TiNi形状記憶合金のサブループ超弾性変形

概要

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The various subloop behaviors for the superelastic deformation of TiNi shape memory alloy were investigated based on the local temperature variation and the surface observation in the tension test. The results obtained are summarized as bellows. (1) The upper and lower stress plateaus during loading and unloading appear accompanying the progress and reduction of the martensitic transformation (MT) band, respectively. In the case of unloading from the upper stress plateau under low stress rate, strain increases due to the progress of the MT band in the initial stage of unloading. (2) If stress is held constant in the upper stress plateau, creep deformation appears. The creep deformation appears based on the progress of the MT band. The volume fraction of the martensitic phase increases in proportion to an increase in strain. (3) If the transformation strain varies in the stress plateau during loading and unloading, the return point memory appears in the reloading stress-strain curve. The progress and reduction of the MT band start from the boundary of the MT band which has appeared in the preceding process. (4) The angle of boundary of the MT band inclined to the tensile axis is 33° for an aspect ratio of 5 and 42° in the central part of the specimen and 37° in the vicinity of the gripping part for an aspect ratio of 10.