Study On Phase Transformation And Deformation Characteristics Of Ti-51.1Ni Shape Memory Alloy

The effects of different heat treatment conditions on the phase composition at room temperature,phase transformation behaviors,microstructure and mechanical behavior of Ti-51.1Ni alloy were studied by means of transmission electron microscope,X-ray diffractometer,differential scanning calorimeter and microcomputer controlled electronic universal testing machine.The results show that the phase compositions of annealed Ti-51.1Ni alloy consists of parent phase A（parent phase B2,CsCl）and martensite M（martensite B19ˊ,monoclinic）at room temperature.The phase composition at room temperature of Ti-51.1Ni alloy aged at 300⁶00℃for 1-50h is parent phase A,martensite M and precipitated phase Ti₃Ni₄.With increasing annealing temperature,the phase transformation types of the Ti-51.1Ni alloy change from A→R→M to A→M upon cooling（R-R phase,rhombohedral）,and the phase transformation type change from R→A to M→R→A to M→A type upon heating;the changes of the R-phase transition temperature and the R-phase temperature hysteresis are very small,the M-phase transition temperature rises first and then tends to be stable,and the M-phase temperature hysteresis decreases sharply.With the increase of aging temperature and time,A→R/R→A phase transformation takes place upon cooling/heating in Ti-51.1Ni alloy aged at 300℃,the phase transformation type changes from A→R/R→A to A→R→M/M→R→A upon cooling/heating in the alloy aged at400℃;the type of cooling/heating in the alloy aged at 500℃does not change,only A→R→M/M→A,A→M/M→A phase transformation takes place upon cooling/heating in Ti-51.1Ni alloy aged at 600℃.The martensite transformation temperature drops sharply below 0℃in the alloy aged at 600℃.The R phase temperature hysteresis is smaller（all below 10℃）of the alloy aged at 300℃and 400℃.With the increase of annealing temperature,the recovery,recrystallization and grain growth take place in deformed Ti-51.1Ni alloy,and the microstructure of the alloy gradually changes from the fibrous during low temperature heat treatment to the equiaxed grains during high temperature heat treatment.With the increase of aging temperature and time,the morphology of the precipitated phase of Ti₃Ni₄ in the alloy changes from fine granular to elliptically flaky,then to lenticular,and finally to coarse flaky.The tensile strength of Ti-51.1Ni alloy annealed at 350⁵50℃is greater than that of the alloy annealed at 600⁷00℃,and that of the alloy aged at 400℃is greater than that of the alloy aged at 300,500 and 600℃.The maximum elongation of the alloy annealed at 650℃and the alloy aged at 600℃for 5h were 77.8%and 69.75%respectively.In one tensile state,the Ti-51.1Ni alloy annealed at 350°C and above 600°C shows superelasticity,and the alloy annealed at 400⁵50°C shows shape memory effect.The alloy aged at 300℃and 400℃for 1h shows superelasticity,and the alloy aged at 400℃for5⁵0h and 500℃⁶00℃for 1⁵0h shows shape memory effect.With increasing deformation temperature T_d,the shape memory behavior of Ti-51.1Ni alloy annealed at400⁶00℃changes from shape memory effect to superelasticity,and the alloy presents shape memory effect when T_d is-20℃and 0℃,presents shape memory effect+superelasticity when T_d is 25℃,and presents superelasticity when T_d is over 25℃.When T_d is over 25℃,the shape memory performance was weakened of the alloy annealed at 700℃.With increasing the cycle number of stress-strain,the superelasticity of the alloy annealed at350℃and aged at 300℃and 400℃for 1h is the most stable and the shape memory effect of the alloy annealed at 500℃is the most stable,and the alloy aged at 400℃for 5¹0h maintains the shape memory effect,and the alloys aged at 400℃for 20h,at 500°C for1⁵0h and 600°C for 1²0h change from shape memory effect to superelasticity gradually.