Lattice orientation effect on the nanovoid growth in copper under shock loading

Abstract

Molecular-dynamics ͑MD͒ simulations have revealed that under shock loading a nanovoid in copper grows to be of ellipsoidal shape and different loading directions ͓͑100͔ and ͓111͔͒ change the orientation of its major axis. This anisotropic growth is caused by preferential shear dislocation loop emission from the equator of the void under ͓100͔ loading and preferential shear dislocation loop emission deviating away from the equator under ͓111͔ loading. A two-dimensional stress model has been proposed to explain the anisotropic plasticity. It is found that the loading direction changes the distribution of the resolved shear stress along the slip plane around the void and induces different dislocation emission mechanisms. DOI: 10.1103/PhysRevB.75.024104 PACS number͑s͒: 61.72.Qq, 62.20.Mk, 62.50.ϩp, 62.20.Fe Void nucleation, growth, and coalescence have long been realized to play a critical role in the initiation of dynamic failure in ductile metals.1,2 The nucleation can initiate from preexisting defects, such as grain boundaries, vacancies, voids, inclusions, etc. under shock loading. 3 Once voids have been formed, they will grow in size and interact with each other, leading to a dynamic fracture. Single void growth models based on continuum theory are extensively employed to investigate shock induced spallation phenomena.4-9 Although this continuum damage model ͑CDM͒ can reproduce the free-surface velocity of experiments well, it ignores the discrete nature of metals and the mechanism of plasticity. It is also questionable to apply this model to microscopic length and time scales that belong to the incipient stage of void growth. In the absence of experimental information, in recent years, atomistic simulations of nanovoid growth have been carried out to investigate the incipient stage of the void growth and the corresponding mechanisms, 10-13 even the coalescence process.14 But their loading conditions have been either quasistatic or strain-rate controlled, which does not take into account the inertial effect. 15,16 Furthermore, the lattice orientation with respect to loading direction has not yet been considered. Recent laser shock experiments illustrated that at high strain rate conditions vacancy diffusion mechanism cannot explain the void fraction in recovered samples, and both prismatic and shear dislocation loop emission mechanisms have been proposed to accommodate the void growth under shock loading.17 But there is still no direct observation of the dynamic process of void growth under shock loading. In polycrystalline metals, each grain has a unique loading direction. How the shock loading direction affects the void growth remains unresolved so far. The main purpose of this work is to study the mechanism of the incipient stage of nanovoid growth under shock loading and how the lattice orientation influences single void growth in monocrystal copper by means of molecular dynamics ͑MD͒ simulation.The interaction between atoms is described by an embedded atom model ͑EAM͒ potential parametrized by Mishin, ...

Growth and collapse of nanovoids in tantalum monocrystals

“…Dávila et al [21] carried out molecular dynamics (MD) simulations of void collapse in copper subjected to shock compression, and found that the emission of loops was responsible for the collapse of voids. Zhu et al [22] modeled void growth during release of shock loading in a face-centered cubic (fcc) metal (copper); shear dislocation loop emission from the equator of the void under [1 0 0] loading was observed. Detailed MD calculations by Bringa et al [23] confirmed that shear loop emission is the principal mechanism of void growth in copper; the effects of void size and load orientation were found to be significant, and one-, two-and three-dimensional loop arrangements were identified and analyzed.…”

Critical Damage Evolution model for spall failure of ductile metals

“…Many authors had explored that the micromechanics of void growth is the dislocation loop emission by externally applied stresses, which carry away the material from the void and are responsible for the plastic deformations needed to accommodate significant void growth, e.g., Rudd and Belak (2002), Lubarda et al (2004), Ahn et al (2006), Zhu et al (2007), Traiviratana et al (2008). Modeling of void growth has been studied extensively at the continuum level.…”

Molecular dynamics modeling and simulation of void growth in two dimensions

“…He concluded that the critical stress for nucleation increased for very small voids but did not analyze void growth. Other authors used 3D atomistic simulations to study the nucleation and growth of nm-sized voids [24][25][26][27][28][29][30][31][32] and Potirniche et al [26] reported that smaller voids grew faster at this length scale, a size effect opposed to that found for micrometer-sized voids. However, atomistic simulations of voids growth were limited to very small voids (void radius was below 10 nm) due to the huge computational costs associated with the simulation of larger voids and it is not clear whether the results reported by the analytical and atomistic simulations will hold for larger voids.…”