Trace element effects on precipitation processes and mechanical properties in an aluminum-copper-lithium alloy

Aluminum alloys for aerospace applications are primarily strengthened by precipitates. In many cases, the material is worked prior to aging in order to introduce dislocation structures into the matrix which will subsequently act as preferential nucleation sites for precipitates. However, many product forms do not lend themselves readily to the use of a pre-age stretch, and even where it is feasible undesirable anisotropy may result. Alternatively, certain trace alloying additions have also been found to aid nucleation of the strengthening phases. This study attempts to determine how certain trace elements affect the microstructure of an Al-Cu-Li alloy and to relate this microstructure to the measured mechanical properties.; Transmission electron microscopy was used to determine the size and distribution of particles in four alloys. Indium and magnesium are both seen to stimulate T{dollar}sb1{dollar} precipitation. Both also affect the {dollar}thetasp{lcub}primeprime{rcub}/thetaspprime{dollar}phases as well, with indium hastening the transition from {dollar}thetasp{lcub}primeprime{rcub}{dollar} to {dollar}thetaspprime{dollar} and magnesium greatly increasing the number density of {dollar}thetasp{lcub}primeprime{rcub}{dollar}. No superposition of the beneficial effects of indium and magnesium was seen.; High-resolution analytical microscopy was used to inspect precipitates and their interfaces with the matrix for segregation of trace elements to precipitates during early stages of aging. No segregation was found within the detection limits of the system. Indium-rich particles were found on intersubgranular tensile fracture surfaces of short-transverse oriented plate specimens.; Variations in heat treatment were made in order to study nucleation kinetics and trace element interactions with vacancies. Based upon aging behavior and TEM observations, indium causes an effective shift in metastable solvus boundaries by altering nucleation kinetics. Indium was not seen to interact with quenched-in vacancies, while magnesium has a strong interaction.; Yield strengths and fracture toughnesses were measured for both sheet and plate materials. Yield anisotropy was also investigated in the sheet material. Magnesium was observed to increase anisotropy, especially in the T8 temper. Models were used to explain the experimental data in terms of texture and precipitate distribution.