was proposed, which should be responsible for the Lüders-like deformation of the composite. The change of deformation mode from Lüders-like localized deformation to homogeneous deformation after the first tensile cycle was attributed to the plastic deformation in Nb nanowire and the formation of internal stress gradient field in NiTi upon pseudoelastic cycling.