PRECIPITATION PROCESSES DURING THE EARLY STAGES OF AGING IN AL-CU-MG ALLOYS

Atom probe field ion microscopy (APFIM), transmission electron microsc opy (TEM), and microbeam electron diffraction have been used to study the early stages of precipitation in an Al-1.1Cu-1.7Mg (at%) alloy. Th is alloy exhibits a large and rapid hardening reaction upon ageing for as short as 60 s at temperatures higher than 130 degrees C. Results f rom APFIM suggest that the formation of Cu-Mg co-clusters, not resolve d in TEM, is responsible for the rapid hardening reaction. A period of constant hardness follows the rapid initial reaction, during which th e clusters develop into ultra fine rod-like precipitates, considered t o be GPB zones and precipitation of the S phase (Al2CuMg) occurs heter ogeneously on dislocations. Like the initial clusters, the GPB zones w ere found to be rich in Cu and Mg. Peak hardness was associated with t he further dispersion of S precipitates in the matrix and further grow th of GPB zones, suggesting the following precipitation sequence: soli d solution --> pre-precipitate stage --> GPB zones + S --> S.