Fadi F. Abdeljawad

Ductility of bulk metallic glass composites: Microstructural effects

Recent experimental findings suggest that the problem of catastrophic failure by shear band propagation in monolithic bulk metallic glasses (BMGs) can be mitigated by forming two-phase composites consisting of a glassy metal matrix phase and a soft crystalline reinforcement phase. Here, we employ a recently introduced phase-field model in two spatial dimensions, capable of capturing shear banding in BMG systems, to address the effects of microstructure on the mechanical properties of BMG composites. We identify an important geometric length scale associated with the dendritic particles and demonstrate that it controls the overall ductility and ultimate strength of such BMG composites.

Unraveling irradiation induced grain growth with in situ transmission electron microscopy and coordinated modeling

Nanostructuring has been proposed as a method to enhance radiation tolerance, but many metallic systems are rejected due to significant concerns regarding long term grain boundary and interface stability. This work utilized recent advancements in transmission electron microscopy (TEM) to quantitatively characterize the grain size, texture, and individual grain boundary character in a nanocrystalline gold model system before and after in situ TEM ion irradiation with 10 MeV Si. The initial experimental measurements were fed into a mesoscale phase field model, which incorporates the role of irradiation-induced thermal events on boundary properties, to directly compare the observed and simulated grain growth with varied parameters. The observed microstructure evolution deviated subtly from previously reported normal grain growth in which some boundaries remained essentially static. In broader terms, the combined experimental and modeling techniques presented herein provide future avenues to enhance quantification and prediction of the thermal, mechanical, or radiation stability of grain boundaries in nanostructured crystalline systems.

Redox instability, mechanical deformation, and heterogeneous damage accumulation in solid oxide fuel cell anodes

Mechanical integrity and damage tolerance represent two key challenges in the design of solid oxide fuel cells (SOFCs). In particular, reduction and oxidation (redox) cycles, and the associated large transformation strains have a notable impact on the mechanical stability and failure mode of SOFC anodes. In this study, the deformation behavior under redox cycling is investigated computationally with an approach that provides a detailed, microstructurally based view of heterogeneous damage accumulation behavior within an experimentally obtained nickel/yttria stabilized zirconia SOFC anode microstructure. Simulation results underscore the critical role that the microstructure plays in the mechanical deformation behavior of and failure within such materials.

A diffuse interface model of grain boundary faceting

Interfaces, free or internal, greatly influence the physical properties and stability of materials microstructures. Of particular interest are the processes that occur due to anisotropic interfacial properties. In the case of grain boundaries (GBs) in metals, several experimental observations revealed that an initially flat GB may facet into hill-and-valley structures with well defined planes and corners/edges connecting them. Herein, we present a diffuse interface model that is capable of accounting for strongly anisotropic GB properties and capturing the formation of hill-and-valley morphologies. The hallmark of our approach is the ability to independently examine the various factors affecting GB faceting and subsequent facet coarsening. More specifically, our formulation incorporates higher order expansions to account for the excess energy due to facet junctions and their non-local interactions. As a demonstration of the modeling capability, we consider the Σ5 〈001〉 tilt GB in body-centered-cubic iron,...

Microstructural coarsening effects on redox instability and mechanical damage in solid oxide fuel cell anodes

In state-of-the-art high temperature solid oxide fuel cells (SOFCs), a porous composite of nickel and yttria stabilized zirconia (Ni/YSZ) is employed as the anode. The rapid oxidation of Ni into NiO is regarded as the main cause of the so-called reduction-oxidation (redox) instability in Ni/YSZ anodes, due to the presence of extensive bulk volume changes associated with this reaction. As a consequence, the development of internal stresses can lead to performance degradation and/or structural failure. In this study, we employ a recently developed continuum formalism to quantify the mechanical deformation behavior and evolution of internal stresses in Ni/YSZ porous anodes due to re-oxidation. In our approach, a local failure criterion is coupled to the continuum framework in order to account for the heterogeneous damage accumulation in the YSZ phase. The hallmark of our approach is the ability to track the spatial evolution of mechanical damage and capture the interaction of YSZ damaged regions with the local microstructure. Simulation results highlight the importance of the microstructure characterized by Ni to YSZ particle size ratio on the redox behavior and damage accumulation in as-synthesized SOFC anode systems. Moreover, a redox-strain-to-failure criterion is developed to quantify the degree by which coarsened anode microstructures become more susceptible to mechanical damage during re-oxidation.

Reduced-Order Microstructure-Sensitive Models for Damage Initiation in Two-Phase Composites

Local features of the internal structure or the microstructure dominate the overall performance of materials. An open problem in materials design with enhanced properties is to accurately identify and quantify salient features of the microstructure and understand its correlation with the material’s performance. This task is exacerbated when dealing with failure related properties that show strong correlations to higher-order details of the material microstructure. This paper presents a novel data-driven framework for quantitatively determining the highly complex correlations that exist between the higher-order details of the material microstructure and its failure-related properties, specifically its damage initiation properties. The enclosed work will address this challenge by significantly extending the Materials Knowledge Systems (MKS) framework and by leveraging concepts in extreme value distributions and machine learning. The developed framework was capable of successfully sorting nine different classes of synthetically generated two-phase microstructures for their sensitivity to damage initiation. The framework and approaches presented here open new research avenues for studying the microstructure-sensitive damage initiation properties associated with heterogeneous materials, and pave the way forward for practical multiscale materials design.