Luders-Like Deformation and Stress Transfer Behavior in an In Situ NiTi-NbTi Composite

A previous study proposed a novel Nb nanowire-reinforced NiTi shape memory alloy composite possessing high yield strength (> 1.6 GPa), low apparent Young's modulus (< 30 GPa), and large quasilinear elastic strain (> 6%). This composite occupies a unique spot on the chart of the mechanical properties of conventional bulk metals, ceramics, and polymer materials. It can be used in dental braces, cardiac pacemakers, implantable devices, and flexible medical instruments. Furthermore, this study suggested that when the NiTi shape memory alloy was adopted as a matrix, the stress-induced martensitic transformation of NiTi would help the embedded nanowire reinforcement to exhibit inherent high strength. Ultralarge elastic strain (4%-7%) of Nb nanowires has been observed in these NiTi-Nb composites. Tailoring superior structural-functional properties by combining a shape memory alloy with other nanoreinforcements have recently gained research attention in materials science research focus. However, in most previous works, the volume fractions of the embedded Nb nanowires were not > 25%. It is reasonable to assume that an increase in the volume fraction of Nb nanowire would further improve the strength of the composite, and make the mechanical performance of the bulk composite much closer to that of a single nano reinforcement. As a result, a study on the high volume fraction of an Nb nanowire-reinforced NiTi shape memory alloy composite is crucial. Herein, an in situ NiTi-NbTi shape memory alloy composite with a high Nb volume fraction was prepared through arc melting, forging, and wire drawing. The microscopic analysis showed that NbTi and NiTi nanofibers were alternatively distributed in the composite along the wire axial direction. In situ synchrotron X-ray diffraction measurements were carried out to study the deformation mechanism of the composite. Results revealed that although the volume fraction of NiTi was only about 30%, the deformation of the composite was mainly controlled by the martensitic transformation of NiTi. The prepared composite showed a homogenous deformation and homogenous martensitic phase transformation before the yielding. It then exhibited LUders-like deformation that originated from the Luders-like stress-induced martensitic phase transformation in the region of yielding. Stress transfer was observed in the Luders band front from the transforming B2-NiTi phase to the NbTi phase and simutaneously to the previously existing B19'-NiTi martensite phase generated during the homogenous martensitic phase transformation process.