Micromechanics of Void Nucleation and Early Growth at Incoherent Precipitates: Lattice-Trapped and Dislocation-Mediated Delamination Modes

Abstract

The initial stages of debonding at hard-particle interfaces during rupture is relevant to the fracture of most structural alloys, yet details of the mechanistic process for rupture at the atomic scale are poorly understood. In this study, we employ molecular dynamics simulation of a spherical Al2Cu θ precipitate in an aluminum matrix to examine the earliest stages of void formation and nanocrack growth at the particle-matrix interface, at temperatures ranging from 200–400 K and stresses ranging from 5.7–7.2 GPa. The simulations revealed a three-stage process involving (1) stochastic instantaneous or delayed nucleation of excess free volume at the particle-matrix interface involving only tens of atoms, followed by (2) steady time-dependent crack growth in the absence of dislocation activity, followed by (3) dramatically accelerated crack growth facilitated by crack-tip dislocation emission. While not all three stages were present for all stresses and temperatures, the second stage, termed lattice-trapped delamination, was consistently the rate-limiting process. This lattice-trapped delamination process was determined to be a thermally activated brittle fracture mode with an unambiguous Arrhenius activation energy of 1.37 eV and an activation area of 1.17 Å2. The role of lattice-trapped delamination in the early stages of particle delamination is not only relevant at the high strain-rates and stresses associated with shock spallation, but Arrhenius extrapolation suggests that the mechanism also operates during quasi-static rupture at micrometer-scale particles.

Voids Development in Metals: Numerical Modelling

“…The atomistic simulation approach is particularly useful in the analysis of the initial phases of void nucleation and the assessment of the accompanying physical phenomena, the experimental capture of which, due to technical difficulties, is troublesome or sometimes even impossible. For instance, Zhao et al [ 52 ], using molecular dynamics (MD) analysis, simulated the separation of a spherical Al 2 Cu θ precipitate in an aluminium matrix. Reaching the atomic level made it possible to distinguish three stages of the early phase of decohesion, namely: a nucleus of excess free volume at the phases’ interface, nuclei growth in the absence of dislocation activity and the emission of Shockley partial dislocations leading to fast microcrack development.…”

Void Nucleation and Growth from Heterophases and the Exploitation of New Toughening Mechanisms in Metals

“…In contrast, the void nucleated within the Mg precipitate in the Al-Mg system and propagated into the Al matrix. Recently, Zhao et al [120] suggested that the initial delamination took place with an increase in the displacement of atoms, without the formation of local dislocation structures. This stage, termed as the lattice-trapped delamination process, was followed by the dislocation-assisted growth of the nucleated void.…”

“…This implies that nucleation is favoured at a higher strain rate, attributing the strain rate sensitivity to the formation of voids. Zhao et al [120] also investigated the effect of temperature and stress on the void formation in FCC Al and demonstrated that the lattice-trapped delamination of the void nucleation was the rate-limiting process.…”