Study On Transformation And Deformation Characteristics Of Ti-50.8Ni Shape Memory Alloy

The microstructure, transformation behavior, shape memory effect (SME) and superelasticity (SE) of Ti-50.8Ni shape memory alloy heat-treated at various processes were investigated by optical micrometry, transmission electron microscope, thermogravimetric, X-ray diffraction, differential scanning calorimetry and tensile test. The results are as follows.The composition phases of Ti-50.8Ni alloy at the room temperature are rhombohedral phase and parent phase, and shows SME and SE characteristics. The microstructure morphology changes from fibrous to equi-axial grains with increasing annealing temperatures (T_a). The Ti₃Ni₄ precipitates appearance in 300-500℃aged Ti-50.8Ni alloy. The evolution rules of the morphology of precipitates are fine particleâ†'lamellarâ†'lenticularâ†'coarse flake with increasing aging temperature or aging time (t_ag).With increasing Ta, the transformation types of Ti-50.8Ni alloy changes from Aâ†'Râ†'M/Mâ†'Râ†'A to Aâ†'Râ†'M/Mâ†'A to Aâ†'M/Mâ†'A (A- parent phase, R-rhombohedral phase, M-martensite phase) upon cooling /heating, the R transformation temperature (T_R) decreases, the M transformation temperature (T_M) increases, the M transformation temperature hysteresis (ΔTM) decreases, and the R transformation temperature hysteresis (ΔT_R) is hardly change (about 4℃). With increasing the annealing time (tan), the transformation types of 400℃annealed Ti-50.8Ni alloy are Aâ†'Râ†'M/Mâ†'Râ†'A consistent, the ones of 500℃annealed alloy change from Aâ†'Râ†'M/Mâ†'Râ†'A to Aâ†'M/Mâ†'Râ†'A upon cooling/heating, the R and M transformation temperatures of 400℃and 500℃annealed Ti-50.8Ni alloy increase firstly then turn to stable, and T_R⁴⁰⁰>T_R⁵⁰⁰,T_M⁵⁰⁰>T_M⁴⁰⁰;ΔT_M⁴⁰⁰>ΔT_M⁵⁰⁰>>ΔT_R⁴⁰⁰≈ΔT_R⁵⁰⁰≈4℃.With increasing T_a, the platform stress of tensile curves in Ti-50.8Ni alloy decreases firstly then increases, the fracture stress increases firstly then decreases, and the alloy characteristics change from SME to SE. With increasing tan, the platform stress of tensile curves in 400℃annealed alloy decrease firstly and then increase and the ones of 500℃annealed alloy decteases thoughoutly. The 400 and 500 annealed alloys show good SME.With increasing T_a, the characteristics of Ti-50.8 Ni alloy springs alter from SME+SE to SE. With increasing T_a and stress-strain cycling numbers, the strain recovery ratio of annealed alloy springs decreases, and the attenuation margin of the strain recovery ratio is reduce when the cycling number is over one hundred times.With increasing the aging time, the transformation types of 300℃aged Ti-50.8Ni alloy are Aâ†'Râ†'M/Mâ†'Râ†'A, the ones of 400℃aged alloy change from Aâ†'Râ†'M /Mâ†'Râ†'A to Aâ†'Râ†'M/Mâ†'Râ†'A, the ones of 500℃aged alloy is Aâ†'Râ†'M/Mâ†'A, and the R and M transformation temperatures and R and M inverse transformation temperatures aged alloys increase gradually. TM 500> TM 400> TM 300, T _R⁴⁰⁰> T _R³⁰⁰> T _R⁵⁰⁰,ΔT_M⁴⁰⁰>ΔT _M⁵⁰⁰>ΔT_M³⁰⁰,ΔT _R³⁰⁰>ΔT _R⁴⁰⁰. With increasing the aging temperature, the platform stress of tensile curves in Ti-50.8Ni alloy decreases, and the alloy characteristics change from SE to SME gradually.