Void Strengthening and Growth in Structural Metals: A Mesoscale Perspective

The impact of voids on structural metal performance has been investigated in the context of two case studies: nanovoids as strengtheners in irradiated materials, and void growth size effects during ductile failure.Nanovoid Strengthening: When nanoscale in diameter, voids can act as obstacles to dislocation motion, resulting in strengthening reminiscent of precipitation hardening. This is especially relevant to irradiated metals in applications such as nuclear power plants. Local regions of very highly concentrated nanovoid defect clusters within irradiated metals are observed to cause material strengthening, meaning bulk yielding behavior is altered from what is expected in the original material design. However, it is not yet possible to reliably predict the consequence of nanovoids in metals with existing models, as they are not informed by the fundamental mechanisms behind this response. To address this, the meso-scale simulation technique Phase Field Dislocation Dynamics (PFDD) is applied to the problem in Chapter 2. This technique is an energetic framework within which rigorous discrete dislocation physics is applied, and which allows for larger simulation cell sizes than are commonly available to lower-length-scale methods.Surprisingly, the critical stress is found to scale linearly with the ratio of the intrinsic to unstable stacking fault energies, and to scale directly with the linear void fraction. This novel empirical strengthening model is named the Linear Fraction (LF) Model, and is thought to be driven by the dynamic constriction and extension of the stacking fault width as the two dissociated partials of an fcc dislocation bow around a nanovoid obtacle. This deviates from the expected line-tension-approximation mechanism due to bowing around an obstacle, which has until now been used to inform analytical predictive models. Remarkably, this new strengthening trend holds for the vast majority of fcc metals and void size arrays tested, and in Chapter 3 it