[Paper Review] The effect of Hydrogen atom on the Screw Dislocation Mobility in BCC Iron: A First-Principles Study

Mitsuhiro Itakura, Hideo Kaburaki+2 more|arXiv (Cornell University)|Apr 2, 2013

Fusion materials and technologies1 references3 citations

TL;DR

This first-principles study investigates hydrogen's influence on screw dislocation mobility in body-centered cubic (bcc) iron, revealing a dual effect: hydrogen softens the dislocation by lowering kink nucleation enthalpy at low temperatures, while causing hardening via kink trapping at higher temperatures. A transition between softening and hardening is predicted at a lower critical temperature, consistent with experimental observations.

ABSTRACT

We investigate the effect of hydrogen on the mobility of a screw dislocation in body-centered cubic (bcc) iron using first-principles calculations, and show that an increase of screw dislocation velocity is expected for a limited temperature range. The interaction energy between a screw dislocation and hydrogen atoms is calculated for various hydrogen positions and dislocation configurations with careful estimations of the finite size effects, and the strongest binding energy of a hydrogen atom to the stable screw dislocation configuration is estimated to be $256\pm32$ meV. These results are incorporated into a line tension model of a curved dislocation line to elucidate the effect of hydrogen on the dislocation migration process. Both the softening and hardening effect of hydrogen, caused by the reduction of kink nucleation enthalpy and kink trapping, respectively, are evaluated. A clear transition between softening and hardening behavior at the lower critical temperature is predicted, which is in qualitative agreement with the experimental observation.

Motivation & Objective

• To understand how hydrogen atoms affect the mobility of screw dislocations in body-centered cubic (bcc) iron at the atomic level.
• To quantify the interaction energy between hydrogen and various screw dislocation configurations, accounting for finite-size effects.
• To determine whether hydrogen promotes dislocation glide (softening) or impedes it (hardening) under different thermal conditions.
• To predict the temperature-dependent transition between softening and hardening behavior in hydrogen-charged bcc iron.

Proposed method

• First-principles density functional theory (DFT) calculations are used to compute the interaction energy between hydrogen atoms and screw dislocations in bcc iron.
• Various hydrogen positions relative to the dislocation core are systematically evaluated to identify the most stable binding sites.
• Finite-size effects are carefully estimated to ensure accurate binding energy values, particularly for the most favorable configuration.
• A line tension model is applied to simulate the curved dislocation line, enabling analysis of dislocation migration dynamics under hydrogen influence.
• The kink nucleation enthalpy and kink trapping energy are calculated to assess the softening and hardening mechanisms, respectively.
• The model predicts a transition temperature where the dominant effect shifts from softening to hardening based on thermodynamic competition.

Experimental results

Research questions

• RQ1 What is the binding energy of a hydrogen atom to the most stable configuration of a screw dislocation in bcc iron?
• RQ2 How does hydrogen influence the kink nucleation enthalpy, and what is its role in dislocation softening?
• RQ3 To what extent does hydrogen cause kink trapping, and how does this contribute to dislocation hardening?
• RQ4 At what temperature does the transition from softening to hardening behavior occur in hydrogen-charged bcc iron?
• RQ5 Is the predicted temperature-dependent behavior consistent with experimental observations?

Key findings

• The strongest binding energy between a hydrogen atom and the stable screw dislocation configuration in bcc iron is estimated at $256 /pm 32$ meV.
• Hydrogen reduces the kink nucleation enthalpy, leading to a softening effect that enhances dislocation mobility at lower temperatures.
• At higher temperatures, hydrogen induces kink trapping, resulting in a hardening effect that impedes dislocation motion.
• A clear transition between softening and hardening behavior is predicted at a lower critical temperature, consistent with experimental trends.
• The combined effect of reduced kink formation energy and increased trapping energy explains the non-monotonic temperature dependence of dislocation mobility.