Orientation-dependent plastic deformation mechanisms and competition with stress-induced phase transformation in microscale NiTi

WS Choi and EL Pang and WS Ko and H Jun and HJ Bong and C Kirchlechner and D Raabe and PP Choi, ACTA MATERIALIA, 208, 116731 (2021).

DOI: 10.1016/j.actamat.2021.116731

Understanding the orientation-dependent deformation behavior of NiTi shape-memory alloys at small length scales is of importance for designing nano- and micro-electromechanical systems. However, a complete understanding of the orientation- and size-dependent competition between the various modes of slip, deformation twinning, and martensitic transformation in NiTi shape-memory alloys is still lacking, especially in micron-scale specimens. In the present study, we perform micro- compression tests on 001- and 112-oriented micro-pillars of a solutionized Ti-49.9at.% Ni alloy. Post-mortem TEM analysis of the deformed pillars reveal that the operating plastic deformation modes are 011<100> slip and 114<22 1 > deformation twinning, which compete with the martensitic transformation, depending on the crystal orientation. Furthermore, in both experiments and molecular dynamics simulations, we consistently find residual B19' martensite in a herringbone microstructure composed of finely spaced (001)(B19') compound twins instead of the generally assumed 011(B19') type II twins common in bulk samples, suggesting that the operative martensitic transformation mode may be size-dependent. Schmid factors in compression are calculated for all commonly reported slip, deformation twinning, and martensitic transformation modes as a function of crystallographic orientation, which rationalize the orientationdependent competition between these deformation modes. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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