MICROSTRUCTURE AND PHASE TRANSFORMATION OF Ni43Co7Mn41Sn9 HIGH TEMPERATURE SHAPE MEMORY ALLOY RIBBON

NiCoMnSn shape memory alloy (SMA) is expected to be a promising high temperature SMA. However, the brittleness has become a big obstacle for its practical application. It is known that, grain refining is effective in improving the ductility of a specific metallic alloy. The aim of this work is to investigate the effect of melt spinning on grain refinement and martensitic transformation and provide a guideline for the development of NiCoMnSn SMA. Ni43Co7Mn41Sn9 high temperature SMA ribbon was prepared by a single roll melt spinning method. The microstructure and martensitic transformation were investigated by means of OM, SEM, TEM, XRD and DSC, respectively. The experimental results showed that, the ribbon had a chemical composition close to the master alloy and exhibited a thermoelastic martensitic transformation at about 160 degrees C. The grains in the as spun ribbon, ranging from 2 mu m to 18 mu m, were remarkably refined compared with the master alloy. In the as prepared ribbon, most of the columnar grains grew along the direction vertical to the ribbon plane. At room temperature, non modulated martensite (tetragonal structure) consisting of twin substructure is determined in the ribbon after relieving the internal stress. Transformation temperatures were lowered by 30 degrees C after heat treatment at 400 degrees C for 1 h and then kept nearly constant with the increase of heat treatment temperatures.