Environmentally Assisted Cracking: Theory and Practice: Hydrogen Embrittlement II
Sponsored by: TMS Structural Materials Division, TMS: Corrosion and Environmental Effects Committee, TMS: Mechanical Behavior of Materials Committee
Program Organizers: Bai Cui, University of Nebraska–Lincoln; Raul Rebak, GE Global Research; Sebastien Dryepondt, Oak Ridge National Laboratory; Srujan Rokkam, Advanced Cooling Technologies

Tuesday 8:30 AM
February 28, 2017
Room: 31A
Location: San Diego Convention Ctr

Session Chair: John Scully, University of Virginia; Brian Somerday, Southwest Research Institute

8:30 AM  Invited
Quantification of Hydrogen-Metal Interactions in Engineering Alloys in Confined Spaces: Challenges and Opportunities: John Scully1; 1University of Virginia
    Significant progress has been made in understanding hydrogen embrittlement (HE) of structural materials over the last few decades. However, considerable gaps remain in the understanding of many important hydrogen-metal surface and bulk metallurgical interactions that often govern HE susceptibility in corrosive environments. One of the needs, gaps, and opportunities in hydrogen embrittlement phenomena includes assessment of diffusible and trapped hydrogen concentrations as well as transport rates in confined spaces such as pits and cracks. This talk will highlight recent progress towards this issue that remains critical to hydrogen embrittlement. One focus will be on quantitative assessments of local hydrogen uptake and transport at pits and cracks, specifically the application of the scanning Kelvin probe and scanning electrochemical microscopy to map local hydrogen concentrations. Other issues will be discussed as time allows

9:10 AM  
The Effect of Microstructural Variation on Hydrogen Environment-Assisted Cracking Susceptibility of Monel K-500: Zachary Harris1; Brendy Rincon Troconis1; John Scully1; James Burns1; 1University of Virginia
    Monel K-500 is prone to hydrogen environment-assisted cracking (HEAC) when stressed in chloride-containing environments under applied potentials more negative than -800 mVSCE (vs. saturated calomel). Intermittent field failures of Monel K-500 components suggest this susceptibility is influenced by differences in environment, applied loading, and/or microstructure. Prior research has established the influence of the former two variables on HEAC, but the effect of lot-to-lot metallurgical variation on HEAC susceptibility is not understood. In this study, slow-rising stress intensity (K) testing and detailed microstructure and H-metal interaction characterization were coupled to establish the microstructural features that govern HEAC susceptibility in Monel K-500. Results suggest that yield strength, impurity segregation to grain boundaries, and hydrogen uptake are the dominant metallurgy-dependent features in determining HEAC susceptibility. Modifications to current micromechanical, decohesion-based models that incorporate the effect of impurity segregation to grain boundaries are proposed and additional experiments to validate such changes are discussed.

9:30 AM  
Factors Causing Hydrogen Embrittlement of Cold-drawn Pearlitic Steel Fractured under Elastic/Plastic Region: Ryosuke Konno1; Toshiyuki Manabe2; Naoki Matsui2; Daisuke Hirakami2; Kenichi Takai1; 1Sophia University; 2Nippon Steel & Sumitomo Metal Corporation
    Factors causing hydrogen embrittlement of cold-drawn pearlitic steel fractured under plastic/elastic region have been investigated from the viewpoint of lattice defects. Tensile tests were conducted for hydrogen-charged specimens containing 1.5 ppm and 4.0 ppm hydrogen to evaluate mechanical properties. The amount of tracer hydrogen, i.e., lattice defects in the specimens unloaded just before tensile fracture strength was measured using a thermal desorption analysis. As a result, specimens containing 1.5 ppm and 4.0 ppm hydrogen fractured under plastic and elastic region, respectively. The specimen fractured under plastic region enhanced the formation of lattice defects corresponding to vacancies, which caused embrittlement directly. In contrast, the specimen fractured under elastic region enhanced no formation of lattice defects. These results revealed that one of the factors causing hydrogen embrittlement under plastic region is due to hydrogen-enhanced strain-induced vacancies, whereas the factors causing hydrogen embrittlement under elastic region are due to others.

9:50 AM Break

10:00 AM  Invited
Factors Governing Hydrogen-Assisted Intergranular Cracking: Ni as a Model System: Brian Somerday1; Samantha Lawrence1; Zachary Harris2; 1Sandia National Laboratories; 2University of Virginia
    Intergranular environmentally assisted cracking can be manifested in a range of engineering alloy systems, including steel, nickel, aluminum, and titanium. Although intergranular cracking in hydrogen-producing environments has been extensively characterized, recent focused studies on nickel as a model system have advanced the understanding of this technologically important fracture mode. In particular, these results have clarified or amplified the factors governing hydrogen-assisted intergranular cracking such as: grain boundary structure, hydrogen-deformation interactions, alloy impurities segregated to grain boundaries, and hydrogen-induced reduction in cohesive energy. The objective of this presentation is to review a series of recent dedicated efforts to address each of these factors and their contribution to hydrogen-assisted intergranular cracking in commercially pure nickel.

10:40 AM  
Stacking Fault Energy Based Alloy Identification for Hydrogen Compatibility: Paul Gibbs1; Patricia Hough1; Konrad Thurmer1; Brian Somerday2; Christopher San Marchi1; Jonathan Zimmerman1; 1Sandia National Laboratories; 2Southwest Research Institute
    While a singular mechanism for hydrogen degradation in austenitic stainless steels is still ambiguous, the deterioration of mechanical properties has been related to two features: 1) hydrogen interactions with dislocation structures and 2) the formation of secondary phases during deformation. Stacking fault energy (SFE) directly correlates with dislocation character and may be used to compare alloys for hydrogen service. The fatigue strength and tensile ductility of austenitic stainless steels are compared using an experimentally informed sub regular solution thermodynamic model for SFE. A transition in the tensile reduction of area is observed at SFE of 40 mJ m-2, below which pronounced hydrogen degradation on tensile ductility is observed. Martensite formation is also discussed to emphasize the importance of second phases on the hydrogen sensitivity of austenitic stainless steels.

11:00 AM  
Hydrogen Embrittlement Mediated by Reaction between Dislocation and Grain Boundary in Iron: Liang Wan1; Wen-Tong Geng1; Jun-Ping Du2; Akio Ishii1; Hajime Kimizuka1; Shigenobu Ogata1; 1Osaka University; 2Kyoto University
    The tensile response of grain boundaries in α-Fe was studied by atomistic modeling method with a density functional theory based empirical interatomic potential together with a bicrystal model. With certain amount of H atoms introduced, fracture of the bicrystal will be accomplished by void nucleation and growth at grain boundary. Importantly, it is recognized that the void nucleation is preceded by a certain amount of dislocations emission and impingement at the grain boundary. Analysis shows that dislocations emission and impingement on the grain boundary makes the grain boundary transform to an activated state of more irregular structure locally, and introduces a local stress concentration. The activation of the grain boundary will facilitates decohesion of the grain boundary with the help of H atoms. The local activation of grain boundaries by reaction with dislocations is deemed to be a key step in the hydrogen embrittlement mechanism in metals.

11:20 AM  
Role of Hydrogen on Metal Plasticity: An Ab-Initio Study: Pulkit Garg1; Ilaksh Adlakha1; Kiran Solanki1; 1SEMTE
    Hydrogen embrittlement (HE) is a phenomenon that affects both the physical and chemical properties of several intrinsically ductile metals. Consequently, understanding the mechanisms behind HE has been of particular interest in both experimental and modeling research. Discrepancies between experimental observations and modeling results have led to various proposals for HE mechanisms. Therefore, to gain insights into HE mechanisms, we aim to examine the effect of hydrogen on the critical resolved shear stress required for dislocation nucleation across a wide range of metals (Fe, Nb, Ta, Ni, Al and Ti) using first principle calculations. The increasing concentration of hydrogen was found to consistently decrease the stress required for dislocation nucleation for all the metals. However, the large decrease was observed for Fe closely followed by other BCC metals. Finally, the quantum charge transfer analysis was found to provide insights into underlying mechanism responsible for the enhanced dislocation nucleation.