Regensburg 2004 – wissenschaftliches Programm

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M: Metallphysik

M 18: Postersitzung

M 18.20: Poster

Dienstag, 9. März 2004, 14:30–16:30, Saal C

Microstructural investigations on single crystalline Ni-rich NiTi shape memory alloys — •Klaus Neuking, Jens Michutta, Alejandro Yawny, Christoph Somsen, and Gunther Eggeler — Ruhr-Universität Bochum, Institut für Werkstoffe, 44780 Bochum

In polycrystalline Ni-rich Ni-Ti shape memory alloys the precipitation of 4 families of Ni₄Ti₃ occurs heterogeneous on grain boundaries. As a result on cooling multiple-step phase transformations can be observed. To overcome this heterogeneous precipitation single crystalline Ni-rich (50.8 at.%) NiTi samples have been produced by using the Bridgman technique. Compression-aging treatments tests at temperatures of 500^∘C and 550^∘C were carried out in order to nucleate only one family of Ni₄Ti₃ precipitates under a compression stress of 50MPa in the [111]-direction. Microstructural investigations with transmission electron microscopy (TEM) reveal that the nucleation of the other three families is suppressed completely under compression aging. Differential scanning calorimetry (DSC) was used to determine the influence of different aging times on the martensitic transformation behaviour on cooling and on heating. For the aging temperature of 500^∘C only a two step transformation can be detected throughout the chosen time range between 2ks and 360ks. By increasing the temperature to 550^∘C a multiple-step transformation occurs at aging times of 8ks and above. At aging times below 8ks a two step transformation is present. The martensitic transformation behavior is analyzed by in-situ cooling TEM investigations.