TRACE-ELEMENT EFFECTS ON PRECIPITATION PROCESSES AND MECHANICAL-PROPERTIES IN AN AL-CU-LI ALLOY

A study has been made of how impurities (Na and K) and trace additions of indium, magnesium, and silicon affect the microstructure and relat ed mechanical properties of an Al-Cu-Li alloy. Transmission electron m icroscopy (TEM) was used to determine the size and distribution of par ticles in four alloys. Indium and magnesium are both seen to stimulate T, precipitation. Indium also modifies theta'' morphology, and magnes ium greatly increases the number density of theta'' precipitates. Stra in localization was observed in underaged Al-Cu-Li-In tensile samples, consistent with observed changes in precipitate structure. No superpo sition of the effects of indium and magnesium was seen. High-resolutio n analytical microscopy was used to inspect precipitates for segregati on of trace elements during early stages of aging, but no segregation was found within the detection limits of the system. Variations in hea t treatment were made in order to study nucleation kinetics and trace element interactions with vacancies. Indium, with a binding energy les s than that of lithium, was not seen to interact with quenched-in vaca ncies, while magnesium, with a binding energy greater than that of lit hium, had a strong interaction. Yield anisotropies and fracture toughn esses were measured. Removal of trace impurities of sodium and potassi um correlated with improved fracture properties. Magnesium was observe d to increase anisotropy, especially in the T8 temper. A model was use d to explain the anisotropy data in terms of texture and precipitate d istribution.