Nanovoid Cavitation by Dislocation Emission in Aluminum

Abstract

This Letter is concerned with the determination of the transition paths attendant to nanovoid growth in aluminum under hydrostatic tension. The analysis is, therefore, based on energy minimization at 0 K. Aluminum is modeled by the Ercolessi-Adams embedded-atom method, and spurious boundary artifacts are mitigated by the use of the quasicontinuum method. Our analysis reveals several stages of pressure buildup separated by yield points. The first yield point corresponds to the formation of highly stable tetrahedral dislocation junctions around the surfaces of the void. The second yield point is caused by the dissolution of the tetrahedral structures and the emission of conventional 1 2 h110if111g and anomalous 1 2 h110if001g dislocation loops. DOI: 10.1103/PhysRevLett.93.165503 PACS numbers: 61.72.-y, 02.70.-c, 46.15.-x Spallation damage under dynamic tensile loading is characterized by the emergence of distributed microcracks or voids in a narrow region of the material, or spall plane, which may result in the catastrophic failure of the specimen. Spall has been widely studied experimentally by means of gas-gun driven plate-impact tests (e.g., [1,2]), high-intensity, pulsed laser shock generators [3][4][5], and other experimental techniques (cf.[6] and references therein). Observations, however, are for the most part restricted to free-surface velocity measurements and post mortem examination of the specimens and, therefore, are necessarily indirect.Molecular dynamics (MD) suggests itself as a natural approach for understanding the intrinsic mechanisms underlying the evolution of nanosized voids [7][8][9]. These studies reveal, among other useful insights, that nanovoids exceeding a pressure and temperaturedependent critical size grow by the emission of dislocations and by coalescence with neighboring voids. However, MD is particularly well suited to the study of void growth at very high strain rates, often greatly in excess of those attained experimentally. In addition, reliance on periodic boundary conditions limits the range of elapsed times which can be examined by MD and may introduce undesirable artifacts. Continuum estimates [10] suggest that dynamic effects are indeed negligible for small voids at moderate-to-high strain rates. This points to the need to complement MD studies with a detailed analysis of the equilibrium energy landscape and transition paths accessible to expanding nanovoids.The technique that we use in order to carry out such an analysis is the quasicontinuum (QC) method. QC is a method for systematically coarse-graining lattice statics models. The method starts with the complete atomistic system and appends kinematic constraints which restrict the configuration space of the crystal. The kinematic constraints are based on the selection of representative atoms and the use of finite-element interpolation. In order to avoid full lattice sums, cluster summation rules are also used. By virtue of these rules, only atoms in small clusters surrounding the representative atoms need to be visite...

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

“…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][11][12][13] even the coalescence process.…”

Atomistic mechanism of shock-induced void collapse in nanoporous metals

“…On the other hand, thanks to the continuous increase in computer power, molecular-dynamics (MD) simulations have become a powerful tool in the study of the high-strain rate response of materials 10 . Due to their importance in the mechanical failure of ductile materials under tensile loading as well as under spall conditions, nucleation and growth of voids under external tension have been studied quite extensively using MD simulations [11][12][13] . In contrast, so far only relatively few MD studies have dealt with the behavior of voids under compression [14][15][16][17] , most of which have focused on hot-spot initiation above the melting pressure.…”

Nanovoid nucleation by vacancy aggregation and vacancy-cluster coarsening in high-purity metallic single crystals

“…In metals at high tensile pressures, the critical void size for plastic cavitation may be in the nanoscale, in which case plastic cavitation occurs by the emission of discrete dislocation loops. [21][22][23][24] Void nucleation in metals under shock loading has been extensively studied by means of molecular dynamics. [25][26][27][28][29][30] While these studies are remarkable for the fidelity and insights that they afford, including the role of inclusions, second-phase particles, dislocations, grain boundaries, and other microstructural features, they often fail to supply an analytical description of nucleation rates that can be effectively integrated into a larger multiscale framework.…”

“…Finally, we resort to simple continuum estimates of cavitation as a function of pressure and temperature, calibrated to quasicontinuum calculations. 21 Based on these estimates, together with the LKMC results, we proceed to calculate the nanovoid nucleation times as a function of temperature and volumetric strain. The results of the analysis are discussed in detail in Sec.…”