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Graduate Student, University of Virginia

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How does the characteristics of a metal affect its susceptibility to hydrogen-assisted cracking?

Hydrogen-induced premature failure of structural components is a significant barrier to the widespread adoption of a hydrogen-based fuel economy. However, despite the fact that this phenomena has been studied for over 100 years, the governing physics by which hydrogen degrades metallic materials remains poorly understood. In particular, our understanding of how variations in a metal's microstructure (i.e. what the metal is composed of) influences a metal's inherent susceptibility to hydrogen-assisted cracking is generally limited and often contradictory. However, recent advances in material characterization techniques offers an exciting opportunity to systematically assess the influence of microstructure variation on hydrogen-assisted cracking. My research is focused on coupling these new/improved techniques with fracture experiment results so as to develop a comprehensive understanding of which microstructural features matter when it comes to hydrogen-assisted cracking. It has long been a goal of the hydrogen community to develop physically-informed, predictive models of hydrogen-assisted cracking, as they would aid the development of hydrogen-resistant alloy and improved lifetime predictions for structural components (like pipelines). By identifying those features which either enhance or degrade a metal's resistance to hydrogen-assisted cracking, my research will provide critical experimental data to help facilitate the development of such models.

11 ITEMS PINNED

Effect of Environment on Fatigue Crack Wake Dislocation Structure in Al-Cu-Mg

Abstract: Fatigue-induced dislocation structure was imaged at the crack surface using transmission electron microscopy (TEM) of focused ion beam (FIB)-prepared cross sections of naturally aged Al-4Cu-1.4Mg stressed at a constant stress intensity range (7 MPa√m) concurrent with either ultralow (~10−8 Pa s) or high-purity (50 Pa s) water vapor exposure at 296 K (23 °C). A 200-to-600-nm-thick recovered-dislocation cell structure formed adjacent to the crack surface from planar slip bands in the plastic zone with the thickness of the cell structure and slip bands decreasing with increasing water vapor exposure. This result suggested lowered plastic strain accumulation in the moist environment relative to the vacuum. The previously reported fatigue crack surface crystallography is explained by the underlying dislocation substructure. For a vacuum, \( \left\{ { 1 1 1} \right\} \) facets dominate the crack path from localized slip band cracking without resolvable dislocation cells, but cell formation causes some off-\( \left\{ { 1 1 1} \right\} \) features. With water vapor present, the high level of hydrogen trapped within the developed dislocation structure could promote decohesion manifest as either low-index \( \left\{ { 100} \right\} \) or \( \left\{ { 1 10} \right\} \) facets, as well as high-index cracking through the fatigue-formed subgrain structure. These features and damage scenario provide a physical basis for modeling discontinuous environmental fatigue crack growth governed by both cyclic strain range and maximum tensile stress.

Pub.: 25 Feb '12, Pinned: 03 Jul '17

Hydrogen embrittlement: the game changing factor in the applicability of nickel alloys in oilfield technology.

Abstract: Precipitation hardenable (PH) nickel (Ni) alloys are often the most reliable engineering materials for demanding oilfield upstream and subsea applications especially in deep sour wells. Despite their superior corrosion resistance and mechanical properties over a broad range of temperatures, the applicability of PH Ni alloys has been questioned due to their susceptibility to hydrogen embrittlement (HE), as confirmed in documented failures of components in upstream applications. While extensive work has been done in recent years to develop testing methodologies for benchmarking PH Ni alloys in terms of their HE susceptibility, limited scientific research has been conducted to achieve improved foundational knowledge about the role of microstructural particularities in these alloys on their mechanical behaviour in environments promoting hydrogen uptake. Precipitates such as the γ', γ'' and δ-phase are well known for defining the mechanical and chemical properties of these alloys. To elucidate the effect of precipitates in the microstructure of the oil-patch PH Ni alloy 718 on its HE susceptibility, slow strain rate tests under continuous hydrogen charging were conducted on material after several different age-hardening treatments. By correlating the obtained results with those from the microstructural and fractographic characterization, it was concluded that HE susceptibility of oil-patch alloy 718 is strongly influenced by the amount and size of precipitates such as the γ' and γ'' as well as the δ-phase rather than by the strength level only. In addition, several HE mechanisms including hydrogen-enhanced decohesion and hydrogen-enhanced local plasticity were observed taking place on oil-patch alloy 718, depending upon the characteristics of these phases when present in the microstructure.This article is part of the themed issue 'The challenges of hydrogen and metals'.

Pub.: 14 Jun '17, Pinned: 30 Jun '17