Study On Improving The Functional Stability Of Coarse-grained NiTi Shape Memory Alloy By Thermal Cycling And Low Temperature Aging Treatment

Shape memory alloys show shape memory effect and pseudoelasticity,which mean it can recover its original shape by proper thermal-mechanical process,making it being an intelligent material which integrates sensing and driving.NiTi shape memory alloys(NiTi alloys)is the most promising shape memory material because of its large recoverable strain and recovery force,good biocompatibility,excellent mechanical properties and so on.NiTi alloys have a wide range of application prospects in aerospace,biomedical and other fields.NiTi devices normally undergo a cyclic phase transformation during service.The accumulation of irreversible strain,which is mainly caused by the plastic deformation,is the main reason for the degradation of functional properties.So the service life and application range of NiTi alloys are severely restricted.To resolve above mentioned problems,domestic and foreign scholars have carried out a lot of research.It is found that the key to improve the functional stability of NiTi alloys is to improve the strength of matrix which could suppress the plastic deformation during cyclic phase transformation.The main methods of improving the functional stability of NiTi alloys are grain refinement strengthening and aging strengthening.However,the above-mentioned methods are very limited by the grain size,all of them need to refine the grain to a certain extent to obtain good properties,the improvement of the properties of coarse-grained NiTi alloys is Ilimited.In recent years,with the rapid development of aerospace,biomedicine and other industries,the demand for complex NiTi alloy structures is gradually increasing.Additive manufacturing and welding are ideal methods to make complex NiTi alloy structures.However,the components achieved by additive manufacturing or welding usually have complex macrostructures and coarse grains,so it is difficult to improve the functional stability by traditional methods.In this paper,a new method to improve the functional stability of coarse-grained NiTi alloys by thermal cycling and low-temperature aging is proposed.The dislocation networks are introduced into the coarse-grained NiTi alloys by thermal cycling.Dislocations serve as nucleation sites for nanoprecipitates,which are formed after subsequent low-temperature aging.With the presence of dislocations,a homogeneous distribution of nanoprecipitates in the grains is expected,enhancing the strength of the NiTi matrix and resisting plastic deformation during the martensite transformation.The functional stability of coarse-grained NiTi alloy components is significantly improved without changing the shape and size.Firstly,thermal cycling is realized by rapid alternation between liquid nitrogen and 90? water,which can lead to repeated thermal-induced martensite transformation.By setting different thermal cycle numbers,different density dislocation networks are introduced into the coarse-grained NiTi alloys with average grain size of 12?m,and followed by aging at 250? for different duration.The effects of thermal cycling and low-temperature aging on microstructure,phase transformation behavior and pseudoelasticity of coarse-grained NiTi alloys are comprehensively analyzed.Finally,it is found that thermal cycling coupled with low-temperature aging can significantly improve the pseudoelasticity of coarse-grained NiTi alloys and eliminate the complex multi-stage martensite transformation in aged coarse-grained NiTi alloys.At the same time,some complex interactions between dislocation networks and nanoprecipitates are found and discussed,and the effect of thermal cycling and low-temperature aging on the functional stability of coarse-grained NiTi alloys has been revealed.Our work will provide a theoretical and technical basis for the further application of NiTi and NiTi-based shape memory alloys.