Study On Phase Transformation And Deformation Characteristics Of Ti-Ni-Zr Shape Memory Alloy

In order to develop excellent Ti-Ni based shape memory alloy and its heat treatment process,Ti-50.8Ni-0.1Zr(atomic fraction)shape memory alloy wires were prepared by doping trace Zr in Ti-Ni alloy.The effects of annealing and aging processes on the phase composition,microstructure,phase transformation behavior,deformation characteristic,superelasticity(SE),shape memory effect(SME)and stress-strain cyclic stability of the Ti-50.8Ni-0.1Zr alloy were studied by means of X-ray diffractometer,scanning electron microscopy,transmission electron microscopy,differential scanning calorimeter and microcomputer controlled electronic universal testing machine.The main results can be described as follows:The Ti-50.8Ni-0.1Zr alloys annealed at 350?700?and aged at 300?600?for 1?50 h are composed of parent phase A(Cs Cl type B2)and martensite phase M(monoclinic B19').With the increase of annealing temperature,the microstructure morphology of the alloy changes from fibrous to equiaxed,and the recrystallization temperature is between 550?and600?.The microstructure morphology of the solution aged alloy is equiaxed,and Ti₃Ni₄precipitated phases are found in the microstructure.With the increase of aging temperature,the morphology of Ti₃Ni₄ precipitates in the alloy changes from the fine particle to the lenticular and then to the thick plate.With the increase of annealing temperature,the reversible phase transformation type of the Ti-50.8Ni-0.1Zr alloy change from A?R?M(A-parent phase B2,Cs Cl;R-R phase,rhombohedral;M-martensite B19',monoclinic)/M?R?A to A?R?M/M?A to A?M/M?A type upon cooling/heating;the phase transformation temperature and temperature hysteresis decreases.With the increase of aging temperature and time,the phase transformation type of the alloy aged at 300?is A?R?M/M?R?A,the transformation type of the alloy aged at 400?changes from A?R?M/M?R?A to A?R?M/M?A;the transformation types of the alloy aged at 500?and 600?are A?R?M/M?A and A?M/M?A,respectively;the R and M phase transformation temperatures of the alloy increase first and then decrease,and the temperature hysteresis of the R phase transformation is stable and changes in the range of 4.8?7.5?.The Ti-50.8Ni-0.1Zr alloy annealed at 350?400?and 600?700?present SE,the alloy annealed at 450?550?present SME.The alloy aged at 300?for 1?50 h,at 400?for 1 h and 600?for 1?50 h present SE,and the alloys aged at 400?for 5?50 h and 500?for 1?50 h present SME.With the increase of annealing temperature,the platform stress of the stress-strain curves decreases first and then increases,the residual strain increases first and then decreases.The maximum value(4.9%)of the residual strain is obtained in the alloy wire annealed at 450?.With the increase of aged time,the platform stress of the alloy aged at 300?and 600?are decrease firstly and then tends to stable,and the energy dissipation is increase.The platform stress and energy dissipation of the alloys aged at 400?and 500?are decrease.The plasticity of the alloy annealed at 600?700?is superior to the alloy annealed at 350?550?,but the strength is less than that of the latter.The maximum value(1489 MPa)of the tensile strength is obtained in the alloy wire annealed at 400?,and the maximum value(35.1%)of the percentage elongation is obtained in the alloy wire annealed at650?.With increasing aging time,the tensile strength is increase and the percentage elongation is decrease in the alloy aged at 300?;the tensile strength is first increase and then decrease,and the percentage elongation is first decrease and then increase in the alloy aged at400?;the tensile strength is decrease and the percentage elongation is increase in the alloy aged at 500?;the tensile strength and percentage elongation decrease in the alloy aged at600?.The fracture morphology of the annealed and aged alloys presents dimple,and subordinates to ductile fracture of the microporous aggregation.With the increase of the number of stress-strain cycles,the platform stress,residual strain and energy dissipation of the Ti-50.8Ni-0.1Zr alloy annealed and aged decreases first and then tend to be stable.With the increase of deformation temperature,the platform stress increases,the residual strain decreases,and the characteristics change from SME to SE in the alloy.With the increase of deformation temperature and rate,the SE of the alloy is improved.