Quench rate effects in Al-Zn-Mg-Cu alloys

The effect of quench rate on alloys with a bcomposition of 6.75 pct Zn, 2.5 pct Mg, 1.2 pct Cu and various levels and combinations of the minor addition elements (MAE’s) Cr, Mn, and Zr is reported. The level of chromium and to a lesser extent the level of zirconium determine quench sensitivity. Slow quenching of chromium- or zirconium-bearing alloys led to the nucleation of (MgZn) plates or laths at MAE-bearing particles which had certain orientation relationships with the matrix. The density of the hardening precipitation (100 to 200Å in diam) was found to be high in slowly quenched, sensitive alloys. Even when the yield strength had fallen to 50 pct of that of the fast-quenched material, a large fraction of the fine precipitate was still formed. A fracture mechanism is proposed to account for the magnitude of quench sensitivity in such materials. An embrittlement, at low quench rates, was found in alloys containing manganese or very low levels of MAE. This embrittlement was thought to be due to precipitation in the grain boundaries, and not to the width of the precipitate-free zone.