Effect Of Cold Working And Annealing On Shape Memory Characteristics Of TiNiFe Alloy

Shape memory alloy coupler are pipeline connectors made of the memory recovery properties of shape memory alloys.Since the 1970s,TiNiFe memory alloy coupler have been successfully applied to the hydraulic pipeline system of military aircraft in foreign countries,which improved the safety and reliability of pipeline connections and fundamentally solved the leakage problem of aviation pipeline connections.The feasibility study of TiNiFe memory alloy coupler was first carried out in China in the 1980s.Due to the poor cold working performance of alloy materials,the material processing basically stayed at the level of hot working,and the microstructure of hot working alloys was not sufficiently refined,which could not meet the needs of the current（28 MPa hydraulic pipeline）high strength,high fatigue,and high reliability of pipeline connections.Therefore,it is imperative to carry out the research in this paper to improve the comprehensive properties of alloy materials and meet the needs of high-pressure pipeline connections.Through the systematic research on the sufficient cold working deformation and annealing process of TiNiFe alloy,this paper aims to obtain the technical process path to refine the alloy microstructure,optimize the microstructure state,improve the mechanical properties and memory recovery properties of the alloy material,and make useful exploration for the engineering application research of memory alloy coupler.The research results show that the hot deformation structure of TiNiFe alloy is coarse and the related properties are poor.After three cold rolling and annealing cycles（30%cold rolling deformation+650℃/1 h annealing）,the alloy microstructure is significantly refined,and the yield strength,tensile strength,and hardness of the alloy are significantly improved;in the lowtemperature martensitic state,the low-temperature mechanical properties,recoverable strain,and memory recovery stress of TiNiFe alloy are significantly enhanced,which is the result of cold rolling processing to refine the alloy microstructure and significantly improve the memory recovery property.On the basis of three cold rolling and annealing cycles,experiments with different cold working deformation（20%→35%→45%）were carried out.The experimental results showed that with the increase of cold rolling deformation,the yield strength,tensile strength,and hardness of TiNiFe alloy increase gradually,and the elongation after fracture decreases.On this basis,matching the appropriate annealing process can further improve the memory recovery property of the alloy,which explains the strengthening effect of work hardening on the alloy.The recoverable strain amount and memory recovery stress of TiNiFe alloy show the same trend as the low-temperature mechanical properties.The higher the alloy pre-strain strength and martensite reorientation stress,the higher the recoverable strain and memory recovery stress.The peak value is reached at 45%cold rolling deformation+450℃/1 h annealing.On the one hand,the critical slip stress of TiNiFe alloy is increased due to the multiple cold rolling+annealing treatment,which makes the alloy retain part of the work hardening on the basis of grain refinement.On the other hand,the grain orientation consistency of the annealed alloys is high when annealed at 450℃,which is concentrated in the[111]orientation.In this paper,the optimal cold rolling annealing process of the alloy is:45%cold rolling deformation+450℃/1 h annealing,the maximum recoverable strain is 6.85%,and the maximum memory recovery stress is 678 MPa.The phase-transition temperature As of TiNiFe alloy decreases with the increase of cold rolling deformation and increases with the increase of annealing temperature.After annealing treatment,the phase-transition temperature As of alloy increases with the increase of the prestrain amount in the determined microstructure state.The recoverable strain amount and memory recovery stress of TiNiFe alloy are the largest at 10%pre-strain.The recrystallization temperature of TiNiFe alloy is about 600℃.