Quenching defects in binary aluminium alloys

Abstract

Abstract Thin foils of binary aluminium alloys containing various amounts of copper, silver, zinc and magnesium in solid solution have been prepared from quenched specimens and examined by transmission electron microscopy. In all the alloys closed loops of dislocation line and complex dislocation networks were observed. The dislocation loops did not show any stacking fault contrast so they are glissile dislocations of prismatic 1/2 <110 > type. These loops are formed by the condensation of vacancies into collapsed discs and the results indicate that this often occurs during the quench. The vacancy concentrations required to produce the defects were calculated from the density and size of the loops and varied from 10−3 to 10−6 as the quenching temperature was decreased from 600°c to 500°c and as the solute content was increased when quenching from the same temperature. This result led to the conclusion that most of the vacancies remain 'quenched in' in the concentrated alloys, i.e. the supersaturation of vacancies decreases with increasing alloys content. The magnitude of this effect decreases in the order copper, silver, magnesium, zinc. This is also the order of increasing mobility of these elements in aluminium. At composition above 0.9 at % Cu, 1.3 at % Ag, 8 at % Mg and > 15 at % Zn most of the vacancies may be retained in solution after quenching the alloys from 550°c. The precipitation of vacancies in the Al-Cu, Al-Ag and Al-Zn alloys is thought to be accompanied by clustering of solute atoms. The annealing process in Al-Mg alloys is complicated and there is no evidence for clustering of magnesium atoms. Dislocations and grain boundaries have been shown to be sinks for vacancies and the possibilities that these defects are also collectors for solute atoms is discussed. Helical dislocations were observed in quenched alloys only when the quenching temperature was close to the limit of solid solubility.