| In order to develop Ti-Ni based shape memory alloy with stable phase transformation and properties,the phase composition,phase transformation behavior,microstructure,mechanical properties,electrochemical corrosion properties,shape memory behavior and stress-strain cyclic stability of Ti-50Ni and Ti-45Ni-5Cu shape memory alloys annealed at350?700 were comparatively investigated using X-ray diffractometer,differential scanning thermal analyzer,scanning electron microscope,universal tensile testing machine and electrochemical workstation and the shape memory behavior and stress-strain cyclic stability of the annealed Ti-50Ni and Ti-45Ni-5Cu alloy springs were comparatively studied also,and the main results are as follows:The room temperature composition phases both of Ti-50Ni and Ti-45Ni-5Cu shape memory alloys annealed at 350?700°C are martensitic phase B19?and parent phase B2.With the increase of annealing temperature,the phase transformation type of Ti-50Ni alloy changes from B2?R?B19?/B19??B2 to B2?B19?/B19??B2(B2-parent phase,Cs Cl type;R-R phase,rhombohedral;B19?-martensitic phase,monoclinic)upon cooling/heating,the reverse martensitic transformation temperature of the alloy changes in the range of 72 and 80,and the temperature hysteresis of the alloy changes in the range of 33 and 41;the phase transformation type of Ti-45Ni-5Cu alloy is B2?B19?/B19??B2 upon cooling/heating,the reverse martensitic transformation temperature of the alloy changes in the range of 46 and65,the temperature hysteresis of the alloy is narrow,and changes in the range of 22 and24.The Ti-50Ni and Ti-45Ni-5Cu alloys annealed at 350?550 are in the recovery stage and the microstructure is fibrous;the alloys annealed at 600?700 are in the stage of grain growth,and the microstructure is equiaxed.The crystallization temperature of the two alloys is between 550 and 600.The tensile strength(Rm)is almost of the same,and the plateau length of martensitic reorientation(LM)is almost equal of Ti-50Ni and Ti-45Ni-5Cu alloys.The martensitic reorientation stress(?M)and elongation(A)of Ti-50Ni alloy are higher than that of Ti-45Ni-5Cu alloy.With the increase of annealing temperature,the Rm and LMdecrease,the A increases,the?M first decreases and then increases in the two alloys.The minimum value(102.8 MPa)of?M of Ti-50Ni alloy was obtained in the alloy annealed at 450,and the minimum value(63.3 MPa)of?M of Ti-45Ni-5Cu alloy was obtained in the alloy annealed at 400.Both of Ti-50Ni and Ti-45Ni-5Cu alloys exhibit shape memory effect at room temperature.With the increase of deformation temperature,the plateau stress of Ti-45Ni-5Cu alloy increases,the residual strain decreases,the shape memory effect gradually changes to superelasticity,and the change temperature ranges are 60?80.The cycle stability of Ti-45Ni-5Cu alloy is better than that of Ti-50Ni alloy.After electrochemical corrosion,the corrosion voltage of Ti-45Ni-5Cu alloy is larger than that of the Ti-50Ni alloy as a whole,and the maximum value-0.283 V is obtained in the Ti-45Ni-5Cu alloy annealed at 600,and the corrosion resistance of Ti-45Ni-5Cu alloy is better.With the increase of deformation temperature,Ti-50Ni and Ti-45Ni-5Cu alloy springs annealed at 350?600?change from shape memory effect to superelasticity,and the change temperature ranges is 60?80 for the former,is 40?60?for the latter.As the number of stress-strain cycles increases,the strain recovery rates of Ti-45Ni-5Cu and Ti-50Ni alloy springs decrease first and then stabilize,and the shape memory stability of Ti-45Ni-5Cu alloy springs is better than that of Ti-50Ni alloy springs. |
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