Masatake Yamaguchi

To elucidate the mechanism of hydrogen embrittlement in steel, quantifying the interaction between hydrogen and lattice defects is essential. However, the hydrogen trapping energy at edge dislocation cores in ferromagnetic body-centered cubic (bcc) iron remains elusive, as the long-range elastic strain field surrounding the cores complicates accurate estimation via first-principles density functional theory (DFT) calculations with small computational cells. In this study, we overcame this limitation by combining DFT with a machine-learning neural network potential (NNP). First, we calculated the hydrogen trapping energy using DFT in a periodic boundary cell containing two oppositely signed dislocation cores, with a cell size of up to approximately 400 atoms. After verifying that the NNP can reasonably reproduce the DFT results, we performed NNP calculations with enlarged cell sizes to quantify the long-range elastic strain interactions between dislocation cores. Subtracting this elastic contribution from the DFT results successfully removed finite-cell-size effects. In addition to the a0/2[-1-11](-101) (a0: lattice constant) edge dislocation core addressed in our previous work, this study expanded the scope of analysis to a broader range of dislocation core structures: the a0/2[111](-101) 71° mixed, the a0/2[111](-1-12) edge, the a0[100](011) and the a0[100](010) binary-junction dislocation cores, as well as a single vacancy. The calculated trapping energies for specific dislocation cores and a single vacancy exhibit excellent agreement with the hydrogen desorption activation energies of the deepest trapping sites reported in recent low-temperature thermal desorption spectrometry on a heavily deformed pure iron subjected to 25% plastic strain.

We attempted to calculate the hydrogen trapping energies at the incoherent interfaces of MgZn2 precipitates and Mg2Si crystallites in aluminum alloys from first-principles calculations. Since the unit cell containing the incoherent interface does not satisfy the periodic boundary condition, resulting in a discontinuity of crystal blocks, the hydrogen trapping energy was calculated in a region far from the discontinuity (vacuum) region. We found considerable trapping energies for hydrogen atoms at the incoherent interfaces consisting of assumed atomistic arrangement. We also conducted preliminary calculations of the reduction in the cohesive energy by hydrogen trapping on the incoherent interfaces of Mg2Si in the aluminum matrix.