What Can Plasticity of Amorphous Silicon Tell Us about Plasticity of Metallic Glasses?
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IT is well known that plasticity in crystalline solids is mediated by dislocations. Their motion and mechanisms of overcoming slip plane obstacles are now part of the classical literature.[1,2] Generalized, multidimensional forms of the mechanics of crystal plasticity in single crystals as well as in polycrystals have also been discussed in numerous publications.[3] While plasticity in amorphous solids such as metallic and polymeric glasses has received much attention,[4–7] many of its aspects are still not entirely clear. On the other hand, the mechanisms of plasticity in covalent glasses have received only scant attention until recently. An exception has been the computer simulations of plastic response of amorphous silicon (a-Si) by Demkowicz and Argon,[8–11] which have gone a long way in clarifying the mechanisms of plastic flow in covalent glasses, in general, and have also led to considerable insight into the corresponding processes in metallic glasses. These simulations are the subject of the present article. Interest in metallic glasses and their plastic response goes back to Duwez and his contemporaries.[12] Early attempts at invoking parallels to crystal plasticity through the motion of generalized dislocations to explain plasticity in metallic glasses have proved to be sterile.[13–16] The first insight into the mechanism of plastic flow in metallic glasses came from experiments A.S. ARGON, Quentin Berg Professor, Emeritus, is with the Massachusetts Institute of Technology, Cambridge, MA 02139. Contact e-mail: [email protected] M.J. DEMKOWICZ, Director’s Post Doctoral Fellow, is with the Los Alamos National Laboratory, Los Alamos, NM 87545. This article is based on a presentation given in the symposium entitled ‘‘Bulk Metallic Glasses IV,’’ which occurred February 25– March 1, 2007 during the TMS Annual Meeting in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee. Article published online January 25, 2008 1762—VOLUME 39A, AUGUST 2008
with amorphous variants of the classical Bragg bubble model,[15,16] which resulted in the introduction of the concept of shear transformations (STs) as the fundamental mechanism of plasticity in metallic glasses.[4,5,17] Many computer simulations of plasticity in metallic glasses have further verified the generality of this concept for glassy systems.[18–21] The concept of an ST in the plasticity of amorphous solids has proved to have wide-ranging applicability not only to metallic glasses, but also to glassy polymers.[22–24] In more recent times, with the introduction of more stable and crystallization-resistant bulk metallic glasses (BMGs), the interest in STs as plasticity carriers has substantially increased.[25,26] In the early work of Falk and Langer[25] and in their later publications, the terminology of shear transformation zones (STZs) has appeared. While this terminology has now taken hold, we will avoid it here because it implies elongated entities linking them to shear bands that are forms of localization that require separate co
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