Localized Shear Deformation of Amorphous Metals
- PDF / 3,095,617 Bytes
- 10 Pages / 417.6 x 639 pts Page_size
- 107 Downloads / 269 Views
B.H.
Kear, B.C. Giessen,
267
and M. Cohen, editors
LOCALIZED SHEAR DEFORMATION OF AMORPHOUS METALS J.C.M. LI, Rochester,
Department of Mechanical Engineering, University of Rochester, N.Y. 14627, U.S.A.
ABSTRACT The characteristics of localized deformation in amorphous metals are reviewed. All the available evidences seem to suggest that dislocations are responsible for shear localization. These dislocations seem to require a stress concentration for their nucleation. Once nucleated, they are capable of producing both forward and reverse shear in a narrow band, much like the dislocations in crystalline materials.
INTRODUCTION Condensed matter can be deformed irreversibly in many ways such as diffusional creep, viscous flow and localized shear. Among these only the viscous flow is truly homogeneous in the sense that every atom or molecule responds to the shear stress and participates in the deformation process. Diffusional creep is also homogeneous but only in the sense that all atoms have a probability of participation although at any one time only a fraction of atoms is diffusing. The diffusing atoms are not responding to the shear stresses. Rather they transport vacancies or excess volumes from sources to sinks which respond to local stresses. Unlike viscous flow which reacts instantly to the action of shear stresses, diffusional creep needs an incubation period in which to develop a concentration distribution of vacancies. Thus, in addition to the shear stress, the rate of diffusional creep depends also on the average distance between sources and sinks of vacancies. The localized shear is of course inhomogeneous; only the atoms or molecules in the localized shear zone take part in the deformation process. Even within such a zone the process is still inhomogeneous as exemplified by a dislocation mechanism or a twinning or phase transformation with a moving front. Among the three condensed states, crystals, amorphous materials, and liquids, the known phenomena are diffusional creep in crystals, homogeneous shear flow in liquids, and inhomogeneous shear in crystals and in amorphous materials. The situation can be summarized in the following table:
TABLE I Deformation of Condensed Matter
Crystals Diffusional Creep Homogeneous Shear Inhomogeneous Shear
Yes No Yes
Amorphous Metals ? ? Yes
Liquids ? Yes ?
While homogeneous shear flow of crystals is not known and it is unlikely to have inhomogeneous shear in liquids, the interesting questions are whether the amorphous metals may have homogeneous shear flow as in liquids and whether they may ave diffusional creep as in crystals. So far there is no evidence of either.
268 In a truly amorphous metal without recognizable microstructural features which can emit or absorb vacancies, partial vacancies or excess volumes, the sources or sinks for these volumetric defects must be everywhere or non-existent at all except at the external surfaces. In a liquid, it is likely that a vacancy can appear or disappear anywhere inside the liquid so that volumetric relaxation
Data Loading...
