Effect Of Thermal Tension And Annealing On Mechanical Behavior And Microstructure Of Ti-50.8Ni Shape-memory Alloy Wire

NiTi alloys are popular in the fields of medical equipment,aerospace and home life due to their unique superelasticity,shape memory and good physical properties.However,due to its faster cold work hardening rate,poor plasticity and high strength at room temperature,the product is usually obtained by hot working in the production process.Therefore,the study of NiTi alloy hot working technology and post-processing heat treatment system can be used as the guidance for alloy production.In this paper,the thermal deformation behavior and microstructure evolution of Ti-50.8Ni(at.%)alloy wires were studied by thermostatic tensile test in the range of 300-600?.The effects of different processing schemes on microstructure,phase transition,mechanical behavior and superelasticity of hot-working alloy wire were studied by single and multiple annealing processes.The main conclusions are as follows:According to the rheological curve of hot stretching,the rheological curve has velocity effect,size effect and temperature effect.In the range of 300-500?,the alloy is mainly dynamic recovery softening.At 600?,dynamic recrystallization is the main softening,and the strain can reach more than 25%.When the strain rate is not less than 0.10s^-1 at 500?,the rheological curve changes dramatically and the peak stress increases significantly.At 500?,when the strain rate is not less than 0.10s^-1,the rheological curve changes abruptly and the peak stress increases significantly.At the same strain rate,as the temperature increases,the elongation first decreases and then increases.According to the fracture analysis,the fracture mechanism is microporous agglomerated dimple fracture.With the increase of deformation temperature,the size of dimples increases,but the number of dimples decreases,and the ductility of alloy increases.With the increase of strain rate,the size and number of dimples decrease and the ductility of the alloy decreases.During the hot stretching process,the dynamic recrystallization(DRX)of NiTi alloy begins at 500?and DRX grains are formed.Full dynamic recrystallization occurs at 600?,and martensite exists in the matrix due to quenching.With the of strain rate increasing,the number and size of martensite increase.At the same strain rate,the microhardness is inversely proportional to the deformation temperature.Strain rates have little effect on microhardness.According to the DSC curves and hysteretic curves of the alloy,the phase transition type after annealing at 300?,600?and 650?is M?A/A?M type,which has the characteristics of superelasticity(SE).The annealing at 400?is M?R'?A/A?M type,which has the characteristics of SE and shape memory effect(SME),and is dominated by SE.The annealing temperature at 500?is M?A/A?R?M type and has SME.The temperature T at the beginning and ending points of M positive/reverse phase transformation is:T⁴⁰⁰>T³⁰⁰>T⁵⁰⁰>T⁶⁵⁰>T⁶⁰⁰.The thermal hysteresis of phase transition?T is as follows:?T⁵⁰⁰>?T³⁰⁰>?T⁴⁰⁰>?T⁶⁰⁰>?T⁶⁵⁰.According to the stress-strain curve,the tensile strength is?_b⁴⁰⁰>?_b⁵⁰⁰>?_b³⁰⁰>?_b⁶⁰⁰>?_b⁶⁵⁰,and elongation is?650>?600>?400>?300>?500.After annealing at 600?,the precipitates in the matrix change after two or three times annealing.The transformation type of the secondary annealed alloy at 300?(process I)is M?R'?A/A?M.The secondary annealing(process II)at 400?is M?R'?A/A?R?M type.After 400?secondary annealing,300?thrice annealing(process III)is M?R₁',M?R₂'and R?A/A?R₁,A?R₂ and R?M type.The tensile strength of the alloy after multiple annealing treatment is?_b⁶⁰⁰<?_b^I<?_b^II<?_b^III,and elongation is?⁶⁰⁰>?^I>?^II>?^III.The alloy after heat treatment of process I&II has excellent SE,and the alloy after heat treatment of process III has SMA.