Oxidation,transformation and deformation characteristics of Ti-49.8Ni shape memory alloy

The oxidation,transformation and deformation characteristics of Ti-49.8Ni shape memory alloy(SMA)were investigated by thermal gravity analyzing,X-ray diffraction,differential scanning calorimetry,tensile test and cycling test.The results show that the oxidation rate of Ti-49.8Ni alloy increases when annealing temperature(Ta)is over 600 ℃.After annealing at 400-600 ℃,the phase transformation type of Ti-49.8Ni alloy is A→R→M/M→A(A-parent phase,R-R phase,M-martensite phase)during cooling/heating cycle.With Ta increasing,the M transformation temperature increases;the R transformation temperature is nearly no change;the temperature hysteresis decreases first and then increases;the martensite reorientation stress decreases first and then increases;and the plasticity increases for Ti-49.8Ni alloy.At the room temperature,the composition phases of Ti-49.8Ni alloy are martensite plus TiO2,and the alloy shows shape memory effect(SME).When the deformation temperature is over 110 ℃,the alloy shows superelasticity.The excellent shape memory properties can be obtained by annealing at 400-550 ℃.The SME of the alloy degrades when Ta is over 600 ℃.The strain recovery ratio of Ti-49.8Ni alloy coil spring decreases with the cycle number increasing,and the decline of the strain recovery ratio is slow when the cycle number beyond 100.