Investigation On Mechamisms Of Amorphization And Crystallization Of NiTi-based Shape Memory Alloys Subjected To Severe Plastic Deformation And Subsequent Heat Treatment

Because of their excellent superelasticity and perfect shape memory effect,NiTi-based shape memory alloys(SMAs)have been widely applied in the fields of aerospace engineering,biomedical engineering,and so on.Nanocrystalline NiTi-based SMAs can substantially enhance their mechanical properties,superelasticity,corrosion resistance and biomedical compatibility.Amorphous phase can be induced when NiTi-based SMAs are proceesed by severe plastic deformation(SPD)at or below room temperature.In the present study,NiTiFe SMA with B2 cubic austenite structure and NiTiCu SMA with B19’ monoclinic martensite structure experience SPD based on local canning compression at room temperature and subsequently they are subjected to annealing at various temperatures.It lays the theoretical foundation for manufacturing high-performance nanocrytalline NiTi-based SMAs to reveal mechamism of amorphization induced by SPD based on local canning compression and crystallization mechanism during subsequent annealing of NiTiFe and NiTiCu SMAs combining rigid-plastic finite element simulation,compressive mechanical test technique,electron back-scattered diffraction(EBSD)technique,differential scanning catoritr(DSC)technique,X-ray diffraction technique,transmission electron microscopy(TEM)technique and high resolution transmission electron microscopy(HRTEM)technique.The results of the present work are as follows.Rigid-plastic finite element simulation is used for macroscale simulating plastic deformation of NiTiFe SMA under local canning compression based on the various can dimensions,which contributes to revealing macroscale plastic deformation behavior of NiTi-based SMAs under local canning compression in order to further predict plastic deformation loading of NiTi-based SMAs under local canning compression and optimize the process parameters of the involved local canning compression.The simulation results show that NiTiFe SMA is in a three-dimensional compressive stress state in the case of local canning compression.The three-dimensional compressive stress state can enhance plastic deformation ability of NiTiFe SMA and suppress the initiation of micro-crack,which lays the mechanics foundation for realizing SPD of NiTi-based SMAs.Amorphization can take place when NiTiFe SMA is subjected to SPD based on local canning compression at room temperature.Deformation twinning,mechanically-induced martensite and dislocation slip play an important role in amorphization of NiTiFe SMA.NiTiFe sample subjected to SPD based on local canning compression by 75%suffers from annealing for 2h at 300℃,450℃ and 600℃,respectively.The results demonstrate that in NiTiFe sample subjected to SPD based on local canning compression and subsequent annealing at 300℃,nanocrystalline is dominant and there exists retained amorphous phase.The phenomenon indicates that annealing for 2h at 300℃ is unable to lead to complete crystallization of amorphous NiTiFe sample.In the case of NiTiFe sample subjected to SPD based on local canning compression and subsequent annealing at 450℃,nanocrystalline is dominant and there exist coarse grains,where a high density of dislocation can be observed.In addition,the aforementioned nanocrystalline NiTiFe samples possess B2 austenite structure.In the case of NiTiFe sample subjected to SPD based on local canning compression and subsequent annealing at 600℃,the mean grain size only is 5.32μm and the strong<111>texture can be found,where {112} twin is observed as well.The {112} twin is a product of inheriting {113} deformation twin in mechanically-induced martensite of NiTiFe SMA.In the same manner,amorphization can also be implemented when NiTiCu SMA is subjected to SPD based on local canning compression at room temperature.Martensite reorientation,detwinning deformation,deformation twinning,dislocation slip and shear deformation play an important role in amorphization of NiTiCu SMA.NiTiCu sample subjected to SPD based on local canning compression by 75%experiences annealing for 2h at 300℃,450℃ and 600℃,respectively.Nanocrystallization can be realized in NiTiCu sample subjected to SPD based on local canning compression and subsequent annealing at 300℃.Nanocrystalline NiTiCu sample possesses B19’ martensite structure and very high yield strength,where As temperature is lower than Ms temperature.{112} twin can be observed in NiTiCu sample subjected to SPD based on local canning compression and subsequent annealing at 450℃.{112} twin is a product of inheriting {113} deformation twin in B19’martensite of NiTiCu SMA as well.{114} twin can be observed in NiTiCu sample subjected to SPD based on local canning compression and subsequent annealing at 600℃.{114} twin is a product of inheriting(201)deformation twin in B19’ martensite of NiTiCu SMA.