Transformation and detwinning induced electrical resistance variations in NiTiCu

Utilizing electrical resistance measurements in combination with strain observations on single crystals, it was possible to distinguish the austenite to martensite transformation from the detwinning of the martensite behavior during deformation of NiTiCu shape memory alloys. It was found that the electrical resistance change associated with the detwinning of the martensite variants was as high as 45% in deformation experiments below the martensite start temperatures. When the conditions led to partial detwinning of the martensite variants (tension at 20 °C) the corresponding electrical resistance changes were 20%. In the tension case at 20 °C, the electrical resistance measurements convincingly identified the end of the stress plateau as the conclusion of the transformation and the onset of detwinning. The detwinning is also activated following austenite to martensite transformation under thermal cycling experiments under a constant stress (shape memory under constant stress). In this case, the electrical resistance change associated with detwinning also reached high levels (45%) and coincided with the change in slope of the strain–temperature curves.