Plastic deformation of metallic glasses: Size of shear transformation zones from molecular dynamics simulations

Abstract

Plastic deformation in metallic glasses well below their glass transition temperatures T g occurs spatially heterogeneously within highly localized regions, termed shear transformation zones ͑STZs͒. Yet, their size and the number of atoms involved in a local shear event, remains greatly unclear. With the help of classical molecular dynamics ͑MD͒ computer simulations on plastic deformation of the model glass CuTi during pure shearing, we address this issue by evaluating correlations in atomic-scale plastic displacements, viz. the displacement correlation function. From the correlation length, a universal diameter of about 15 Å, or, equivalently, approximately 120 atoms is derived for a variety of conditions, such as variable strains, strain rates, temperatures, and boundary conditions. The physics of plastic deformation and flow in glassesone of the oldest engineering materials of mankind-is still poorly understood when compared to their crystalline counterparts, and thus has attracted increasing experimental and computational interests during the past decades. [1][2][3][4][5] In particular the highly heterogeneous, cooperative dynamics on multiple time and length scales, 6 and its influence on macroscopic dimensions is still greatly unclear, presumably due to the lack of an atomic-scale picture. This includes the impact on important material properties, such as elastic modulus, fracture behavior, and plastic flow. Adam and Gibbs 7 were the first to invoke the concept of a cooperative rearranging region ͑CRR͒ back in 1965 by proposing clusters that reorganize during shearing. As a main weakness of their treatment, however, they fail to predict a specific cluster size. Subsequent ideas developed by Cohen et al., 8 Spaepen,9 Argon, 10,11 and others describe flow and creep in metallic glasses as activated transformations in intrinsic dynamic heterogeneities, termed shear transformation zones ͑STZs͒. 12These models characterize plastic flow in metallic glasses during shearing as a local event appearing as a spontaneous and cooperative reorganization of individual clusters ͑STZs͒.13-16 As a major problem, however, the size of STZs still remains unclear, in particular for realistic material systems. A first hint to remedy this deficiency was provided by Debenedetti and Stillinger 17 who introduced a potential energy landscape model for the amorphous state and yielding in glasses. Johnson and Samwer 18 extended their treatment and proposed a size of STZs of the order of 100 atoms from energetic considerations.In the present investigations we employ molecular dynamics ͑MD͒ computer simulations to corroborate these predictions and link atomic-scale kinetics and mechanical properties across these length scales by determining the size of STZs as a function of macroscopic quantities. This is accomplished by investigating shear events below T g for the model glass CuTi. Evaluations of the plastic contribution to atomicscale displacements facilitate to identify cooperative motions, as expected in dynamical heterogeneities or ...

Nanoindentation investigation on the creep mechanism in metallic glassy films

“…Computed results of V * and Ω using the statistical method are summarized in Table1. The STZ volumes of the both samples are consistent with those reported by experimental calculation and simulation prediction [13,29,30]. 60% enlargement on the STZ volume, from 0.552 nm 3 to 0.884 nm 3 with 1.5% Ti addition, could explain the significant improvement of ductility in the bulk one in turn [7,12].…”

Experimental characterization of shear transformation zones for plastic flow of bulk metallic glasses

“…In fact, because the nanoindentation tests were performed at ambient temperature far below the glass transition points of the model BMGs, activations of individual STZs in an embryonic shear band are considered to be independent of each other under applied stresses. Interactions between these activation units of plastic flow in the BMGs are thus unlikely to alter the mechanism of plastic flow in the consecutive deformation of metallic glasses (12,15,19,30), implying the physical feasibility of the experimental evaluation of STZs using the experimental methodology proposed in this study.…”

“…Historically, several rheological theories have been developed to describe the heterogeneous plasticity of glasses. These models are mainly based on two possible atomic-scale mechanisms, i.e., deformationinduced dilatation or free volume (8)(9)(10) and local events of cooperative shearing of atomic clusters termed shear transformation zones (STZs) (11)(12)(13)(14)(15)(16). Recently, a cooperative shearing model (CSM) of STZs by Johnson and Samwer (17), together with work of Falk and Langer (13,14) and others (18,19), has been shown to provide an effective interpretation of plasticity in metallic glasses well below their glass temperatures.…”

Crossover from random three-dimensional avalanches to correlated nano shear bands in metallic glasses

“…In the picture of microscopic plastic events, like 'shear transformation zones' (STZs) as introduced by Argon 15 , stress relaxation is localized in ensembles of about 100 atoms 16,17 . Although, in mean field descriptions of viscoplasticity 18 , STZs are often regarded to act independently, it is known that they indeed carry a long-range elastic stress field-like an Eshelby inclusion 19 causing cooperative effects among single plastic events 20,21 .…”