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MM: Fachverband Metall- und Materialphysik
MM 40: Nanomaterials III
MM 40.2: Talk
Thursday, March 17, 2011, 16:30–16:45, IFW B
Martensitic transformation of nanostructured NiTi made by crystallization of a deformation induced amorphous phase — •Martin Peterlechner1, Christoph Gammer2, Gerhard Wilde1, and Thomas Waitz2 — 1Institut für Materialphysik, Universität Münster, 48149 Münster — 2Physik nanostrukturierter Materialien, Fakultät für Physik, Universität Wien, 1090 Wien
Nanocrystalline NiTi shape memory alloys with a tailored grain size are of interest for applications. In this work, bulk nanostructured NiTi was processed by nanocrystallization of an intermediate amorphous phase made by repeated cold rolling (RCR). The structures and their phase stability were studied using transmission electron microscopy and calorimetry. RCR deformation causes grain refinement and amorphization; at high deformation degrees an almost completely amorphous phase is achieved. Upon heating nanocrystallization occurs. Combinations of the deformation degree and annealing condition allow to control the final grain size. Grains are stable up to ~370°C where grain growth occurs. The occurring martensitic phase transformation was analyzed using calorimetry, showing that the grain size strongly impacts the transformation path and temperatures. The transformation from the B2 austenite to the B19' martensite occurs via the intermediate R-phase. The effect of the grain size on the transformation temperature of the R-phase is small; this is in contrast to the transformation to the B19' martensite, which strongly depends on the grain size. With decreasing grain size, both the forward transformation (from B2 to B19') and the reverse transformation shift to lower temperatures.