Evolution of the Free Volume during Homogeneous Flow of a Metallic Glass
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Evolution of the Free Volume during Homogeneous Flow of a Metallic Glass
M. Heggen1, F. Spaepen2, M. Feuerbacher1 1 Institut für Festkörperforschung, Forschungszentrum Jülich, D-52425 Jülich, Germany 2 Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138 USA ABSTRACT Glassy Pd41Ni10Cu29P20 was creep-tested in compression at constant true stress. The defect concentrations were shown to be reversible with reproducible kinetics. The results could be fit better with a defect creation rate that is proportional to the applied power density than with one that depends on the strain rate only. The dilatation mechanism for creation of free volume is inefficient in energy and generation of free volume. INTRODUCTION At low temperature or high stress, plastic deformation of a metallic glass is inhomogeneous: it is concentrated in very thin shear bands, the number and geometry of which depends on the constraints of the deformation. At higher temperatures and lower stresses, the deformation is homogeneous: each volume element undergoes the same strain. The latter mode includes the flow of the liquid. Inhomogeneous flow is an instability caused by progressive softening brought about by deformation-induced disordering. The atomistic mechanism of the disordering process is still under investigation. Density measurements [1], positron annihilation [2, 3] and electron microscopy [4, 5] show that the formation of shear bands is accompanied by dilation. That shear of random close-packed particles causes dilatation has been known for years in soil mechanics [6, 7]. This dilatation is linked naturally to enhanced plastic flow through the free volume theory [8, 9]. The precise mechanism of free volume generation, however, is still not known. For example, the rate of free volume (or flow defect) generation has been proposed to depend on stress [8, 10], strain rate [11-14] or their product (applied power density) [10]. One expects deformation-induced disordering to occur during homogeneous flow as well. At sufficiently high stress, even though the deformation remains spatially uniform, there is evidence of softening: a maximum in the stress-strain curve during tensile testing [11, 14, 15] or a lowering of the viscosity in creep experiments [12]. The advantage of studying the disordering process in the homogeneous regime is that the process is sufficiently slow to allow detailed studies of its dependence on stress, strain rate and temperature. The disadvantage is that the concurrent process of re-ordering must be taken into account as well. The ordering process is one of densification, and is known as structural relaxation as a result of (stress-free) annealing. The kinetics of ordering are
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known from creep experiments at low stress [16] to be bimolecular: annihilation of free volume occurs by collapse of two adjacent free volume fluctuations [17, 18]. ATOMISTIC MECHANISMS Plastic deformation of metallic glasses is modeled by the superposition of the shear of localized groups of 10-100 atom
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