Berlin 2005 – wissenschaftliches Programm
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M: Metallphysik
M 13: Flüssige und amorphe Metalle IV
M 13.1: Vortrag
Freitag, 4. März 2005, 16:30–16:45, TU H111
Shear-band propagation in fully amorphous and partially crystallised Mg-based alloys studied by nanoindentation and TEM — •A. Castellero1,2, S.J. Lloyd1, Zs. Kovacs3, S.V. Madge1, M. Baricco4, J.F. Löffler2, and A.L. Greer1 — 1Dept. of Materials Science and Metallurgy, University of Cambridge, Cambridge, UK — 2Lab. of Metal Physics and Technology, ETH Zurich, Zurich, Switzerland — 3Dept. of General Physics, Eötvös Loránd University, Budapest, Hungary — 4Dip. di Chimica IFM, Università di Torino, Torino, Italy
Initiation and propagation of discrete shear bands in metallic glasses can be observed as constant-load steps in the loading curve of nanoindentation measurements. For Mg60Cu30Y10 bulk metallic glass such steps, that can be easily observed in the as-quenched sample, have been found even in partially crystallised samples with a very high density of crystals (30-80 nm in diameter). Since such a grain size is comparable with the width of shear bands (10-60 nm) the band-propagation cannot be inhibited. TEM dark-field images show a variation in contrast to the amorphous phase in the region beneath the indent, suggesting the presence of medium-range order induced by relaxation. In the case of Mg66Ni20Nd14 the steps disappear for a crystalline fraction of about 50 percent and a grain size of 200 nm. Corresponding to a low density region around the indent tip, the indent profile becomes steeper suggesting that the material cannot recover elastically after the deformation. We propose that crystals larger than the width of a shear band are able to stop the bands originating from the indent tip, leading to a high concentration of free volume that cannot relax.