Direct atomic-scale evidence for shear–dilatation correlation in metallic glasses

Abstract

Direct atomic-scale evidence is presented for the shear-dilatation correlation in metallic glasses via molecular dynamics and first-principles calculations. A quantitative parabolic relationship is established between the atomic local shear and hydrostatic volumetric strains by carrying out statistical analysis on a deformed glass model. The correction is further verified by density functional theory. Our atomistic demonstration of shear-dilatation correlation collaborates with the experimentally observed a few percent volume change in shear bands. It brings quantitative insights into the unique correlation between shear transformation and cavitation in metallic glasses.© 2015 Elsevier Ltd. All rights reserved.Metallic glasses (MGs) are a category of promising high strength structural materials with many other superior physical and chemical properties [1]. The deformation mechanisms of such amorphous solid state are in sharp contrast with their crystalline counterparts since there are no long-range atoms packing pattern in MGs [2,3]. No conventional mechanisms, such as ordinary dislocation plasticity or deformation twinning in crystals, exist in MGs which can carry plastic deformation. Therefore MGs usually suffer from the notorious catastrophic failure with a strong strain localization (shear banding) phenomenon [4,5].Several models have been proposed to rationalize the inelastic deformation of MGs, which include free volume [6], shear transformation zone (STZ) [7][8][9], cooperative shear model (CSM) [10,11], flow unit [12], vibrational soft spot [13], shear-diffusive transformation [14,15], and our recently proposed tensile transformation zone (TTZ) [16][17][18]. All of these models are based on the local structural rearrangement of atoms via the interplay of atomic-scale shear and dilation/contraction in MGs [19][20][21][22][23][24]. Among them, Argon's STZ model involves local rearrangement of a cluster of atoms undergoing a stress-driven, and thermally activated shear distortion [25,23,24]. Whereas Spaepen's free volume model is based on a dynamic equilibrium between the stress-assisted creation and annihilation of free volume [6]. The free volume behaviors can be regarded as dilation and contraction of local atomic environments. Since both models have been successfully adopted to describe the homogeneous and inhomogeneous flows in MGs, there should be some intrinsic correlation between shear and dilatation/compaction during deformation of glasses [21,26].Generally, shear-dilatation correlation is an intrinsic nature of deformation in amorphous alloys although local compaction is also allowed [27,28]. For example, intuitively derived law between shear strain and local dilatation has been used in constitutive modelings of MGs [29]. Shear is not necessarily the only deformation mode accommodating the local atomic rearrangement. The microscopic scenario is that the STZ operations redistribute stress spatially which usually leads to the creation of free volume via atomic-scale dilatation [21,1]. So that local ...

Effect of strain rate on yielding strength of a Zr-based bulk metallic glass

“…Under the quasi-static strain rates, the yielding strength is mainly dominated by structural STZ operations. Due to its inherent dilatational nature [55][56][57][58], the STZ occurs easier in tension than in compression, and thus the plastic yielding is achieved in the former with a relatively lower strength. With increasing strain rate into the dynamic range, an additional thermal softening should contribute to the plastic yielding that, although, is still governed by STZs.…”

Crack initiation in metallic glasses under nanoindentation

“…In our simulation, two methods with atomic-level accuracy based upon atomic dilatation [1,56,57] and local density change [58] have been used here to directly identify the event of crack initiation and the corresponding loading threshold (L c ). In our density analysis, the indentation samples were divided into small through-thickness volume elements (1.35 Â 2.7 Â 1.35 nm 3 ).…”

“…Fig. 6C plots the local shear strain and atomic dilation [1,59] evolution during the indentation at the circular region in Fig. 6B.…”

How the toughness in metallic glasses depends on topological and chemical heterogeneity

“…The authors found that cavitation plays an essential role in the initiation of fracture, where they found a crack to evolve by successive void nucleation events along an operating shear band. In the context of molecular dynamics (MD) simulations, we and others proposed that cavitation precedes the onset of crack formation at the core of an operating shear band (12)(13)(14)(15).…”

“…Furthermore, Falk et al showed that our revised CNT remains valid down to picosecond time scales, whereas strainaging effects become important at longer time scales in MGs (13). Murali et al found that fracture in a brittle MG is governed by nanoscale void nucleation and coalescence (14), whereas Wang et al established a correlation between shear transformations and dilatation (15). However, despite the success in numerical calculations, the atomistic origins of the cavitation process in MGs remains obscure, especially the role of topological structure and chemistry.…”