Study On Transformation And Deformation Characteristics Of Ti-Ni-Cr Superelastic Alloy

Effects of annealing and aging treatment on transformation, deformation and microstructure of Ti-50.8Ni-0.3Cr(% atomic fraction)Shape Memory Alloys (SMAs) were investigated by X-ray diffraction, differential scanning clorimetry, tensile test, optical microscopy and electonic microscopy in this paper. The results are as follows.Aâ†'R/Râ†'A type reversible transformation occurred in 350～450℃annealed alloy upon cooling/heating, Aâ†'Râ†'M/Mâ†'Râ†'A occurred in 500℃annealed alloy, Aâ†'Râ†'M/Mâ†'A occurred in 550～600℃annealed alloy, and no transformation occurred in the above 650℃annealed alloys. With increasing aging time, the transformation type of 300℃aged alloy is Aâ†'R/Râ†'A, the one of 400℃aged alloy is from Aâ†'R/Râ†'A to Aâ†'Râ†'M/Mâ†'Râ†'A, and the one of 500℃aged alloy is from Aâ†'Râ†'M/Mâ†'Râ†'A to Aâ†'Râ†'M/Mâ†'A. With increasing T_an, the R transformation temperature T_R increases first and then decreases, the M transformation temperature T_M increases, and the M temperature hysteresiseΔT_M decreases. After aging at 300～500℃, the T_R⁴⁰⁰>T_R³⁰⁰>T_R⁵⁰⁰. With increasing t_ag, the T_R and T_M increases first and then tend to stable, and theΔT_M decreases first and then tend to stable. Futhermore, the R temperature hysteresis in the annealed and aged alloys is all about 4℃.The Ti-50.8Ni-0.3Cr alloys took place the recovery and the recrystallisation procedure during annealing treatment due to the cold working as recived. When T_an =350～500℃, the alloy is at the recovery procedure, the microstructure is fibriform; when T_an =550～590℃, the alloy is at the recrystallisation procedure, the microstructure turns to the unstrained equi-axed grains; When T_an =600～800℃, the alloy is at grain growth procedure, the microstructure became larger and asymmetrical grain. With increasing T_an, the critical stress for inducing martensiteσ_M descends firstly and then rises; when T_an =350～590℃, the residual strainεR has no obvious change and keeps at a low value all the time. When T_an >590℃, theεR rises rapidly. Furthermore, the recrystallisation temperature of the alloy is about 570℃. In order to getting excellent SE, the annealing temperature of the alloy should be below recrystallization temperature. After solution and aging treatment, the 800℃solution annealed Ti-50.8Ni-0.3Cr alloy exhibits well plasticity and is advant_aged to cold deformation. With increasing t_ag, the tensile strengths (σ_b³⁰⁰ andσ_b⁴⁰⁰) of 300℃and 400℃aged alloys increase sharply firstly and then tend to a stable value, here,σ_b³⁰⁰ <σ_b⁴⁰⁰; the % elongationδ_k³⁰⁰ andδ_k⁴⁰⁰ decrease sharply firstly and then tend to stable. After aging at 500℃, theσ_b⁵⁰⁰ decreases sharply firstly and then tends to stable, theδ_k⁵⁰⁰ increases sharply firstly and then tends to stable. With increasing t_ag, the critical stress for inducing martensitic transformationσ_M300 andσ_M400 of 300℃and 400℃aged alloys decrease gradually, whileσ_M500 decreases firstly and then increases sharply, finally decreases gradually again and the maximum is reached at t_ag=10h. Furthermore, with increasing t_ag, the morphology of Ti3Ni4 precipitates of 300℃aged alloy is fine particle shape; the one of 400℃aged alloy evolves from particle to lenticular shape; the one of 500℃aged alloy evolves from lenticular to needle to plate shape. The influencing of the aging temperature on the morphology of Ti3Ni4 precipitates is more significant than that of the aging time.With increasing the test temperature Td, the critical shear stress for inducing martensitic transformationÏ„_M of both the annealed and aged Ti-50.8Ni-0.3Cr alloy spring increase gradually; with increasing T_an, theÏ„_M decreases firstly and then increases; with increasing t_ag, theÏ„_M of 300℃aged alloy spring decreases gradually; with increasing the stress-strain cycles numbers N, the strain recovery ratio declines quickly at the beginning and then slowly.