Environmentally Assisted Cracking: Theory and Practice: Hydrogen Embrittlement I
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

Monday 8:30 AM
February 27, 2017
Room: 31A
Location: San Diego Convention Ctr

Session Chair: Ian Robertson, University of Wisconsin-Madison; Petros Sofronis, University of Illinois at Urbana-Champaign

8:30 AM Introductory Comments Speaker: Prof. Ian Robertson / Bai Cui

8:45 AM  Invited
Linking Hydrogen-enhanced Plasticity to Hydrogen-induced Failure Mode: Kelly Nygren1; Shuai Wang2; Ian Robertson2; 1University of Illinois; 2University of Wisconsin-Madison
    Hydrogen embrittlement of metals is a sudden and catastrophic failure event and, yet, despite over a century of research there is still debate about the fundamental mechanisms and the relationship between the mechanisms and the ultimate hydrogen-induced failure mode. In this talk, the ongoing efforts to bridge from the hydrogen-enhanced plasticity mechanism, which is based on individual dislocations and the hydrogen shielding concept, and hydrogen-induced failure will be made. Specifically, the relationship between the evolved microstructural state beneath fracture surfaces, the morphological features on the fracture surface, and the macroscopic mechanical properties will be made. This connection will establish the key role of the hydrogen influence on plasticity in establishing the local conditions, hydrogen concentration and local stress, that determine the ultimate failure path. In addition, it will be proposed that there exists a unique evolved microstructural state for each of the observed fracture surface features.

9:25 AM  
Effects of Trace Impurities on the Strength and Fracture of Hydrogen-Charged Ni-201: Samantha Lawrence1; Richard Karnesky1; Khalid Hattar1; Stephen Foiles1; Brian Somerday2; 1Sandia National Laboratories; 2Southwest Research Institute
    The susceptibility of commercially-pure nickel to hydrogen-induced intergranular fracture depends on the concentration of hydrogen, the structure of grain boundaries, and the concentration of trace impurities in the microstructure. In particular, sulfur (S) and lead (Pb) are potent amplifiers of hydrogen-induced intergranular fracture. In this work, we assess the mechanical properties and fracture behavior of Ni-201 with varied chemistries and added hydrogen contents. In particular, Pb content is systematically varied by means of implantation using a 15MeV ion beam. Mechanical testing reveals that solid solution strengthening is the predominate strengthening mechanism regardless of hydrogen content. Importantly, a systematic increase of the Pb content in the Ni-201 alloy enhances susceptibility to hydrogen-induced non-ductile fracture modes when compared to the non-charged condition.

9:45 AM  
Macro- and Micro-scale Study of Hydrogen Susceptibility of Advanced High Strength Sheet Steels: Yiran Lu1; Shrikant Bhat2; Clyde Briant1; Sharvan Kumar1; 1Brown University; 2ArcelorMittal, Global R&D
    Hydrogen susceptibility of advanced high strength steels (AHSS) with tensile strengths greater than 1 GPa is a potential concern for automotive applications. Understanding the hydrogen susceptibility of constituent phases of AHSS can provide valuable insights into alloy design to mitigate the problem. Recent advances in microscale experimental techniques have made this multiscale approach possible. In this work, we performed tensile tests on M900, M1300, and M1700 martensitic sheet steels under different hydrogen charging conditions. M1700 with high hydrogen content showed a reduced total elongation of 1% compared to 7% without charging. M900 and M1300 showed less severe susceptibility. Quantitative fracture surface analysis revealed a 15% smaller dimple size in the hydrogen-charged conditions. Results from microfluid cell coupled with nanoindentation performed within grains isolated by EBSD, together with TEM provide insight into plastic deformation and possible cracking under the indent. These results will be presented and their implications discussed.

10:05 AM Break

10:20 AM  Invited
Hydrogen-Induced Fracture: From Fundamentals to Prognosis: Petros Sofronis1; Mohsen Dadfarnia1; Akihide Nagao2; Shuai Wang3; May Martin1; Brian Somerday4; Reiner Kirchheim5; Robert Ritchie6; Ian Robertson3; 1University of Illinois; 2JFE Steel Corporation; 3University of Wisconsin; 4South West Research Institute; 5Georg-August-Universität Göttingen; 6University of California-Berkeley
    A summary of recent developments on fracture prognosis for various materials is presented that accounts for microscale deformation mechanisms. Recent studies of structural materials fractured in hydrogen suggest that dislocation structures and hydrogen transported by mobile dislocations play important roles in the evolution of fracture processes/events. A model for hydrogen/deformation interactions that accounts for dislocation transport along with stress driven diffusion and trapping at microstructural defects is introduced. As an example, an approach to quantify the effect of hydrogen on the fracture of a martensitic steel through a statistically-based micromechanical model for the critical local fracture event is also presented. The model accounts for the synergistic effect of hydrogen on decohesion and the effect of dislocations on hydrogen distribution. Lastly, a mitigation strategy for the hydrogen effect on ferritic systems subjected to cyclic loading is suggested: dissolution of a few ppm by volume of oxygen in hydrogen gas.

11:00 AM  
Atomic Insights on Hydrogen Embrittlement in Iron: Ilaksh Adlakha1; Kiran Solanki1; 1Arizona State University
    In order to gain insights into Hydrogen embrittlement (HE) mechanisms in α-Fe, we aim to discuss several key issues involving HE such as: a) the dislocation nucleation; b) the crack tip deformation; c) the cohesive strength of grain boundaries (GBs) and d) the dislocation-GB interactions. The presence of hydrogen was found to decrease the critical resolved shear stress required for dislocation nucleation. The co-existence of hydrogen enhanced plasticity and decohesion mechanisms was observed during the examination of the crack tip deformation. Next, the segregation of hydrogen along the interface was found to decrease the cohesive strength by varying magnitude based on the GB character. Finally, we examined the effect of hydrogen on the interactions between a screw dislocation and <111> tilt GBs in α-Fe. The introduction of hydrogen along the GB was found to increase the dislocation pileup size, thereby increasing the susceptibility for the intergranular failure.

11:20 AM  
Effects of Internal and External Hydrogen Environments on Crack Growth in an Iron Based Superalloy: Neville Moody1; Warren Garrison2; S. Robinson1; M. Perra1; William Gerberich3; 1Sandia National Laboratories; 2Carnegie Mellon University; 3University of Minnesota
    Austenitic superalloys are often used in hydrogen and hydrogen-producing environments due to their good resistance to hydrogen effects. Nevertheless, these alloys exhibit significant reductions in ductility and crack growth resistance when exposed to high fugacity hydrogen environments. As shown by crack growth tests in a high strength iron-based superalloy, there are also significant differences in crack growth susceptibility and fracture modes from internal and external hydrogen. Crack growth rates were significantly higher and the thresholds much lower in hydrogen precharged samples than in samples tested in hydrogen gas. Furthermore, fracture initiated at matrix carbides followed by slip band failure in precharged samples while fracture occurred intergranularly in hydrogen gas at high pressures but along slip bands at low pressures. In both environments, crack tip hydrogen concentrations at threshold were much higher than unstressed equilibrium levels. There have been several studies on hydrogen affects thresholds in bcc iron and steels and fcc superalloys, but very few on crack growth rates. In this presentation, crack growth and threshold models will be used to show that the differences in crack growth rates and fracture modes from internal and external hydrogen are due to the combined effects of microstructure, hydrogen diffusivity, and time-dependent stress enhancement on crack tip hydrogen concentrations. This work is supported by Sandia National Laboratories, a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

11:40 AM  
Hydrogen Embrittlement and Hydrogen-enhanced Strain-induced Vacancies in α-iron: Yuya Matsumoto1; Nami Kurihara1; Hiroshi Suzuki1; Kenichi Takai1; 1Sophia University
    Clarifying states of hydrogen present in iron and steel is important in order to understand hydrogen embrittlement mechanisms and develop materials with high resistance to hydrogen embrittlement. Though it is widely recognized that fracture strain of iron and steel decreased with increasing the amount of hydrogen, it has not been understood that hydrogen affects the decrease in fracture strain directly or not. Therefore, the objective is to clarify the atomic-scale changes in α-iron specimen strained with hydrogen. A thermal desorption spectroscopy which can heat from lower temperature (L-TDS) was used to separate peak temperatures and hydrogen states corresponding to various lattice defects in α-iron. The L-TDS results show that new hydrogen trap sites in α-iron specimen strained with hydrogen were enhanced compared with that without hydrogen. These sites were not dislocations but hydrogen-enhanced strain-induced vacancies since these sites were annihilated during aging at 30 °C.