Theory of Large-Scale Plastic Deformation in Amorphous Materials: A Progress Report
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1048-Z01-06
Theory of Large-Scale Plastic Deformation in Amorphous Materials: A Progress Report James S. Langer Physics, University of California, Broida Hall, Santa Barbara, CA, 93106 ABSTRACT The goal of recent shear-transformation-zone (STZ) theories has been to construct a phenomenological description of amorphous plasticity that is based on physical principles and molecular models, and yet is simple enough to be useful in predicting the performance of real materials. In reporting progress toward this goal, I focus on the dynamic role played by the effective disorder temperature (a generalization of the free-volume) in controlling relaxation rates and predicting shear-banding instabilities. ELEMENTS OF AN STZ THEORY For over a decade, my coworkers and I have been developing a shear-transformationzone (STZ) theory of plastic deformation in noncrystalline solids. [1-5] Our goal has been to construct a phenomenological description of amorphous plasticity that is based on physical principles and molecular models, and yet is simple enough to be useful for predicting the performance of real materials. At the molecular level, amorphous solids are structurally no more complicated than fluids. They do, of course, exhibit highly non-fluidlike properties such as rigidity, jamming, and the like. Nevertheless, their underlying simplicity implies that their behaviors may exhibit some degree of universality. My purpose in this talk is to report briefly on progress in moving the STZ theory toward such a description. In particular, I argue that it is possible to go remarkably far using just symmetry, conservation laws, and thermodynamics. From its inception, the STZ theory was intended to be an extension of the flow-defect picture of Turnbull, Cohen, Spaepen, Argon and others. [6-10] A deforming amorphous solid is a persistently noisy environment within which rare, localized fluctuations of the molecular configurations undergo irreversible rearrangements in response to applied stresses. The STZ’s are, in effect, the transition states that enable these rearrangements. When an STZ appears, if it is properly oriented with respect to the stress, it rapidly undergoes a shear transformation. Otherwise, it rapidly disappears, and no irreversible deformation takes place. An especially important feature of the STZ’s is that they carry memory. For some time after an STZ undergoes a transition, before it disappears, it causes the system to resist further deformation in the original direction. That time is short in a steadily deforming system; but it may be long if the system is jammed, i.e. if the stress is below the yield stress, in which case the system exhibits Bauschinger effects and the like.
A key dynamical variable in recent versions of the STZ theory is an effective disorder temperature. The relation between the population of STZ's and an intensive variable such as the effective temperature has a long history. Earlier investigators, notably Cohen and Turnbull [6] and Spaepen [8], described the intrinsically disordered state of n
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