Degradation of recovery properties after heat treatment in hot-forged NiTiFe alloy

NiTiFe is a kind of low-temperature shape memory alloy (SMA) with great application potential in aerospace, such as pipe joints. Heat treatment can effectively regulate the microstructure and recovery properties (recovery strain and recovery stress) of SMAs. In this study, the microstructure, recovery properties, and transformation temperatures of hot-forged Ni ₄₇Ti ₅₀Fe ₃ shape memory alloy (SMA) before and after heat treatment (HT) with different HT temperatures (550–950 ℃) were analyzed. The results show that when the HT temperature is above 650 ℃, the density of high- and low-angle grain boundaries was reduced. The recovery properties of the hot-forged NiTiFe alloy deteriorate with increasing HT temperature. The initial hot-forged samples showed the best recovery strain of 8.32% and recovery stress of 449 MPa, which were reduced to 5.99% and 299 MPa, respectively, after 950 ℃ HT. The R↔B2 phase transformation temperature changes little with the increasing HT temperature. But the reverse martensitic phase transformation finish temperature increased from − 75 ℃ to − 50 ℃ after tensile deformation at − 180 ℃. The EBSD and HRTEM analyses of the microstructure after the free and constrained recovery tests were carried out. The {114} _B2 and {112} _B2 twins remained after the free recovery process, and more of those twins remained after the constrained recovery process. Although the recrystallization and grain growth can reduce the defects density which are the obstacles to the form of detwinning martensite, it also makes the deformation twins prone to form during the tensile and constrained recovery process, thus leading to poor recovery properties.