H. Kato, K. Sasaki, Hokkaido University, Sapporo, Japan; T. Mori, University of Manchester, Manchester, United Kingdom
NiTi alloy polycrystals can exhibit superelasticity during which the flow stress shows no work hardening. When a plate or rod of polycrystal specimen is deformed in tension, the superelastic deformation does not proceed in grains simultaneously, but develops in an inhomogeneous manner as the propagation of Lüders’ band, which can be observed by eyes on surface. First, the surface observation showed that the front line of the band, a Lüders’ line, was straight and inclined 45 deg. to the loading axis, and the Lüders’ band increased its width in proportion to the increase in the superelastic strain, during which the flow stress was constant. All the results support the idea such that simple shear occurs in the band, and the shear strain in the band should be constant from the beginning to the end of superelastic deformation. Next, it was observed that the Lüders’ band increased its numbers with increasing the tension speed. It is likely that the velocity of Lüders’ line has an upper bound, above which new band may be formed in the untransformed gauge part to come up with the increase of flow speed. The probable reason is discussed.
Summary: This study observed Lüders’ band deformation during superelasticity in NiTi alloy polycrystals. Photographs of the Lüders’ lines are shown. The result of surface observation and the flow stress-strain curve suggest that simple shear occurs in the bands and the strain does not change during superelastic deformation. The kinetics of the mobility of the bands was also discussed in terms of the velocity of propagation.
