Atomistic simulations of the interactions of hydrogen with dislocations in fcc metals

The interactions of hydrogen with both edge and screw dislocations in face-centered-cubic (fcc) metals are investigated using molecular statics simulations of nickel-hydrogen as a model system. It is shown that the most energetically favorable sites for H occupation are octahedral sites in the perfect fcc lattice, tetrahedral sites in the stacking fault, and both octahedral and tetrahedral sites in the Shockley partial cores of dislocations. Moreover, the diffusion barrier for H is relatively high except for pipe diffusion near the Shockley partial cores. It is also shown that partial dislocation cores have the strongest interactions with hydrogen. The hydrogen-dislocation interactions (attractive or repulsive) and the change in stacking width (increase or decrease) depend on the hydrogen-occupying sites (octahedral or tetrahedral) and the positions of the hydrogen atoms relative to the strongest binding energy sites. In particular, on the dislocation glide plane, only hydrogen atoms in both of the Shockley partial core regions can result in increasing the stacking fault width, while those in the stacking fault region or in the perfect fcc lattice region have no observable effects. On the other hand, uniformly distributed hydrogen in the tension region near the center of a dislocation can result in decreasing the stacking fault width. The stable stacking fault energy also decreases with increasing hydrogen concentration due to the resulting negative binding energy of hydrogen to the stacking fault, while the unstable stacking fault energy increases with increasing hydrogen concentration. This may result in pinning the dislocation due to the nature of short-range interaction between interstitial hydrogen atoms and their neighboring Ni atoms.

Figure 1

Schematic of the simulation boxes showing dissociated screw and edge dislocations. The Shockley partials are denoted by T or inverse T symbols, and the shaded areas represent the stacking fault.

Figure 2

(a) and (b) The side and top views, respectively, of octahedral (O) and tetrahedral (T) occupying sites (shown by small purple spheres) in relation to the ABCABC packing sequence of a perfect fcc lattice. (c) and (d) The side and top views, respectively, of octahedral (${\mathrm{O}}^{\prime }$) and tetrahedral (${\mathrm{T}}^{\prime }$) occupying sites (shown by small purple spheres) in relation to the ACAC packing sequence of a stacking fault. Pink, blue, and orange spheres denote A, B, and C stacking planes, respectively. In the top view, only A and B planes are shown, with the atoms in the A plane enlarged for better visualization of the occupying sites.

Figure 3

Binding energy for H occupying: (a) an O site moving through a T site to its neighboring O site along the $\left[\overline{1}\overline{1}2\right]$ direction; and (b) a ${\mathrm{T}}^{\prime }$ site in the SF moving through an ${\mathrm{O}}^{\prime }$ site to its neighboring ${\mathrm{T}}^{\prime }$ site along the $\left[11\overline{2}\right]$ direction.

Figure 4

Top view of a (111) slip plane of the equilibrium configuration of a screw dislocation. The fcc and non-fcc atoms in the lower A plane are visualized by large pink (light gray) and red (dark gray) spheres, respectively; while fcc and non-fcc atoms in the upper B plane are visualized by small blue (medium gray) and green (gray) spheres, respectively. Regions of perfect fcc crystal, SF, and Shockley partial cores (i.e. transition region from a perfect fcc to a SF) are indicated.

Figure 5

(a) A hydrogen atom occupying a ${\mathrm{T}}^{\prime \prime }$ site on the glide plane near a partial core of an edge dislocation moved along two $\langle 112\rangle$ paths to neighboring ${\mathrm{O}}^{\prime \prime }$ and ${\mathrm{T}}^{\prime \prime }$ sites and (b) the resulting binding energy. (c) A hydrogen atom occupying an ${\mathrm{O}}^{\prime \prime }$ site on the glide plane near a partial core of a screw dislocation moved along two $\langle 112\rangle$ paths to neighboring ${\mathrm{T}}^{\prime \prime }$ and ${\mathrm{O}}^{\prime \prime }$ sites and (d) the resulting binding energy.

Figure 6

Binding energy of interstitial H to (a) edge and (b) screw dislocations. The positions of the Shockley partial cores are shown by T and inverse T symbols.

Figure 7

Initial and equilibrium configurations showing the interaction of an H array (small purple spheres) with an edge dislocation. The H array is above the glide plane in (a) and (b), below the glide plane in (c) and (d), and on the glide plane in (e) through (h). View is along the dislocation line direction ($y$ axis). Only non-fcc atoms are shown (red [dark gray] and green [gray] denote atoms in lower and upper A and B planes). Arrows indicate the motion of the entire dislocation during energy minimization.

Figure 8

Initial and equilibrium configurations showing the interaction of an H array (small purple spheres) with a screw dislocation. The H array is on the glide plane in (a) through (d), above the glide plane in (e) and (f), and below the glide plane in (g) and (h). View is along dislocation line direction ($y$ axis). Only non-fcc atoms are shown (red [dark gray] and green [gray] denote atoms in lower and upper A and B planes). Arrows indicate the motion directions of the entire dislocation during energy minimization.

Figure 9

Initial and equilibrium configurations showing the change in the SF width of an edge dislocation due to the interactions with two arrays of H atoms (small purple spheres) on the glide plane in (a) and (b) and off the glide planes in (c) through (f). The SF width is also shown, and the increase or decrease in the SF width is indicated by arrows. View is along the dislocation line direction ($y$ direction).

Figure 10

Initial and equilibrium configurations showing the change in the stacking fault width of a screw dislocation due to the interactions with two arrays of H atoms (small purple spheres) on the glide plane in (a) and (b) and off the glide plane in (c). The SF width is also shown, and the increase or decrease in the SF width is indicated by arrows. View is along the dislocation line direction ($x$ direction).

Figure 11

Initial and equilibrium configurations showing the change in a screw dislocation SF width due to interactions with: (a) uniform distribution of H arrays (small purple spheres) in the SF and the Shockley partial cores; only in the SF (b) with a moderate and (c) with a high H line density; and only near the partial cores (d) with a moderate and (e) with a high H line density. The SF width is also shown, and the increase or decrease in the SF width is indicated by arrows. View is along the dislocation line direction ($x$ direction).

Figure 12

Generalized stacking fault energy of $\left[11\overline{2}\right]$(111) slip system (a) without and (b) with H on the glide plane.

Figure 13

(a) Energy change for selected individual atoms and (b) energy change per unit area in the absence (solid black line) and presence of one H atom on the glide plane (red [dashed-dotted] and green [dashed] lines) during shear of the upper half along $\left[11\overline{2}\right]$ direction. Inset in (a) shows numbering of the selected five Ni atoms around an H, as well as the shear direction.