Effects of VIM frequency on chemical composition, homogeneity and microstructure of NiTi shape memory alloy

The concentration distribution and microstructure of NiTi shape memory alloys produced in vacuum induction furnaces operating at high and medium frequencies are compared with those of materials melted in vacuum resistance furnaces. The production procedure consisted of charging pure metals into a graphite crucible; raising the temperature to 1450°C under vacuum; holding the materials in high and medium frequency induction and resistance furnaces for 3, 5, and 120 min under vacuum; and allowing alloys to solidify in the crucibles under the same vacuum. The procedures in each of the three furnaces were made as similar as possible. Chemical composition, microstructure, and surface hardness of alloys were determined at different locations along the length of solidiŽ ed specimens. Standard deviations in chemical analyses and hardness numbers were taken as criteria for specimen homogeneity. Results indicate that a high vacuum induction furnace operating at medium frequency is an appropriate equipment for melting NiTi shape memory alloys of the desired homogeneity, microstructure, and hardness. MST/5915