Study On Microstructural Modification And Superelastic Stability Of NiTi Shape Memory Alloy Via Deformation Thermal Treatment And Electric Pulse Process

As a promising functional material integrating sensing and actuation,NiTi shape memory alloy has been widely used in mechanical engineering and biomedical fields due to its unique shape memory effect and superelasticity.In the field of interventional medicine,more than 80%of products take advantage of superelasticity.However,the application of NiTi alloys still faces serious challenges,especially the problem of the reduced functional stability under cyclic loading,which severely limits the service life of NiTi products.At present,NiTi alloys are mostly used in the form of thin plates,filaments and bars with a thickness or diameter between 0.1 and 10.0 mm.These NiTi alloys are usually produced by multi-pass rolling or drawing processes,and some of the NiTi alloys are directly used as raw materials.The superelasticity index obtained by conventional methods,namely the accumulative residual strain,is usually at a high level.With the rapid development of modern technology,the functional properties of NiTi alloys are increasingly required.For instance,in the electromechanical system,NiTi products are used as sensors,control switches,valves,important connecting devices,etc.,requiring NiTi materials with high sensitivity,high resilience and higher functional stability.It directly leads to the fact that NiTi alloys produced by conventional processes will not meet the needs of modern technology.Therefore,for the rolled NiTi sheets or drawn NiTi filaments,an appropriate process should be proposed and microstructural modification research should be carried out to reduce the accumulative residual strain and improve the functional stability of NiTi alloys,which will have important theoretical significances and application values.The Ti-50.8 at.%Ni alloy sheets and filaments with a thickness or diameter of 0.5 mm were taken as the research object in this study.Deformation devices and process methods suitable for NiTi sheets and filaments were independently designed and adopted.Deformation thermal treatment and electric pulse process were respectively employed to explore the influence of process parameters on the microstructure,phase transformation behavior and superelasticity of NiTi alloys under different treatment methods.Molecular dynamics simulation was subsequently used to determine the microstructural evolution of NiTi sheets and filaments during the plastic deformation processes at the atomic scale,which provided theoretical guidance for formulating an effective treatment method and a reasonable process scheme to improve the superelasticity of NiTi alloys.The main research work is as follows:A plastic deformation process and the corresponding deformation device suitable for NiTi sheets were designed.Aging treatments under different deformation conditions were formulated.The effects of deformation pass,aging temperature and duration on the microstructure,phase transformation behavior and superelasticity of NiTi alloys were studied.The experiment results indicated that the repeated stretching-bending deformation(SBD)effectively refined the grains and introduced a large number of dislocations.During the thermal-induced martensitic transformation,the deformed NiTi alloy underwent one-stage phase transformation of A?M.The SBD process coupled with aging treatment indicated that with the increase of aging temperature and time,the average grain size of NiTi alloy increased,the number of dislocations decreased,the size and number of Ni4Ti3 precipitates increased,and the corresponding superelasticity increased first and then decreased.Through the analysis of the above microstructural evolution,the intrinsic mechanism of the SBD process coupled with aging treatment to improve the superelasticity of NiTi alloy was revealed.The optimal process path was obtained,namely,7 SBD passes supplemented by the aging treatment at 350℃ for 0.5-1 h.The corresponding accumulative residual strain was reduced from 3.6%in the initial state down to 0.7%.A method of torsion deformation suitable for NiTi filaments was used.The effects of torsion turn and annealing temperature on the grain morphology and dislocation distribution of NiTi alloys were carried out.The influence law of microstructural evolution on the phase transformation behavior and superelasticity of NiTi alloys were obtained under different conditions of torsion deformation and annealing treatment.The experiment results showed that torsion deformation broke the elongated austenite grains into fine grains and introduced dislocations.Two-stage phase transformation of A→R→M occurred during cooling,and the transformation peak gradually broadened or even disappeared with the increase in torsion turns.The superelasticity first increased and then decreased.After annealing treatment,the phase transformation sequence of NiTi alloy did not change during the thermal-induced martensitic transformation.With the increase in annealing temperature,the phase transformation peaks were getting sharper,dislocation density decreased and the superelasticity deteriorated.Through the above microstructure and property analysis,the optimal process to improve the superelasticity of NiTi wire was obtained,which was 20 turns of torsion deformation supplemented by annealing treatment at 350℃ for 0.5 h.It was clarified that fine grains and proper density of dislocations were the basis microstructure for good superelasticity of NiTi wire.Electric pulse(EP)treatment and stretching-bending/torsion deformation coupled with EP treatment under different pulse frequencies,switch-on times,and pulse durations were carried out.The effects of the microstructural evolution on the phase transformation behavior and superelasticity were analyzed,including grain size,dislocations,stacking faults,R-phase and Ni4Ti3 precipitates.The experiment results showed that the thermal-induced martensitic transformation was promoted by the short-time EP treatment,and multi-stage phase transformation was prone to take place.With the increase in pulse frequency,the dislocation density decreased,the size of Ni4Ti3 precipitates increased,and the superelasticity changed little in the SBD-treated NiTi alloy.After the multi-process coupling treatment of ’annealing,plastic deformation,electric pulse and aging’,superelasticity was significantly improved.With the increase in pulse frequency and switch-on time,the dislocation density decreased,and superelasticity first increased and then decreased in the deformed NiTi alloy.On the basis of the above research,the optimal process parameters of EP treatment for the deformed NiTi alloy under different conditions were obtained respectively.Thermal and athermal effects of EP treatment were analyzed.The effects of torsion deformation coupled with thermal or EP treatments on the microstructure and superelasticity of NiTi alloys were compared.The mechanism of EP treatment was revealed.Considering the stress characteristics of SBD process,the microstructural evolution in different regions divided by the neutral layer was studied at the atomic scale.The single crystal models of compressive/combined/tensile zones were established.The strain rate(velocity)with a linear variation was applied,which was more in line with the actual stress situations.The microstructural evolution of the above different zones was shown during the SBD process.The simulation results indicated that martensitic transformation occurred in different zones,forming various microscopic morphologies.The martensitic morphology and structure were mainly affected by the deformation mode and strain level.The content of each phase and the average dislocation density in different zones were quantitatively clarified.The main source of dislocations was from the SBD process and martensitic transformation.The stretching-bending simulations and experiments were compared and the accuracy of molecular dynamics simulations was confirmed.Based on the analysis of the above microstructural evolution results,it was revealed that the microscopic mechanism that coordinates the SBD process of NiTi sheet was the twin formation and dislocation movement.A large bending angle and multi-pass deformation should be adopted to gradually introduce deformation during the SBD process,which provided theoretical guidance for regulating the microstructure and improving the superelasticity of NiTi sheet through SBD.Aiming at the features of torsion deformation,a NiTi polycrystalline molecular dynamics model was established.Grain morphology evolution,martensitic transformation,dislocation distribution,and atomic force motion during the torsion deformation were observed.The microstructural evolution of NiTi wire during the torsion deformation was studied at the atomic scale.The simulation results showed that the polycrystalline torsion simulation revealed the migration and movement of grain boundary and the change of grain morphology.The stress-induced martensitic transformation during the torsion deformation was explored.The phase content and average dislocation density were quantitatively calculated.The average dislocation density first increased,then decreased,and finally remained unchanged.The relationships between the dislocation density and grain size,as well as grain boundary ratio,were analyzed.The model size,grain number,torsion speed,length of clamping end,and torsion direction had no significant effect on the microstructural evolution law.The comparison between torsion simulation and experiment showed the accuracy of molecular dynamics torsion simulation.On the basis of the above simulation results,it was revealed that the microscopic mechanism of coordinating torsion deformation was the grain rotation and dislocation motion.It was proposed that a moderate amount of deformation should be introduced in the torsion process,which offered a theoretical basis for the microstructural regulation of superelasticity and the formulation of a reasonable torsion process.