Attention was focussed upon stress-induced transformation of solutionized and overaged single-crystal NiTi alloys. The potential role of de-twinning in recoverable strain was investigated theoretically, and by means of temperature-cycling experiments. The de-twinning involved the growth of one variant within a martensite at the expense of the other. It was shown that the experimental recoverable strains in tension (near to 8.01% in the [123], to 9.34% in the [111] and to 7.8% in the [011] orientations) exceeded the theoretical martensite correspondent-variant pair formation strains (6.49% in [123], 5.9% in [111] and 5.41% in [011]). This again supported the idea that partial de-twinning of martensite had occurred in both the solutionized and overaged specimens. In compression, the experimental recoverable strains were lower than the theoretical martensite (correspondent-variant pair) formation strain. In the compression case, the de-twinning strain contribution was estimated to be negligible in most orientations. The transformation strains observed in overaged NiTi were similar to those in solutionalized NiTi; thus suggesting that incoherent precipitates did not restrict de-twinning of the martensite. In the case of the [123] orientation, it was demonstrated that the thermal hysteresis was higher in solutionized NiTi; as compared with overaged NiTi.

De-Twinning in NiTi Alloys. H.Sehitoglu, R.Hamilton, D.Canadinc, X.Y.Zhang, K.Gall, I.Karaman, Y.Chumlyakov, H.J.Maier: Metallurgical and Materials Transactions A, 2003, 34[1], 5-13