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

The effects of annealing and aging treatment on the transformation and deformationcharacteristics of Ti-50.8Ni-0.5V（atomic fraction,%）shape memory alloy （SMA） wereinvestigated by means of thermo gravimetric analyzer （TG）, X-ray diffraction （XRD）, opticalmicroscopy （OM）, transmission electron microscopy （TEM）, differential scanning calorimetry（DSC） and tensile test machine in this paper. The results are as follows:With the increase of annealing temperature (T_an), the microstructure of the alloytransforms from fiber style to equiaxial style. The effect of aging temperature （Tag） on thegrowth rate of the precipitation is stronger than that of aging time (t_ag), and t_agalso hassignificant influence on the size and morphology of the precipitation. With increasing t_ag, themorphology of the alloy aged at400℃evolves from fine partical to lenticular, and the alloyaged at500℃evolves from fine partical to coarse flake and then to fine needles.With increasing T_an, the transformation types of Ti-50.8Ni-0.5V alloy annealed at350～800℃change from Aâ†'R/Râ†'A （A—parent phase, CsCl structure; R—R phase,rhombohedral structure） to Aâ†'Râ†'M/Mâ†'Râ†'A （M—M phase, monoclinic structure） andthen to Aâ†'Râ†'M/Mâ†'A and at last to Aâ†'M/Mâ†'A, and both the R transformationtemperature （T_R） and M temperature hystersis （ΔT_M） decrease, M transformation temperature（T_M） increases. With the increase of t_ag, the Aâ†'R/Râ†'A type transformation occures in thealloy aged at300℃, it changes from Aâ†'R/Râ†'A to Aâ†'Râ†'M/Mâ†'Râ†'A in the alloy agedat400℃, while it remains Aâ†'Râ†'M/Mâ†'A in the alloy aged at500℃. With the increase oft_ag，both T_Mand T_Rincrease，ΔT_Mdecreases, T_M⁵⁰⁰＞T_M⁴⁰⁰，T_R⁴⁰⁰＞T_R³⁰⁰＞T_R⁵⁰⁰，ΔT_M³⁰⁰＞ΔT_M⁴⁰⁰. ΔT_Ris hardly affected by annealing and aging processes, it keeps at a low level（about4℃）.With the increase of T_an, the tensile strength （σ_b） of Ti-50.8Ni-0.5V alloy increases firstand then decreases; the critical stress for stress-induced martensite （σ_M） decreases first andthen increases; the fracture elongation （δ_k） increases; the residual strain （ε_R） increases first andthen decreases and increases at last. With the increase of t_ag, the σ_bof the alloy aged at300 and400℃increase first and then tend to stable; while the σ_bof the alloy aged at500℃decreases first and then tends to stable. The δ_kof the alloy aged at300℃increases first andthen decreases; it decreases first and then tends to constant in the alloy aged at400℃, and itincreases first and then tends to constant in the alloy aged at500℃.With the increase of deforming temperature （T_d）, the property of the alloy annealed atdifferent temperatures changes from shape memory effect （SME） to superelasity （SE）, and itstransformation temperature is influenced by T_an. With the increase of T_d, σ_Mincreases linearly;the ε_Rof the alloy annealed at350～600℃decreases first and then tends to stable, while itdecreases first and then increases in the alloy annealed at650and700℃; the one cyclingenergy dissipation （WD） increases first and then decreases and increases at last, while itincreases all the time in the alloy annealed at650and700℃.With the increasing of cycle number N, the stress-strain cycling characteristics of thealloy annealed at400℃is stable, it shows completely non-linear SE after only one cycle. Thecyclical stability of the alloy annealed at500and600℃is inferior to that of the alloyannealed at400℃. With the increase of N, σ_Mand WDdecrease.The alloy aged at300℃for various time keeps SE all the time, and with the increase oft_ag, WDdecreases. The stress-strain curves change from SE to SME in the alloy aged at400and500℃with the increase of t_ag, and the transformation time of the alloy aged at500℃isshorter than that of500℃aged alloy. With the increase of t_ag, the critical stress forstress-induced martensite （or martensite reorientation） in the alloy aged at differenttemperatures decreases, σ_M³⁰⁰＞σ_M⁴⁰⁰＞σ_M⁵⁰⁰; the ε_Rof the alloy aged at300℃decreases firstand then tends to stable, while it increases first and then tends to stable in the alloy aged at400and500℃.