Abstract
In an attempt to bridge the gap between atomistic and continuum plasticity simulations of hydrogen in iron, we present three dimensional discrete dislocation plasticity simulations incorporating the hydrogen elastic stress and a hydrogen dependent dislocation mobility law. The hydrogen induced stress is incorporated following the formulation derived by Gu and El-Awady (2018) which here we extend to a finite boundary value problem, a microcantilever beam, via the superposition principle. The hydrogen dependent mobility law is based on first principle calculations by Katzarov et al. (2017) and was found to promote dislocation generation and enhance slip planarity at a bulk hydrogen concentration of 0.1 appm; which is typical for bcc materials. The hydrogen elastic stress produced the same behaviour, but only when the bulk concentration was extremely high. In a microcantilever, hydrogen was found to promote dislocation activity which lowered the flow stress and generated more pronounced slip steps on the free surfaces. These observations are consistent with the hydrogen enhanced localized plasticity (HELP) mechanism, and it is concluded that both the hydrogen elastic stress and hydrogen increased dislocation mobility are viable explanations for HELP. However it is the latter that dominates at the low concentrations typically found in bcc metals.
Highlights
Hydrogen can cause premature failure accompanied by a decrease in ductility in metals, which is often referred to as hydrogen embrittlement (HE) (Ayas et al, 2014)
L i, j l l where fkilj (Xk ) is the elastic force due to segment i → j integrated along segment k → l evaluated at node k. This is summed over all segments i → j inside the domain, including the self force due to segment k → l, this is summed over all nodes l which are connected to node k. fkl (Xk ) is the corrective elastic force on node k integrated along segment k → l obtained using FEM to account for the applied and image stresses
The formulation proposed by Gu and El-Awady (2018) to incorporate the hydrogen elastic force was implemented in discrete dislocation dynamics (DDD) to simulate a Frank-Read source and within discrete dislocation plasticity (DDP) to simulate a microcantilever
Summary
Hydrogen can cause premature failure accompanied by a decrease in ductility in metals, which is often referred to as hydrogen embrittlement (HE) (Ayas et al, 2014). A novel hydrogenenhanced-plasticity mediated decohesion mechanism was proposed for HE in martensitic steels (Nagao et al, 2018; 2012). It claims that hydrogen-enhanced mobility of dislocations leads to local stress build-up and hydrogen concentration elevation close to material interfaces where decohesion is promoted. In this mechanism, hydrogen enhanced plasticity is the driving force for HE, which highlights the importance of understanding how hydrogen affects dislocations
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