UC Santa Barbara

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 is discovered to fit compellingly with a number of MD studies as well as for nano-precipitates in PFDD. The boundaries of when the LF model will apply to nano-obstacle strengthening has only briefly been investigated, and the extent to which PFDD results will agree with other atomistic techniques such as MD is still an open question. With this in mind, a number of future work directions are discussed in Chapter 5, Sections 5.1 and 5.2.

Meso-Scale Void Growth: Voids are present in more than just irradiated structural metals, and evidence exists that nanovoids can also be found in large quantities during plastic deformation. While some of these may strengthen at the nanoscale, some may also grow during plastic deformation. It is still an open question which voids will grow and which will not, but growth into the hundreds of nanometers and beyond increase the likelihood that the presence of these voids will degrade the structural performance of the metal and possibly lead to part failure. Given the severity of the consequences, an understanding of this void growth behavior is of significant interest. A novel in-situ testing method is discussed next in Chapter 4, which was designed to target a specific gap in experimental void growth evidence at these lower, transitional length scales.

The design of this experiment was meant to be as versatile as possible while also providing as much control as possible over material, void size, void-void interactions, and void/grain size interactions. Two primary goals of this approach were to 1) directly observe the evolution of voids at length scales below what has been achieved in literature, and 2) to fabricate these intentional void defects within tensile specimen without the use of FIB which is known to damage film surfaces. For these goals moderate success was achieved, and this technique was deployed for the problem of void growth size effects at sub-micron void sizes. Existing literature on fundamental size effects are limited to computational theories for very simplified model systems. When applied to the two defected specimen for this work, the fundamental size effect theories proved elusive because of the dominance of a third size effect consideration: the ratio of the void size to the thin film grain size. As such, promising future work avenues to push the boundaries of what is currently known experimentally for void growth are provided in Chapter 5, Section 5.3.