CHANGE IN THE FINE STRUCTURE OF ALLOY V-93 DURING AGING.

The chemical composition of the alloy was as follows: 6. 5-7. 3% Zn, 1. 6-2. 2% Mg, 0. 8-1. 2% Cu, less than equivalent to 0. 2% Si, 0. 2-0. 4% Fe. Alloy V-93 differs from other alloys of the Al-Zn-Mg-Cu system in that it contains Cr, Mn, and Zr, and therefore has no intermetallic precipitates within the grains. A high dislocation density was observed in large parts of quenched alloy V-93, which indicates high internal stresses due to quenching. The distribution of dislocations in pile-ups against the grain boundaries is unfavorable, since locally high stresses are created in the grain boundaries. Quenching dislocations are completely annihilated in the second stage of aging at 185 degree for 6-8 h and are pinned by particles of second phase after two-stage aging at 120 degree for 3 h plus 175 degree for 4 h.