CAREER: Identifying the Micromechanisms Leading to Hydrogen-Induced Intergranular Fracture in Metals

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This Faculty Early Career Development (CAREER) Program project will identify the underlying deformation and failure mechanisms of nickel (Ni) and its alloys under coupled environmental and mechanical conditions. Nickel and nickel-based alloys are commonly used in many crucial service applications due to their high strength and fracture toughness. In many cases these materials are used in energy generating, conversion or storage systems. In such conditions a loss of toughness associated with exposure to hydrogen can occur. This award supports fundamental research on the effect of hydrogen on the deformation and fracture of metals, and will contribute to engineering practice via advances in the structural integrity of energy systems. The education and outreach tasks through this grant will contribute to efforts aiming to improve STEM achievement in Baltimore elementary public schools with a high minority student population. Practical engineering problems and solutions will be presented and discussed in the classroom with the goal to stimulate interest in engineering. Undergraduates from a local historically black college will obtain research internships allowing for active involvement in this CAREER research project. This will allow students to develop interest and foundations for careers in mechanics of materials. The primary research objectives of this CAREER project are to fundamentally identify the influence of H-diffusion on dislocation microstructure evolution, damage accumulation, and subsequent H-induced intergranular fracture of Ni crystals. We hypothesize that, unlike conventionally presumed, dislocation plasticity plays a major role in controlling material response and subsequent failure even in high-pressure H environments. We will perform unprecedented large scale 3D discrete dislocation dynamics (DDD) simulations coupled with finite element method to study dislocation evolution in H-charged single, bi, and poly-crystals. Details of the dislocation-H interactions, dislocation grain boundary interactions, and H pipe/bulk diffusion will be identified through molecular dynamics (MD) simulations, then hierarchically informed into DDD. In particular, this work will address two fundamental questions: (1) How does H influence dislocation multiplication/evolution? and (2) What is the role of H-diffusion on the evolution of the dislocation microstructure? The MD simulations will: (1) quantify H effects on the activation parameters of cross-slip; and (2) quantify H-diffusion coefficients and dislocation grain boundary interactions. Coupled H-diffusion/DDD simulations will be used to identify effects of H concentration and grain size on: (1) flow strength, and slip-morphology; and (2) dislocation evolution ahead of H-induced intergranular cracks. Simulations will be validated by comparisons with key experimental results from literature.