Grain-size gradient NiTi ribbons with multiple-step shape transition prepared by melt-spinning

A grain-size gradient NiTi ribbon with multiple-step shape transition was papered by means of melt-spinning. The ribbons contain coarse and fine grains in the free surface side and copper roller surface side, respectively. The grain-size gradient microstructure induces a two-stage phase transformation behavior in the ribbons during heating or cooling. After tensile deformation pre-treatment, the ribbons exhibit a back-and-forth shape change (shape A→B→A) upon a single heating or cooling process, resulting from the sequential phase transformation through the thickness of the ribbon as dictated by gradient grain size. The activating performance of the ribbons, i.e. shape transition amplitude and speed, can be customized by controlling the pre-deformation strain. This work offers a new opportunity for innovative designs to reach a novel shape memory behavior in NiTi alloys conveniently and efficiently.