Degradation of Shape Memory Effects after Annealing Treatment in Hot-Forged Nitife Alloy

The microstructure and shape memory effects (recovery strain, recovery stress, transformation temperatures) of hot-forged Ni47Ti50Fe3 shape memory alloy (SMA) before and after heat treatment with different heating temperatures (550~950℃) were compared and analyzed in this study. The results show that when the annealing temperature is above 650℃, the density of high- and low-angle grain boundaries was reduced and the shape memory effects degenerated. The initial hot-forged specimens showed the best recovery strain of 8.32% and recovery stress of 449MPa, which was reduced to 5.99% and 299MPa after 950℃ annealed. With the increasing annealing temperature, the change of R↔B2 phase transformation temperature was not significant, but after tensile deformation at -180℃, the reverse martensitic phase transformation finish temperature increased from -75℃ to -50℃. Through the EBSD analysis of the microstructure after the free and constrain recovery, it was found that the {20-1}B19' and {113}B19’ twins are responsible for the irreversible strains, and more {114}B2 and {112}B2 twins remained after the constraint recovery process. Although the recrystallization and grain growth can reduce the defects density which are the obstacles of the form of detwinning martensite, it also makes the {20-1}B19' and {113}B19’irreversible deformation twins prone to form during the tensile and constraint recovery process, thus leading to poor shape memory effects.