Detwinning in NiTi Alloys

This work focuses on the stress-induced transformation in solutionized and overaged single-crystal NiTi alloys. The potential role of detwinning on the recoverable strains was investigated both theoretically and also with temperature-cycling experiments. The detwinning is the growth of one variant within a martensite in expense of the other. It is shown that the experimental recoverable strains in tension (near 8.01 pct in the [123], 9.34 pct in the [111], and 7.8 pct in the [011] orientations) exceed the theoretical martensite (correspondent-variant pair (CVP) formation strains (6.49 pct in [123], 5.9 pct in [111], and 5.41 pct in [011]), lending further support that partial detwinning of martensite has occurred in both the solutionized and overaged specimens. In compression, the experimental recoverable strains are lower than the theoretical martensite (CVP) formation strain. In the compression cases, the detwinning strain contribution is calculated to be negligible in most orientations. The transformation strains observed in overaged NiTi are similar to the solutionalized NiTi, suggesting that incoherent precipitates do not restrict the detwinning of the martensite. For the [123] orientation, it is demonstrated that the thermal hysteresis is higher in solutionized NiTi compared to the overaged NiTi. The higher thermal hysteresis can be exploited in applications involving damping and shape stability, while the lower hysteresis is suited for actuators.