Regensburg 2013 – scientific programme
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MM: Fachverband Metall- und Materialphysik
MM 32: Topical Session: TEM-Symposium - Structure-Property / In-Situ I
MM 32.3: Talk
Wednesday, March 13, 2013, 11:00–11:15, H25
Combined use of ex-situ and in-situ TEM for the analysis of the nanocrystallization of bulk amorphous NiTi — •Martin Peterlechner1, Thomas Waitz2, and Gerhard Wilde1 — 1Institute of Materials Physics, WWU Münster, Germany — 2Physics of Nanostructured Materials, University of Vienna, Austria
A NiTi shape memory alloy was subjected to severe plastic deformation by repeated cold rolling (RCR). RCR yields almost complete amorphization. A small volume fraction of nanocrystalline debris is embedded heterogeneously in the amorphous matrix. Upon annealing nanocrystallization occurs. The crystallization kinetics were analyzed using ex-situ and in-situ transmission electron microscopy (TEM) experiments. Ex-situ TEM investigations were carried out of bulk samples crystallized at 307°C (270 min) in a calorimeter that allows precise control of the temperature. Ex-situ TEM specimens were analyzed at room temperature. Additional, in-situ TEM heating was carried out at a nominal temperature of 307°C. From in-situ TEM it can be concluded that hard-impingement is present. Therefore, the grain sizes formed by ex-situ nanocrystallization can be used to analyze the kinetics based on hard impingement models. The crystallization causes grains with a bimodal size distribution. This is explained considering the heterogeneous distribution of nanocrystals left in the amorphous matrix that can act as nucleation sites of new grains. Since the number density of the nanocrystalline debris depends on the degree of deformation, both the degree of the deformation and the crystallization temperature can be used to tailor the grain size.