Investigation Of Plastic Deformation Mechanism Of NiTiFe Shape Memory Alloy By Equal Channel Angular Extrusion

Ni-Ti-based shape memory alloy(SMA)is a promising candidate which can be widely employed in aeronautical,engineering,and biomedical fields on account of its excellent shape memory effect,superelasticity,mechanical properties,corrosion resistance and biocompatibility.Moreover,NiTiFe SMA possesses low phase transformation temperature and good stability at high temperature.It is a perfect candidate for manufacturing composite pipe which is used in the field of aviation.As one of sever plastic deformation process,equal channel angular extrusion(ECAE)can effectively refine the grain without changing the size of material,and therefore improve the mechanical properties of material.In the present study,the microstructure evolution and plastic deformation mechanisms in the process of ECAE plastic deformation of NiTiFe SMA were analyzed based on ECAE experiment,finite element simulation,optical microscopy(OM)experiment,electron backscatter diffraction(EBSD)experiment and transmission electron microscope(TEM)experiment.These investigations and observations lay the theoretical foundation for the grain refinement of NiTiFe SMA during ECAE process.(1)The true stress-strain curves were obtained from the hot compression experiments of NiTiFe SMA conducted at different temperatures including 300?,400?,500? and 600?and at different strain rates including 0.001s-1 and 0.01s-1.Then the optical microstructure analysis was conducted on these deformed samples.These experimental results indicate that during the process of hot compression at 300? and 400?,NiTiFe SMA is in the state of work hardening,while during the process of hot compression at 500?,dynamic recovery happens within the NiTiFe SMA.Moreover,dynamic recovery and dynamic recrystallization coexist within the NiTiFe SMA during the process of hot compression at 600?.As a consequence,in order to avoid dynamic recrystallization during the ECAE process of NiTiFe SMA,the manufacture temperatures are determined as 400? 450? and 500?.Moreover,the stress-strain curves of NiTiFe SMA during hot compressions can serve as the corresponding material constitutive models in macroscale finite element simulations of ECAE process.(2)Based on the theory of elastic-plasticity finite element,macroscale finite element simulations of the ECAE process of NiTiFe SMA with various inner corner(0=90°,105° and 120°)various inner corner radius(r=0.5mm,1mm and 1.5mm)and various outer corner radius(R=lmm,2mm and 3mm)are conducted on the finite element code ABAQUS.Simulation results indicate that during the ECAE process of NiTiFe SMA,extrusion force undergoes three characteristic stages,firstly the stage with sharp increase in extrusion force,then the stage with gradual increase in extrusion force,lastly the stage with subsequent stabilization in extrusion force.In addition,distribution of equivalent stress and strain are inhomogeneous and the severe shear plastic deformation happens in the corner.Therefore,the equivalent stress in the corner reaches the maximum value.Moreover,the equivalent strain in the inner side of sample is bigger than the one in the outer side of sample.The numerical modelling results also demonstrate that extrusion force and equivalent strain gradually reduce with the increase of inner corner,while along with the increase of inner corner radius,extrusion force and equivalent strain gradually increase and the inhomogeneity within the deformed sample is reduced.Moreover,in the case with relatively small outer corner radius,equivalent strain possesses relatively large value and its distribution shows less inhomogeneity within the deformed sample.However,the equivalent stress sharply increases and this issue goes against the usage of die.Finally,the optimal die parameters are determined as r=1mm and R=2mm.(3)One-pass ECAE experiments of NiTiFe SMA with 120° torsion angle were conducted at temperature 400?,450? and 500?.Then the corresponding microstructure are characterized by OM,EBSD and TEM experiments.The observations indicate that the substructure within individual grains of the deformed sample of NiTiFe SMA is reduced along with the increase of extrusion temperature,so the effect of grain refinement is also reduced.In the deformed sample subjected to ECAE at 400?,nanocrystalline phase can be observed.Dislocation slip is the main mechanism to sustain plastic deformation during ECAE of NiTiFe SMA.The dislocation density increases along with the increase of plastic strain,forms dislocation wall within grains and gradually turns into dislocation cells and finally forms new subgrains with low angle grain boundary.With the increase of plastic deformation,the subgrains with low angle grain boundary continually absorb dislocations,and gradually rotate to form the new grains with large angle grain boundary.The aforementioned process is repeated until the individual grains finally are refined.