Microstructure, phase transformation and shape memory behaviors of aged Ti-50.8Ni-0.5V alloy

To reveal the effects of aging temperature and aging time on the superelasticity (SE) and shape memory effect (SME) of Ti-50.8Ni-0.5V shape memory alloy, the microstructure, phase transformation and shape memory behavior of Ti-50.8Ni-0.5V alloy aged at 300 ℃, 400 ℃, 500 ℃ for 0.5 h, 1 h, 5 h, 10 h, 20 h. 50 h, respectively, were studied by the transmission electron microscopy, differential scanning calorimeter and tensile test. The microstructures of 300−500 ℃ aged Ti-50.8Ni-0.5V alloy are composed of the matrix and the precipitated phase of Ti3Ni4, and the precipitated phase of Ti3Ni4 is lenticular and distributed in the matrix and grain boundary. With the increase of aging temperature and time, the precipitates of Ti3Ni4 increase in size and decrease in density. The B2→R→B19′/B19′→R→B2 (B2—Parent phase, CsCl-type structure; R—R phase, rhombic structure; B19′—Martensite, monoclinic structure) phase transformations occur in 300 ℃ and 400 ℃ aged alloys, and the B2→R→ B19′/B19′→B2 phase transformations occur in 500 ℃ aged alloys upon cooling/heating. With the increase of aging time, the R transformation temperature TR and the martensitic transformation temperature TM of the alloy increase, the martensitic transformation temperature hysteresis ΔTM decreases, and the R transformation temperature hysteresis ΔTR changes little, and TR400℃>TR300℃>TR500℃, TM500℃>TM400℃>TM300℃, ΔTM400℃>ΔTM300℃>ΔTM500℃, ΔTR300℃≈ΔTR400℃≈4 ℃. At room temperature, Ti-50.8Ni-0.5V alloys aged at 300 ℃ for 0.5−50 h, at 400 ℃ for 0.5−10 h and at 500 ℃ for 0.5−1 h present SE, aged at 400 ℃ for 10−50 h and at 500 ℃ for 1−50 h present SE + SME. The Ti-50.8Ni-0.5V alloy can be aged at 300 ℃ for 0.5−50 h or at 400 ℃ for 0.5−10 h or at 500 ℃ for 0.5−1 h to obtain excellent SE at room temperature. © 2023 Central South University of Technology. All rights reserved.