Regensburg 2010 – wissenschaftliches Programm
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MM: Fachverband Metall- und Materialphysik
MM 28: Nanostructured Materials I
MM 28.1: Vortrag
Mittwoch, 24. März 2010, 10:15–10:30, H16
Microstructure and thermodynamics of nanocrystalline NiTi alloy processed by high pressure torsion — •reeti singh1, harald rösner1, ruslan z. valiev2, sergiy divinski1, and gerhard wilde1 — 1Institut für Materialphysik, Westfälische Wihelms-Universität Münster, Wilhelm-Klemm-Str. 10, 48149, Münster, Germany — 2Institute of Physics of Advanced Materials, Ufa State Aviation University, 12 K. Marx Street, 450000 Ufa, Russian Federation
An equiatomic nano-NiTi alloy, deformed by high-pressure torsion (HPT) was investigated. By HPT, almost complete amorphization is obtained in bulk NiTi containing B2 austenite. Crystallization and structural changes during annealing were investigated by differential scanning calorimetry (DSC), X-ray diffraction analysis and transmission electron microscopy (TEM). The DSC signals observed during continuous heating experiments from 323 K to 723 K indicate an unusually large separation between the nucleation and the growth stages. A large variation in the grain size is observed after annealing at 723 K. This behavior is discussed with respect to the nanoscale microstructural heterogeneity after initial deformation processing. The activation energy for grain growth was determined to be 289 kJ/mole. Additional TEM studies reveal amorphous bands sandwiched between crystalline parts after heating at lower temperatures below 623 K, where the atomic mobility is sufficiently low to prevent long-range diffusion. The heat release during annealing in the temperature range from 453 K to 623 K shows an unexpected decrease at 423 K, possibly due to a reverse amorphization, takes place at lower temperatures.