Delayed fracture and the nature of substructural defects formed in aluminum alloys due to sustained loading

We analyze and generalize experimental and theoretical data on substructural defects associated with dislocation pileups, hydrogenation, and excess vacancies formed in Al-Mg, Al-Mg-Li, and Al-Zn-Mg-(Cu) alloys subjected to long-term loading under normal conditions. It is shown that the key role in the formation of defects of the last two kinds is played by selective oxidation of magnesium and/or lithium. We investigate the interaction of defects of various kinds, including the interaction of dislocations with vacancies and vacancies with hydrogen in the processes of delayed fracture and deterioration of mechanical properties of alloys. One should especially mention the possibility of formation of substitutionally dissolved hydrogen (SDH) as a result of the interaction of the second type. The maximum possible amount of SDH accumulated in the entire service life of the alloy is equal to the concentration of the elements susceptible to selective oxidation. It is proposed that SDH embrittles grain boundaries of the alloy by the mechanism of diffusion creep and facilitates shifts of dislocations in pileups. The ideas suggested in the present work enabled us to explain, by using experimental data on the distribution of alloying elements in the vicinity of grain boundaries, all known facts about the influence of metallurgical factors on the susceptibility of alloys subjected to long-term loading under normal conditions to delayed fracture and embrittlement.