ON THE EFFECT OF STRESS ON NUCLEATION AND GROWTH OF PRECIPITATES IN AN AL-CU-MG-AG ALLOY

A study has been made of the effect of an externally applied tensile s tress on Omega and Theta' precipitate nucleation and growth in an Al-C u-Mg-Ag alloy and a binary AI-Cu alloy which was used as a model syste m. Both solutionized and solutionized and aged conditions were studied . The mechanical properties have been measured and the microstructures have been characterized by transmission electron microscopy (TEM). Th e volume fraction and number density, as well as the precipitate size, have been experimentally determined. It was found that for as-solutio nized samples aged under stress, precipitation occurs preferentially p arallel to the stress axis. A threshold stress has to be exceeded befo re this effect can be observed. The critical stress for influencing th e precipitate habit plane is between 120 and 140 MPa for Omega and bet ween 16 and 19 MPa for Theta' for the aging temperature of 160 degrees C. The major effect of the applied stress is on the nucleation proces s. The results are discussed in terms of the role of the lattice misfi t between the matrix and the precipitate nucleus.