Atomic-scale investigation of point defects and hydrogen-solute atmospheres on the edge dislocation mobility in alpha iron

Abstract

In this study, we present atomistic mechanisms of 1/2 [111](11¯0) edge dislocation interactions with point defects (hydrogen and vacancies) and hydrogen solute atmospheres in body centered cubic (bcc) iron. In metals such as iron, increases in hydrogen concentration can increase dislocation mobility and/or cleavage-type decohesion. Here, we first investigate the dislocation mobility in the presence of various point defects, i.e., change in the frictional stress as the edge dislocation interacts with (a) vacancy, (b) substitutional hydrogen, (c) one substitutional and one interstitial hydrogen, (d) interstitial hydrogen, (e) vacancy and interstitial hydrogen, and (f) two interstitial hydrogen. Second, we examine the role of a hydrogen-solute atmosphere on the rate of local dislocation velocity. The edge dislocation simulation with a vacancy in the compression side of the dislocation and an interstitial hydrogen atom at the tension side exhibit the strongest mechanical response, suggesting a higher potential barrier and hence, the higher frictional stress (i.e., ∼83% higher than the pure iron Peierls stress). In the case of a dislocation interacting with a vacancy on the compressive side, the vacancy binds with the edge dislocation, resulting in an increase in the friction stress of about 28% when compared with the Peierls stress of an edge dislocation in pure iron. Furthermore, as the applied strain increases, the vacancy migrates through a dislocation transportation mechanism by attaining a velocity of the same order as the dislocation velocity. For the case of the edge dislocation interacting with interstitial hydrogen on the tension side, the hydrogen atom jumps through one layer perpendicular to the glide plane during the pinning-unpinning process. Finally, our simulation of dislocation interactions with hydrogen show first an increase in the local dislocation velocity followed by a pinning of the dislocation core in the atmosphere, resulting in resistance to dislocation motion as the dislocation moves though the hydrogen-solute atmospheres. With this systematic, atomistic study of the edge dislocation with various point defects, we show significant increase in obstacle strengths in addition to an increase in the local dislocation velocity during interaction with solute atmospheres. The results have implications for constitutive development and modeling of the hydrogen effect on dislocation mobility and deformation in metals.

Atomic-scale investigation of triple junction role on defects binding energetics and structural stability in α-Fe

“…[41,49,[56][57][58][59][60]). The cohesive energy predicted by all the potentials for α-Fe was approximately 4.01 eV/atom, which compares well with 4.2 eV/atom reported by Bhatia et al [61] using density functional theory. In this work, the TJs were constructed using <110> symmetric tilt grain boundaries (GBs) (see Table 1).…”

Discrete dislocation plasticity HELPs understand hydrogen effects in bcc materials

“…In their multiscale quantum-mechanics/molecular mechanics simulation, Zhao and Lu (2011) observed hydrogen lowered the Peierls energy barrier for screw dislocations significantly, indicating enhanced mobility. Comparing this conclusion to their observation of hydrogen decreased mobility in edge dislocations, Bhatia et al (2014) proposed that hydrogen has a tendency to make the crystal more isotropic in terms of dislocation motion, i.e. increasing screw mobility while decreasing edge mobility (Itakura et al, 2013), which was also hypothesized by Moriya et al (1979) in an experimental investigation.…”

“…In case I, hydrogen promotes dislocation generation via increased screw mobility and the shape of loops becomes more circular; in case II, hydrogen accelerates dislocation evolution but has little influence on the final dislocation structure in quasi static loading. Case I reduces the anisotropy in the dislocation motion, this was noted by Bhatia et al (2014) and observed experimentally in Moriya et al (1979). Case II, however, finds no direct evidence in the literature except for a very general statement in Robertson (2001), besides, case II implies hydrogen does not alter the dislocation structure.…”

“…Exactly how hydrogen influences edge mobility is unclear, since no quantitative description is available and there is consensus in the literature. Robertson (2001) concluded that hydrogen increases dislocation mobility equally for edge and screw dislocations, while atomistic simulations predict hydrogen reduces edge mobility (Song and Curtin, 2014;Bhatia et al, 2014) and increases screw mobility (Itakura et al, 2013;Katzarov et al, 2017). Experimentally, Xie et al (2016) observed hydrogen decreased edge dislocation mobility in Al during in situ micropillar cyclic loading test, and the same author (Xie, 25 May 2018) most recently observed hydrogen enhanced screw mobility in Fe using similar technique.…”

Critical assessment of hydrogen effects on the slip transmission across grain boundaries in α -Fe

“…The presence of a corrosive environment such as hydrogen increases the tendency for a transition from a transgranular to an intergranular fracture mode [4][5][6][7]. For example, hydrogen has a strong bias to segregate around defects such as cracks, dislocations and GBs [8][9][10]. The segregation of hydrogen along the GBs leads to a reduction in the cohesive strength [7,10], thereby promoting an intergranular failure [11][12][13][14].…”