Production and Nondestructive Characterization of Shape Memory Alloys

In this study, Ti-Ni-based and Fe-based shape memory alloys were fabricated by traditional powder metallurgy (press sinter). The green specimens were sintered, quenched, and precipitation hardened in a horizontal sliding flange tube furnace. This thermal process combines the sintering, quenching, and aging in a single step. Effects of the fabrication and heat treatment parameters on the mechanical properties were investigated. Elastic modulus of the alloys was characterized by destructive compression tests and non-destructive ultrasonic tests and eddy current tests. Nondestructive ultrasonic tests and eddy current tests were used in order to investigate the elastic modulus and microstructure. As the martensite and austenite phases have different elastic modulus values, martensite-austenite phase transformation temperature can be determined by ultrasonic velocity measurements and electrical conductivity measurements. Microstructure and corrosion properties were also investigated. Increasing Co and Sn content of the Ti-Ni alloy decreased the corrosion potential and increased the corrosion rate. Co and Sn additions to the Ti-Ni alloys enhanced the sinterability by liquid phase formation. Controlling the sintering and aging parameters was determined the mechanical properties. Elimination of the individual austenitizing, quenching, and aging steps can provide a cost advantage. Optimum aging temperatures were 400 degrees C for Ti-Ni-based alloys and 650 degrees C for Fe-Mn-Si-based alloys. The lowest transformation temperature values were in Ti-Ni-Co-Sn and Ti-Ni-Cr alloys, while the highest values were in Ti-Ni and Ti-Ni-Cu alloys. The lowest transformation temperature values were in Fe-Ni-Co-Sn and Fe-Mn-Si-Co alloys, while the highest values were in the Fe-Mn-Si alloy.