Investigation Of Plastic Deformation Mechanism And Microstructure Evolution Of Ni Ti Fe Shape Memory Alloy During Plane Strain Compression

NiTi shape memory alloy(SMA)is widely used in variant engineering fields owing to its good shape memory effect and superelasticity.The mechanical properties and phase transformation behaviors of NiTi SMA are sensitive to its components.The addition of other elements to NiTi SMA has deserved more increasing attention in the field of SMA.Among them,NiTiFe SMA is widely used as pipe couplings,fasteners and connecters in the field of aerospace due to its good mechanical property,lower martensite transformation temperature,and stability of the B2-type austenite.The substitution of a Ni atom for a Fe atom in binary NiTi SMA could stabilize the B2 phase and result in the suppression of martensite transformation,which leads to a decrease in martensite transformation temperature.With the development of science and technology,the application of NiTi-based SMA becomes diversified,and the influence of adding the fourth element to ternary NiTi-based SMA on the mechanical properties and phase transformation behaviors of NiTi-based SAM has become one of new topics in the field of SMA.In the present study,the influence of the fourth element on the microstructure,phase transformation behaviors and mechanical properties of NiTi-based SMA were investigated based on NiTiFe SMA.Plastic processing is always an important forming method for NiTiFe SMA.The mechanical properties,microstructure and phase transformation behavior of SMAs could be changed by means of thermomechanical treatment including cold working and subsequent annealing.Plane strain compression is a common stress state in plastic processing and widely employed in the research about metal flow and texture evolution.In the present study,NiTiFe SMA undergoes exactly plane strain compression based on channel die compression.The effect of plane strain compression and subsequent annealing on the microstructure,deformation texture and phase transformation behavior of NiTiFe SMA is investigated in detail through a combination of crystal plasticity finite element method(CPFEM),transmission electron microscope(TEM),electron backscatter diffraction(EBSD)and differential scanning calorimetry(DSC).The plane strain plastic deformation mechanism of NiTiFe SMA is revealed,which can lay the scientific foundation for the plastic processing of NiTiFe SMA.The main achievements of the study were summarized as follows.The mechanical properties and phase transformation behaviors of NiTiFe SMA are sensitive to its components.In the present study,Nb and Ta elements were added to NiTiFe SMA in order to research the properties of quaternary NiTi-based SMAs.The experiment results show that the addition of Nb and Ta elements could obviously enhance the yield strength,ultimate strength and decrease the plasticity of NiTiFe SMAs.Addition of Nb and Ta elements does not change phase transformation path of NiTiFe SMAs,but it does have a certain effect on transformation temperatures of NiTiFe SMAs.The TEM results show that strain-induced martensite appears in the matrix at the deformation degree by 10% and 30%.Nanocrystalline phase appears in the matrix at the deformation degree by 30% and 50%.Amorphous phase appears in the matrix at the deformation degree by 50%.It is known by combining TEM with EBSD results that the appearance of nanocrystalline and amorphous phases is attributed to the effect of grain refinement,the mechanism of which is based on the formation of subgrain boundaries and grain boundaries.The static recrystallization mechanism is discussed in detail according to the different microstructure after plane strain compression.The influence of various deformation degrees on static recrystallized microstructure of NiTiFe SMA is researched.The phase transformation path and temperatures are sensitive to the grain size of austenite.From the results of DSC experiments,the microstructure and grain size of NiTiFe SMA has a deep influence on the phase transformation path and temperatures.Coupling crystal plasticity constitutive theory with finite element method leads to an extensive application of crystal plasticity model,which is based on the establishment of the polycrystalline finite element model.Research shows that the grain size distribution in polycrystalline finite element model has a great influence on the accuracy of simulation results.Therefore,the establishment of polycrystalline finite element model that could respond to the real micro morphology of NiTiFe SMA is very important for exact predicting the deformation behavior.In the present study,the 2-D and 3-D polycrystalline finite element models were built based on Voronoi tessellation scheme and EBSD data respectively.The difference between two models is discussed in detail to ensure the accuracy of the simulation results.In the present study,2-D and 3-D crystal plasticity finite element method(CPFEM)is used to simulate microstructure,texture evolution and stress-strain response during uniaxial compression and plane strain compression of NiTiFe SMA respectively.The simulation results have a good agreement with experimental results.The main texture formed in NiTiFe SMA during uniaxial compression is γ-fiber texture,which is fiber texture with the <111> axis parallel to the ND.The main slip system activated during uniaxial compression is <100>/{110}.Main textures during plane strain compression are comprised of α-fiber and γ-fiber texture,which are fiber textures with <110> axis parallel to RD and with the <111> and <001> axis parallel to ND.The main slip systems activated during plane strain compression are <100>/{110} and <111>/{110}.By establishing the relationship between rotation angle and initial orientation of grains,through a combination of activated slip systems in plastic deformation,the evolutionary process and formation mechanism of deformation textures is revealed based on the simulation results of CPFEM.The intergranular and intragranular deformation heterogeneity during uniaxial compression and plane strain compression is analyzed quantitatively by comparing the stress-strain curves.The deformation ununiformity during plane strain compression is more serious than that during uniaxial compression.