T. Waitz et al. Acta Materialia 52(2004)137-147 MT2 001 MT1,2 30nm M2 08642 汤MT2 Fig. 7. Ni-50.3at %Ti. Nanocrystalline structure; martensite. (a)SA diffraction pattern of a grain containing twinned martensite; (001) pound twinning is indicated by twin reflections(marked by mtI and width [nm reflections along 001 作wy如mdhm t % Ti. Nanocrystalline structure; martensite(a) TEM wins(BD=[IOMTI=[ 10Mr)(b)Two martensite variants MI and pound twins. The twin lamellae show bright and dark contrast(b) M2 that are twinned on(00 1)occur within one grain(BD=[110 M1,2). Histogram of the observed twin widths 4. Discussion rolling of NiTi a martensite microstructure refined by dislocation accumulation and strain induced twinning 4.1. HPT induced amorphization was observed containing numerous intersecting bands of nanocrystalline and amorphous phase arising locally by The results of Figs. I and 2 show that by hpt de- a shear strain instability [5, 6, 9, 10]. As compared to cold formation Ni-50.3at %Ti transforms from B19 mar- rolling the HPT strain is much higher and dominated by tensite to a nanostructured amorphous phase. It a shear deformation [11] causing shear bands and strong proposed that a localized deformation process proceeds crystal refinement [12]. Therefore, the HPt deformation the transformation by leading to a nanocrystalline of NiTi facilitates both the crystal refinement and the structure. The latter is fragmented by bands of an continuous accumulation of amorphous shear bands amorphous phase(cf Fig. 1; S=6.7). Similar, after cold When s increases the crystalline volume fraction4. Discussion 4.1. HPT induced amorphization The results of Figs. 1 and 2 show that by HPT de￾formation Ni–50.3at.%Ti transforms from B190 mar￾tensite to a nanostructured amorphous phase. It is proposed that a localized deformation process proceeds the transformation by leading to a nanocrystalline structure. The latter is fragmented by bands of an amorphous phase (cf. Fig. 1; S ¼ 6:7). Similar, after cold rolling of NiTi a martensite microstructure refined by dislocation accumulation and strain induced twinning was observed containing numerous intersecting bands of nanocrystalline and amorphous phase arising locally by a shear strain instability [5,6,9,10]. As compared to cold rolling the HPT strain is much higher and dominated by a shear deformation [11] causing shear bands and strong crystal refinement [12]. Therefore, the HPT deformation of NiTi facilitates both the crystal refinement and the continuous accumulation of amorphous shear bands. When S increases the crystalline volume fraction Fig. 7. Ni–50.3at.%Ti. Nanocrystalline structure; martensite. (a) SA diffraction pattern of a grain containing twinned martensite; (0 0 1) compound twinning is indicated by twin reflections (marked byMT1 and MT2) related by a rotation of p around ½001. Streaks and elongated reflections along ½001MT1;2 are caused by the very small width of the twins (BD ½
110MT1 ½1
1 0MT2) (b) Two martensite variants M1 and M2 that are twinned on (0 0 1) occur within one grain (BD ½
110M1;2). Fig. 8. Ni–50.3at.%Ti. Nanocrystalline structure; martensite. (a) TEM bright field image of a grain containing a high density of (0 0 1) com￾pound twins. The twin lamellae show bright and dark contrast. (b) Histogram of the observed twin widths. T. Waitz et al. / Acta Materialia 52 (2004) 137–147 143