In–situ NiTi shape memory alloy–Nb nanowire nanocomposite was revealed to exhibit exceptional mechanical properties. The properties of high strength and ultra-large elastic strains are closely related with the high performance of in situ-formed Nb nanowire reinforcements. In this study, the morphology evolution and crystallographic feature of Nb nanowires in NiTi shape memory alloy matrix during cold drawing were investigated by means of transmission electron microscopy（TEM）. It is revealed that, as a result of the severe work hardening and amorphization of NiTi matrix, the cold work reduction of nanowire reinforcements can exceed that of the entire composite. The irregular-shaped Nb nanowires are reshaped to straight and smooth nanowires during cold drawing process. By means of TEM nanobeam diffraction,it is revealed that the in situ-formed Nb nanowire in such top-down processed NiTi–Nb composite keeps single crystalline over 1,000 nm at length direction. As a consequence, the aspect ratio of nanowire reinforcements in NiTi matrix exceeds 25：1.
Zhen-Yang LiuFang-Min GuoZhong-Qiang WangLi-Shan Cui
The microstructure of stress-induced martensite （SIM） in the nanocrystalline NiTi alloy was investigated by means of transmission electron microscopy （TEM）. The result shows that the multi-variant structure of the martensite is suppressed and only single-variant martensitic twins form after tensile deformation when the grain size is smaller than 80 nm. The normal directions of the （001）B19, twin planes are all. within the range of 45° from the axial direction of the wire. The angle between twin crystals （111）M and （111）x of the SIM is also found to be smaller than that of thermally induced martensite in nanocrystalline NiTi.
Xiao-Bin ShiLi-Shan CuiZhen-Yang LiuDa-Qiang JiangXiao-Dong Han
