Atomic Insights into Hydrogen-Grain Boundary Interactions

Abstract

Derived from the most abundant element in the universe, hydrogen is the smallest element and, at the same time, a clean, mobile, and efficient energy carrier. Nowadays, hydrogen is regarded as the fuel of future and could help the world transform into a zero-emissions scenario. However, the popularity of hydrogen-based energy economy has also put the problems associated with hydrogen storage and transport to the forefront. Hydrogen embrittlement (HE), the phenomenon where dissolved hydrogen in metals causes dramatic degradation of mechanical properties leading to sudden and catastrophic failure, was first observed in 1875. However, even after one-century research, the fundamental mechanisms of HE are still in the dark forest mainly due to the lacking of effective methodology for tracking hydrogen experimentally. In this thesis, the HE phenomenon is studied in a substituted way, by using atomistic simulations, to get a comprehensive understanding of the nanoscale mechanism.

Essentially, HE is all things about the interactions between hydrogen atoms and multiple types of microstructures in material, including vacancies, dislocations, grain boundaries (GBs) and crack tips. Of them, GB is the interface separating differently oriented crystallites and plays a central role in deformation and fracture mechanisms. In polycrystalline materials, the HE is often accompanied by a transition from transgranular to intergranular fracture. However, the nanoscale interactions between hydrogen and GB largely remain illusive. And there is a large knowledge gap in the connection between the microscale hydrogen-GB interactions and macroscale observed fracture transition. Starting from this, uniaxial straining is applied to bi-crystalline Ni with a Σ5(210)[001] GB and a transgranular to intergranular fracture transition facilitated by hydrogen is directly elucidated by atomistic modeling. Hydrogen is found to form a local atmosphere in the vicinity of GB, which induces a local stress concentration and inhibits the subsequent stress relaxation at the GB during deformation. It is this local stress concentration that promotes earlier dislocation emission and generation of additional vacancies that ultimately facilitate nanovoiding. The nucleation and growth of nanovoids finally lead to intergranular fracture at the GB, in contrast to the transgranular fracture of hydrogen-free sample. This hydrogen-controlled plasticity mechanism provides a rationale for macroscale fracture transition.

To further validate the universality of this mechanism under various conditions and quantify the hydrogen-induced fracture transition process, uniaxial straining is applied to Ni Σ5(210)[001] and Σ9(1-10)[22-1] GBs with various hydrogen concentrations and temperatures based on a large statistical repetition. Without hydrogen, vacancy generation at GB is limited and transgranular fracture mode dominates. When charged, hydrogen as a booster can enhance strain-induced vacancy generation by up to ten times. This leads to the superabundant vacancy stockpiling at the GB, which agglomerates and nucleates intergranular nanovoids. While hydrogen tends to persistently enhance vacancy concentration, temperature plays an intriguing dual role as either an enhancer or an inhibitor for vacancy stockpiling. These results show a good agreement with positron annihilation spectroscopy experiments. Importantly, an S-shaped quantitative correlation between the proportion of intergranular fracture and vacancy concentration was for the first time derived, highlighting the existence of a critical vacancy concentration, beyond which fracture mode will be completely intergranular.

Besides GB fracture, hydrogen could also influence the migration behavior of GBs. The effect of solute hydrogen on shear-coupled GB migration is investigated with the dislocation-array type Σ25(430)[001] GB and a dual role of hydrogen on GB mobility is unraveled. In the low temperature and high loading rate regime, where hydrogen diffusion is substantially slower than GB motion, GB breaks away from the hydrogen atmosphere and transforms into a new stable phase with highly enhanced mobility. In the reverse regime, hydrogen atoms move along with GB, exerting a drag force on GB and decreasing its mobility. This helps to understand the coexistence of hydrogen hardening and softening in experiments.

Finally, we make an attempt to extend the results in bicrystal to polycrystal model. The trapping and diffusion of hydrogen in polycrystal was analyzed by elucidating the hydrogen-GB segregation spectrum. The spectrum shows three peaks corresponding to GB core sites and hybrid GB surface-octahedron/ tetrahedron sites, respectively. The low migration energy inside GB core and high energy barrier between different types of sites indicate the coexistence of short-circuit diffusion and GB trapping. In the end, the spectrum, combined with a thermodynamic model, is utilized to derive the equilibrium hydrogen concentration at GBs. This framework has significant implications for applications to industrial material, which enables the transposition from the atomic scale to real-world scenarios.