Theodore Zirkle 1 Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332 e-mail: tzirkle6@gatech.edu Luke Costello Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332 e-mail: lcostello@ab.mpg.de Ting Zhu Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332 e-mail: ting.zhu@me.gatech.edu David L. McDowell Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332 e-mail: david.mcdowell@me.gatech.edu Modeling Dislocation-Mediated Hydrogen Transport and Trapping in Face-Centered Cubic Metals The diffusion of hydrogen in metals is of interest due to the deleterious inﬂuence of hydrogen on material ductility and fracture resistance. It is becoming increasingly clear that hydro- gen transport couples signiﬁcantly with dislocation activity. In this work, we use a coupled diffusion-crystal plasticity model to incorporate hydrogen transport associated with dislo- cation sweeping and pipe diffusion in addition to standard lattice diffusion. Moreover, we consider generation of vacancies via plastic deformation and stabilization of vacancies via trapping of hydrogen. The proposed hydrogen transport model is implemented in a physi- cally based crystal viscoplasticity framework to model the interaction of dislocation sub- structure and hydrogen migration. In this study, focus is placed on hydrogen transport and trapping within the intense deformation ﬁeld of a crack tip plastic zone. We discuss the implications of the model results in terms of constitutive relations that incorporate hydrogen effects on crack tip ﬁeld behavior and enable exploration of hydrogen embrittle- ment mechanisms. [DOI: 10.1115/1.4051147] Keywords: hydrogen transport, hydrogen trapping, hydrogen embrittlement, crystal plasticity, ﬁnite element analysis, constitutive relations, elastic behavior, environmental effects, fracture, mechanical behavior, microstructure property relationships, plastic behavior 1 Introduction Literature regarding the inﬂuence of hydrogen (H) on metals dates back to 1875 [1]. Many subsequent experimental and theore- tical studies have investigated the effects of H on the mechanical response of metals [2]. Complete understanding remains elusive despite this rich history of investigation, and resolution of the problem remains desirable due to the common exposure of struc- tural metals to H in a variety of energy applications [3,4]. Hydrogen fundamentally changes the effective response of a material, most frequently causing components to exhibit loss of ductility and increasing susceptibility to fatigue failure [5]. Lower length scale investigations often reveal more complex interactions between H and the host material, e.g., competition between material hardening and softening [6]. To effectively design for the inﬂuence of H based on a broad range of experimental observations, a wide variety of mechanisms have been proposed. The most commonly cited mechanisms include hydride precipitation, hydrogen enhanced localized plasticity [7,8], adsorption induced dislocation emission [9–12], hydrogen enhanced decohesion [13,14], and hydrogen enhanced strain induced vacancy creation [15,16]. While each mechanism has a reasonable basis, experimental inves- tigations are typically based on averaged, macroscale behavior, pre- cluding direct observation of the proposed mechanism(s) at appropriately small length and time scales. Therefore, accurate computational modeling at operative sub-micron length scales is key to elucidating the governing processes. Regardless of the mechanism(s) subscribed to, the deleterious effects of H are directly linked to the elevated concentration of H in small volumes of material in critically stressed or strained regions such as notches and crack tips. Faithful representation of the H distribution is prerequisite to more complete consideration of the mechanistic aspects of hydrogen embrittlement. Computa- tional simulations of H transport and trapping are therefore essen- tial. Sofronis and McMeeking [17] considered the distribution of H ahead of a crack tip, accounting for diffusion driven by the H chemical potential. This approach has been adopted by a number of researchers in a variety of computational frameworks [18–22]. These works, applicable to length scales well above those of dislo- cation substructure, do not capture important mesoscale transport and trapping processes. These processes, affected by the evolution of certain material defect populations, have been experimentally observed to increase the rate of H transport. Saintier et al. [23] found enhanced H transport at a crack tip in face-centered cubic (FCC) stainless steel, attributed to dislocation-mediated processes due to the lack of appreciable martensitic transformation. Disloca- tion transport processes were partially accounted for in the recent computational work of Dadfarnia et al. [24] via introduction of the convective transport of H via mobile dislocations. Dadfarnia et al. [24] found that accounting for this mechanism enhanced the H transport, supporting the notion that mesoscale transport pro- cesses play an important role in the overall distribution of H at suf- ﬁciently small length scales. This paper extends the current state-of-the-art computational H transport framework to more com- prehensively account for a variety of line and point defect popula- tions and their inﬂuence on the distribution of H ahead of a crack tip in an FCC metal. 2 Hydrogen Transport 2.1 Mesoscopic H Transport Mechanisms. As noted in the experimental observations of Murakami et al. [25] and Nagumo [16], dislocations and vacancies (Va) play an important role in the H distribution problem. As such, we consider mobile disloca- tions, dislocation wall substructures, and Va to be among possible H traps. Grain boundaries are not considered in this treatment. We label the H concentration according to respective sites, namely, in the lattice C L , at mobile dislocations C HMD , at 1 Corresponding author. Contributed by the Materials Division of ASME for publication in the JOURNAL OF ENGINEERING MATERIALS AND TECHNOLOGY. Manuscript received January 5, 2021; ﬁnal manuscript received May 6, 2021; published online May 31, 2021. Assoc. Editor: David Field. Journal of Engineering Materials and Technology JANUARY 2022, Vol. 144 / 011005-1 Copyright © 2021 by ASME Downloaded from http://asmedigitalcollection.asme.org/materialstechnology/article-pdf/144/1/011005/6702391/mats_144_1_011005.pdf by guest on 21 December 2021