Cavitation in Amorphous Solids

Abstract

Molecular dynamics simulations of cavitation in a Zr 50 Cu 50 metallic glass exhibit a waiting time dependent cavitation rate. On short time scales nucleation rates and critical cavity sizes are commensurate with a classical theory of nucleation that accounts for both the plastic dissipation during cavitation and the cavity size dependence of the surface energy. All but one parameter, the Tolman length, can be extracted directly from independent calculations or estimated from physical principles. On longer time scales strain aging in the form of shear relaxations results in a systematic decrease of cavitation rate. The high cavitation rates that arise due to the suppression of the surface energy in small cavities provide a possible explanation for the quasibrittle fracture observed in metallic glasses. DOI: 10.1103/PhysRevLett.110.185502 PACS numbers: 63.50.Lm, 62.20.mm, 64.70.pe Amorphous materials, commonly termed glasses when quenched from the melt, occur in every class of material including ceramics, metals, and polymers. While the shear response of amorphous solids has received a significant amount of attention in the theoretical physics and molecular simulation literature over the past decade [1-8], significantly less attention has been devoted to hydrostatic loading in such systems [9,10]. This omission appears significant since experimental studies in metallic glass (MG) and other amorphous solids reveal nanocavities [11,12] that form during or subsequent to deformation and strongly implicate cavitation in the physics of the fracture process zone, even when the fracture behavior is relatively brittle [13][14][15]. The importance of cavitation in fracture is supported by recent molecular dynamics (MD) simulations in glassy Cu 50 Zr 50 and Fe 80 P 20 [16].Theory and simulation studies have been applied to understand this process in liquids more commonly than in glasses. Two recent studies [17,18] simulated the process of homogeneous nucleation in liquids in comparison with experimental data. These studies observed evidence of the curvature dependence of the surface energy on the measured cavitation rate, an effect that has itself been the subject of a significant amount of study [19][20][21][22]. One particularly notable contribution from simulation was the mapping out of the point at which the gas-liquid spinodal dips below the glass line and the glass must become unstable to cavitation [23].Continuum mechanics approaches to modeling the kinetics of cavitation have been developed over many years by numerous researchers [6,[24][25][26][27][28][29][30]. Often preexisting cavities are assumed to exist due to voids, inclusions, or intersections of grains, and the effect of surface energy is neglected. Various assumptions have been considered regarding the plastic constitutive behavior around the cavity [6,[24][25][26][27][28][29][30]. In these cases, because plastic and elastic energy both scale with the volume of the void, above a critical stress cavity growth becomes unbounded. Here, as in some earlier ...

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

“…The calculated dilatation is consistent with experiments where a few percent volume change is associated with shear band initiation [30,45,28]. Note that the shear-induced dilatation could be probably enhanced in brittle glasses, where the dilation can be as much as 10% which may eventually lead to fracture via cavitation (or TTZ) mechanisms [35,31]. For a detailed examination, the average atomic volumetric strain is plotted against average atomic local shear strain in Fig.…”

“…For the present 'ductile' CuZr glass with smaller G/B ratio, the correlation is relatively weaker which indicates that shear could be the dominating deformation mode [35]. While it is most likely to find brittle behaviors in stronger shear-dilatation correlation glass (with bigger G/B ratio) since the great propensity to nucleate cavitation (or TTZ) via shear-induced dilatation, which has been proposed as a brittle fracture mechanism in MGs [33,35,31,18,17,34]. as demonstrated by Fig.…”

“…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 structural dilatation [19,30,31], density change [27,28], and even nanovoids [32,33,21,34] have been observed within the shear bands of MGs. Although the concept of shear-dilatation correlation is widely accepted in the glass community, and various experiments and simulations have indicated STZ and cavitation as important deformation and fracture mechanisms of glassy alloys [33,35,31,18,17,34], there still lacks a direct microscopic evidence and no quantitative relationship established for such an intimate correlation.…”

Cryogenic-temperature-induced transition from shear to dilatational failure in metallic glasses

“…This results in the emergence of 10-nm-scaled shear bands, which macroscopically leads to shear-dominated failure [14,15]. However, recent experiments [14,[16][17][18] and simulations [1,3] have revealed that the dilatation itself, whether induced by shear or hydrostatic tension, can dominate the brittle failure of metallic glasses. In this case, the crack tip propagates via cavitation events that involve a series of nanoscale void nucleation and coalescence processes with very limited plastic growth.…”

“…Mechanical failure of metallic glasses continues to fascinate researchers [1][2][3][4][5], since dislocations, grain boundaries, crystallographic planes, etc., are not defined in this class of non-crystalline materials [6][7][8][9]. Instead, the shear transformation zone (STZ), i.e.…”