Microstructure, Phase Transformations, and Properties of Hot-Extruded Ni-Rich NiTi Shape Memory Alloy

Processing of NiTi shape memory alloys strongly influences their microstructure, phase transformations, mechanical, and shape memory properties. Hot forging, hot swaging, or hot rolling are efficient techniques for obtaining the desired shape, but during multiple operations the material must be heated and worked in the temperature range from 700 to 900 °C. During these processes, intense oxidation takes place. In order to reduce it, the hot-pack working is applied. The hot extrusion is more effective for reduction of ingot, billet, and rod diameters than hot forging, hot swaging, or hot rolling. Also, during hot extrusion the material surface undergoes considerably less oxidation. In the present work, results of the characterization by differential scanning calorimetry, low-temperature x-ray powder diffraction, and three-point bending and free recovery ASTM F2082-06 tests of the samples after hot direct extrusion and heat treatment are presented. The obtained alloy after hot direct extrusion exhibits desired shape memory effect. The phase transformations during cooling and heating cycle occur with the presence of the R phase. The range of the characteristic temperatures for the obtained material gives possibility for further medical applications. After annealing at 400 and 500 °C, the characteristic temperatures shift to higher values.