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 alpha-iron for loads far exceeding the critical values. Pairwise interaction potentials calculated within the Hei ne-Abarenkov-Animalu pseudopotential approximation were applied. It was sho wn that cracks do not propagate in the pure metals. Their tips become blunt , mouths broaden, and internal stresses are released owing to arising dislo cations and necking. This means that mechanisms of viscous fracture come in to play. In the presence of hydrogen impurity, the situation is quite diffe rent. In aluminum, hydrogen desorbs and the material retains its ductility. In alpha -iron, hydrogen forms Cottrell clouds around dislocations, thus s uppressing their movement and generation. In addition, an increase in the h ydrogen concentration in iron near the crack mouth makes the material more prone to alpha --> gamma phase transition. As a result, crack propagation i s observed; i.e., the material embrittles. (C) 2000 MAIK "Nauka/Interperiod ica".