Study On Diffusion Bonding Of Dissimilar Shape Memory Alloys Subjected To Severe Plastic Deformation

NiTi shape memory alloy has good shape memory effect and superelasticity,and thus it has been widely used in aerospace and biomedical fields.With the development of industry,the performance of some materials must meet the higher requirements.The results show that the nanocrystalline NiTi shape memory alloy has the advantages of better performance than the coarse-grained one,such as mechanical properties,superelasticity,biocompatibility,connectivity and so on.Therefore,the development of new shape memory alloy has important scientific significance and engineering value.In this paper,a new idea of the diffusion bonding of dissimilar NiTi-based shape memory alloys is proposed,and the nanocrystallization and amorphization of the interface of the dissimilar shape memory alloy are realized by severe plastic deformation in order to facilitate the diffusion bonding of the interface.In this paper,NiTiNb and NiTiCu shape memory alloys undergoing solid solution treatment were studied.The compressive mechanics experiment,local canning compression technique,heat treatment technology,transmission electron microscopy(TEM),scanning electron microscopy(SEM),differential scanning calorimetry(DSC)and solid diffusion bonding technology were used to investigate diffusion bonding of dissimilar shape memory alloy subjected to severe plastic deformation.The nanocrystallization and amorphization mechanism of NiTiNb and NiTiCu shape memory alloys were investigated.The crystallization mechanism of NiTiNb shape memory alloy subjected to heat treatment was investigated.The mechanical properties of the NiTiNb shape memory alloy were investigated.Finally,the diffusion bonding behavior of NiTiNb and NiTiCu shape memory alloys was revealed.The results of this paper are as follows.The NiTiNb and NiTiCu shape memory alloys in the hot-rolled state were subjected to solution treatment,where it was held at 850? for 2h.X-ray diffraction(XRD),SEM and TEM observation of NiTiNb and NiTiCu shape memory alloys in solid solution state were performed.The NiTiCu shape memory alloy was found to be in a complete lath martensitic state,and the NiTiNb shape memory alloy was composed of NiTi(B2)and ?-Nb phase.The NiTiNb and NiTiCu shape memory alloys were subjected to local canning compression experiments.When the NiTiNb shape memory alloy undergoes local canning compression by 25%,it is found that a small amount of nanocrystalline phase exists in the alloy.When the NiTiNb shape memory alloy undergoes local canning compression by 50%,nanocrystals and amorphous phases can be found to coexist and the number of amorphous phases increases.When the NiTiNb shape memory alloy undergoes local canning compression by 75%,the NiTiNb shape memory alloy has been severely amorphized.When the NiTiCu shape memory alloy undergoes local cannnig compression by 50%,amorphization and nanocrystallization occur,and there is lath martensite in the local region.The NiTiNb shaped memory alloy subjected to severe plastic deformation based on local canning compression by 75% was subjected to heat treatment at 300 °C,400 °C,500 °C and 600 °C for 2h,respectively.TEM observation showed that the grain size increased with the increase of the crystallization temperature in the case of heat treatment.In the case of crystallization at 300?,the amorphous phase of NiTiNb shape memory alloy is dominant,and the nanocrystalline phase exists locally.In the case of crystallization at 400?,NiTiNb shape memory alloy suffers from serious nanocrystallization,and almost complete nanocrystalline phase is induced.In the case of crystallization at 500?,the grain size is obviously increased,where the main body is coarse phase,but there are still a few nanocrystalline phases.In the case of crystallization at 600?,the grain size is further increased,and no nanocrystalline phase can be observed.The true stress-strain curves of the crystallized NiTiNb shape memory alloys were obtained by means of compression experiment.The experimental results show that the NiTiNb shape memory alloy is fractured instantaneously at room temperature under heat treatment at 300?,and no plastic deformation occurs.The yield strength of NiTiNb shape memory alloy is the highest during heat treatment at 400?.Under the condition of heat treatment at 500?,the yield strength of NiTiNb shape memory alloy is lower,but it is higher than that of the original NiTiNb shape memory alloy,and the plasticity is also good.The yield strength of Ni Ti Nb shape memory alloy is further reduced in the case of heat treatment at 600?,but it is still higher than that of original NiTiNb shape memory alloy.For NiTi Nb and NiTiCu shape memory alloys undergoing local canning compression by 50%,diffusion bonding was performed based on the bonding temperature of 670°C,the bonding pressure of 5 MPa and the various holding times.The results showed that when the holding time was 0.5h,Ni Ti Nb and NiTiCu shape memory alloys with amorphous and nanocrystalline interface were not bonded.In the case of 1h holding time,NiTiNb and NiTiCu shape memory alloys with amorphous and nanocrystalline interface were bonded.In the case of holding time for 1.5h,amorphous and nanocrystalline interfaces of NiTiNb and NiTiCu shape memory alloy are bonded.In particular,with the increase in the holding time,the distance of element diffusion increases and the content increases.As a consequence,the higher the bonding quality of the interface is,the greater the microhardness near the interface is.