| Nickel-titanium shape memory alloys (hereinafter referred to as NiTi alloys) are widelyused in medicine, aviation, aerospace, military and civilian fields such as engineering becauseof their excellent shape memory effect and superelasticity. Plastic working is an indispensablemeans to make the NiTi alloys ingot from the as-cast into engineering applications and has aninfluence on shape memory effect and superelasticity of NiTi alloys. In particular, cold plasticdeformation is able to form some substructures such as a high density of dislocations andtwins in the NiTi alloys. In addition, with the increase of plastic strain, severe plasticdeformation (SPD) can lead to the nanocrystalline or amorphous NiTi alloys, which is of greatimportance in improving the superelasticity of NiTi alloys. However, it is in the case of coldplastic deformation that NiTi alloys have lower plasticity and higher deformation resistanceand are more difficult to form. Furthermore, the occurrence of stress-induced martensite phasetransformation in the case of austenite state shall lead to more complicated deformationmechanism of NiTi alloys.Therefore, the domestic and foreign scholars devote themselves toexplorating new cold plastic deformation methods of NiTi alloys, performing severe plasticdeformation of NiTi alloys, and preparing nanocrystalline and amorphous NiTi alloys. In thepresent study, a plastic deformation technology with respect to NiTi alloys based on localcanning compression is put forward. Local canning compression causes NiTi alloys to be in athree-dimensional compressive stress state, which contributes to enhancing the plasticity ofNiTi alloys and preparing the nanocrystalline or amorphous NiTi alloys. In the paper, basedon near-equiatomic NiTi alloys bar, by combining local canning compression process testwith metal forming mechanics and by using the optical microscope, X-ray diffraction (XRD),differential scanning calorimetry (DSC), scanning electron microscopy (SEM) andtransmission electron microscope (TEM), deformation behavior and microstructural evolutionof NiTi alloys under local canning compression are sysmetically investigated, which lays animportant theoretical foundation for making nanocrystalline and amorphous NiTi alloys.The uneven and multilateral equiaxed grains distribute in the near-equiatomic NiTi alloyand the average grain size is about25μm. The NiTi alloy matrix mainly beongs to theaustenitic cubic structure. The austenitic phase transition temperatures are determinted asA_s=-17.3℃and A_f=-4.1℃and the martensite phase transition temperatures are determinted as M_s=-27.2℃and M_f=-41.7℃.Based on the metal forming mechanics, by combining the differential equilibriumequations with the plastic yield criterion, the plastic mechanics with respect to local canningcompression of NiTi alloy is solved. Accordingt to the stress-strain curves of NiTi alloy underlocal canning compression, the equivalent stress and hydrostatic pressure of NiTi alloys underlocal canning compression are obtained. The theoretical analysis and the experimental resultsreveal that the hydrostatic pressure increases with the increase of the diameter of metal can,which contributes to supressing the brittle fracture of NiTi alloys and improving the plasticityof NiTi alloys. When the diameter of metal can is equal to or greater than10mm, NiTi alloyscan be subjected to severe plastic deformation. when the strain is less than0.15, NiTi alloyswill experience the elastic deformation and stress-induced martensite phase transformation.When the strain ranges from0.15to0.5, NiTi alloys meet the Mises yield criterion. When thestrain is greater than0.5, NiTi alloys do not satisfy the Mises yield criterion due to occurrenceof nanocrystalline or amorphous phase.Microstructural evolution of NiTi alloys under local canning compression at the variousdeformation degrees at room temperature is investigated. The deformation process of NiTialloys at room temperature deals with the elastic deformation of austenite, the stress-inducedmartensitic transformation, the elastic deformation of stress-induced martensite and plasticdeformation of stress-induced martensite by dislocation slip. Stress-induced martensitictransformation, deformation twinning and dislocation slip play a dominant role in thedeformation process. When the deformation degree is25%, at the initial stage of deformation,the stress-induced martensite phase transformation occurs in the austenite NiTi alloy and thenthe sub-structures such as deformation twins and dislocations arise. When the deformationdegree is50%, the dislocation density continues to increase and a small amount ofnanocrystalline and amorphous phase appear. When the deformation degree is80%, an almostcomplete mixture of nanocrystalline and amorphous NiTi alloy occur.Microstructural evolution of NiTi alloys under local canning compression at the variousdeformation degrees at low temperature (-150℃) is investigated. The deformation process ofNiTi alloys at low temperature deals with the elastic deformation of martensite, martensitereorientation (along with detwinning of martensite), the stress-induced martensitictransformation, and plastic deformation of martensite by dislocation slip. After the plastic deformation, the start of slip systems lead to a large number of dislocations. The inhibition ofmartensite interface makes dislocation pile-up at the interface so that the microstructuralmorphology of martensite is retained. The NiTi alloy exhibits the martensite morphology afterunloading at room temperature and goes back to the austenite state by thermal driving sincethe dislocations does not have an influence on the crystal structure. The NiTi matrix mainlyconsists of the cubic austenite structure and a small amount of retained nanocrystallinemartensite in the NiTi sample after large deformation at low temperature. Furthermore, a greatdeal of amorphous and nanocrystalline NiTi phase is obtained in the severe plasticdeformation zone. |
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