Atomic mechanisms of microcrack propagation in pure and hydrogen-containing FCC and BCC metals

A molecular dynamics method was used to simulate crack propagation in pure and hydrogen-containing aluminum and α-iron for loads far exceeding the critical values. Pairwise interaction potentials calculated within the Heine-Abarenkov-Animalu pseudopotential approximation were applied. It was shown that cracks do not propagate in the pure metals. Their tips become blunt, mouths broaden, and internal stresses are released owing to arising dislocations and necking. This means that mechanisms of viscous fracture come into play. In the presence of hydrogen impurity, the situation is quite different. In aluminum, hydrogen desorbs and the material retains its ductility. In α-iron, hydrogen forms Cottrell clouds around dislocations, thus suppressing their movement and generation. In addition, an increase in the hydrogen concentration in iron near the crack mouth makes the material more prone to α → γ phase transition. As a result, crack propagation is observed; i.e., the material embrittles.