Study On Transformation And Deformation Characteristics Of TiNiV Shape Memory Alloy

NiTi-based shape memory alloys are widely used in aerospace,biomedical and daily life,due to its unique“shape memory”effect and superelasticity.Because the transformation behaviors and deformation characteristics of NiTi-based shape memory alloys are influenced by many factors,such as alloy content and processing methods,therefore,the key of NiTi-based shape memory alloys in recent years is control the alloy content accurately and adopt reasonable processing methods.In this paper,TiNiV shape memory alloy was taken as the research project,on which carried out the microstructure,transformation behavior,deformation characteristic of cold-rolled sheet and drawn wire before and after annealing treatment.The main works and conclusions can be described as follows:Transformation behavior and deformation characteristic of TiNiV cold-rolled sheet were investigated systematically.The results showed that the structure of original TiNiV sheet at the room temperature was composed of NiTi parent phase and?Ti,V?₂Ni second phase,Ti₃Ni₄strengthening phase was precipitated with the increase of annealing temperature in the temperature range of 300?⁶50?,and the structures of the alloys changed from fibrous to equiaxed crystal;the types of phase transformation changed from two-stage martensitic transformation?A?R?M/M?R?A?to one-stage transformation?A?M/M?A?;all stress-strain curves showed stress-induced martensitic transformation plateau,the critical stress of stress-induced martensitic transformation(?_SIM)tended to desrease,and the fracture was characterized by ductile fracture;in addition,the superelasticity of alloy first improved and then decreased,after annealed at 450?,the superelasticity of the alloy was better.Transformation behavior and deformation characteristic of TiNiV wire were investigated systematically.The results showed that with the decrease of straightening temperature,microstructures of the TiNiV wires changed from othoscopic equiaxed to fibrous and partly recrystallized structure to fibrous;the transformation types changed slightly,and both occured A?R?M and M?A in cooling ang heating processes;all alloys stress-strain curves showed stress-induced R and M transformation plateau,and superlasticity of hot-staightened wire was worse than of the warm and cold-staightened wire.The microstructures and properties of different straightened TiNiV wires varied with the annealing temperature as follows:the microstructures varied greatly;the resistance of phase transformation decreased and R phase disappeared gradually;the stress-induced R phase transformation disappeared on stress-strain curves of the hot-straightened,but R and M phase appeared on the stress-strain of warm and cold-straightened,all specimens showed ductile fracture;superelasticity of the different straightened wires was not same and hot straightened was poor after the same annealing temperature.The superelasticity changes of warm-straightened TiNiV wire under the different strain and cyclic deformation were investigated.The results showed that it was affected by strain amplitude and number of cycles,and complete superelasticity can be obtained after several times cycles on given stress and strain.