Internal stresses and plastic strains introduced into surface layers of bent NiTi ribbon by low-temperature shape setting

Engineering applications of NiTi alloys frequently require the customized shapes that can be achieved through shape setting i.e. applying heat treatment while constraining a NiTi element in a desired shape. This works reliably with cold worked alloys which change its microstructure during the shape setting in the whole volume. When an already superelastic NiTi alloy subjected to shape setting, it depends very much on the shape setting temperature. When the temperature is lower than ~450 °C, only the martensite phase is affected, the austenite microstructure and functional properties remain intact. This represents the basis of the selective Low Temperature Shape Setting process /LTSS. When the temperature is higher, microstructure within the whole volume of the shape set NiTi element is affected as a result of which its functional properties change. Shape setting NiTi wires and ribbons into bent shapes is one of the most frequent technological steps in the field. In the present work we analyze plastic strains and internal stresses introduced into surface layers of bent superelastic NiTi ribbon by the shape setting.