Dresden 2014 – scientific programme

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MM: Fachverband Metall- und Materialphysik

MM 2: Topical Session: Nanomechanics of nanostructured materials and systems I - Grain size effects

MM 2.4: Talk

Monday, March 31, 2014, 11:15–11:30, BAR 205

Stress induced martensitic phase transformation during nanoindentation in NiTi shape memory alloys — •Guillaume Laplanche, Gunther Eggeler, and Janine Pfetzing-Micklich — Institut für Werkstoffe, Ruhr-Universität Bochum, D-44780 Bochum, Germany

NiTi shape memory alloys (SMA) exhibit remarkable functional properties which rely on a stress and/or temperature induced martensitic phase transformation. Our study is motivated by the need to mechanically characterize small NiTi components, such as micromechanical systems like stents and microactuators. In this study, we use a NiTi alloy with specific phase transformation temperatures. In our NiTi alloy, stress induced martensite is stable at room temperature and this allows its post-mortem characterization. We use EBSD to select grains with specific crystallographic surface normals. In those grains, we perform nanoindentation with spherical indenter tips, in order to avoid symmetries others than those of the crystal structure. The topography of the remnant indents were characterized using atomic force microscopy (AFM). The stress induced formation of martensite shows a crystallographic anisotropy which manifests itself in orientation dependent surface patterns. These patterns reflect the well-known symmetries of the cubic crystal lattice and vanish upon heating, when the martensite transforms back to austenite. AFM investigations of the topography of the regions surrounding the remnant indents directly after nanoindentation, reveal that the stress induced formation of martensite results in the formation of crystallographic sink-in regions surrounding the contact.