Martensitic Transformation of Titanium-Rich NiTi Prepared by Rapid Solidification
Hitoshi MATSUMOTO and Nobuyuki ISHII
Abstract:In order to reveal effects of rapid solidification on a shape memory NiTi alloy, ribbons of NiTi rich in titanium are prepared by a single roll method and experiments of X-ray diffraction and thermal analysis are performed to characterize the rapidly-solidified NiTi ribbon. The ribbon has the same crystal structure as that of furnace-cooled NiTi and the surface of the ribbon shows the markedly prefered <100>orientation in the high-temperature phase. The lattice constant of the high-temperature phase tends to decrease with increasing composition of titanium and the Ti2Ni phase precipitates at compositions above 53 at.% titanium, of which the behaviors are almost unchanged even by annealing up to 800. The shift of the transformation temperature caused by rapid solidification is small, though the transformation behavior of the NiTi ribbon and its recovery process due to annealing are complicated. The prepared NiTi ribbon shows a shape memory, even without a heat treatment. Therefore, the transformation is not practically influenced by the rapid solidification. It is concluded that the ribbon available for the shape memory material can be directly made from the melt by the single roll method. Key Words:Single roll method, Rapid solidification, NiTi, Transformation, DSC, Lattice constant, Ti2Ni, Annealing