Quantum mechanical modeling of hydrogen assisted cracking in aluminum

We report multiscale quantum mechanical modeling of hydrogen assisted cracking in aluminum which is central to H embrittlement phenomena. We find that dislocation emission and brittle cleavage can occur simultaneously. H embrittlement takes place when H occupies the top sites on the crack front surface and even a very low H coverage at 0.2 monolayers can lead to brittle cleavage. H atoms adsorbed on the crack surfaces tend to suppress dislocation emission, whereas the solute H atoms on the slip plane can promote dislocation emission. Top-site H atoms at the front surface are found to facilitate the migration of other H atoms towards the front surface, providing a mechanism for H accumulation at the crack tip. The study resolves a long-standing puzzle of why H embrittlement could occur in Al where the equilibrium H solubility is extremely low under normal conditions.

Figure 1

(a) An overview of the entire simulation box with finite-element mesh. (b) A blown-up view of the central box in (a) showing the DFT region (red), EAM region (blue), and the finite-element region. (c) A displacement contour plot showing four edge dislocations that have been emitted and subsequently propagated 400 Å away from the crack tip. All lengths are in Å.

Figure 2

(a) Schematic diagram showing the three crack surfaces: the upper side surfaces (U), the lower side surfaces (L), and the crack tip front surface (F). The slip plane positions are indicated by $x^{\mathrm{U}}$ in the upper half crystal and $x^{\mathrm{L}}$ in the lower half crystal relative to the crack front. (b) Graphical definition of the crack opening width $\Delta ={d_y}^{\prime }-d_y$, where ${d_y}^{\prime }$ is the maximum interplanar distance in the $\mathit{y}$ direction and $d_y=1.41$ Å is the equilibrium interplanar distance.

Figure 3

The atomic configurations used in the calculations of H adsorption energy $E_{\mathrm{ad}}$: (a) for a H atom on the B site and (b) for a H atom on the T site on Al (111) surface. (c) Partially relaxed H adsorption energy $E_{\mathrm{ad}}$ versus the interplanar separation $\delta$, showing the crossover between the B site and the T site. (d) Fully relaxed $E_{\mathrm{ad}}$ versus $\delta$. When $\delta$ reaches 0.9 Å, the H atom shifts from the B site to the T site, accompanied by an energy jump. The red and blue spheres represent H and Al atoms, respectively.

Figure 4

Schematic diagrams showing dislocation emission and brittle cleavage under $K_{\mathrm{I}}=0.32$ eV/Å${}^{2.5}$. Red and blue circles represent H and Al atoms, respectively. All lengths are in Å. The sheared finite-element mesh is the result of dislocation(s) passage along the slip plane; more emitted dislocations yield more severe shears. (a) Pure Al; (d) solute H atoms at the slip plane (crack ix); (b) and (e) H atoms at the B sites of the side surfaces (crack i) and the front surface (crack iii), respectively; (c) and (f) H atoms at the T sites of the side surfaces (crack ii) and the front surface (crack v), respectively. The bond rupture in (f) is represented by the dashed lines.

Figure 5

Schematic diagrams showing dislocation emission and brittle cleavage. (a) The initial cleavage for crack viii under $K_{\mathrm{I}}=0.30$ $\mathrm{eV}/{\AA }^{2.5}$ (crack viii). (b) The crack extends one layer forward with H at the T sites of the fresh front surface under $K_{\mathrm{I}}=0.35$ $\mathrm{eV}/{\AA }^{2.5}$. (c) The crack halts without H on the fresh front surface. (d) The crack halts with H at the B sites of the fresh front surface. The red and purple dashed lines indicate the bond rupture on the initial and new crack front, respectively.

Figure 6

(a) The atomic structure of the crack tip used in the NEB calculations with one H atom at the T site of the front surface and another H migration towards the front surface. The arrow indicates the approximate initial and final positions of the migrating H atom. (b) H migration energy barriers from the bulk interior to the crack front surface with and without a H atom at the T site. $E^{\mathrm{b}}$ is the H binding energy as defined in Ref. 41. The first Al layer is taken as the front surface.