Relationship between hardness after furnace cooling and local strains in Al-6Zn-0.79Mg alloy

Hardness of furnace-cooled Al-6Zn-0.79Mg (mass%) alloy is larger than that of the water-quenched alloy. Just after the furnace cooling, strains remain as a high density of dislocations, leading to nanoscale local lattice distortions of the fcc matrix. This phenomenon happens in conjunction with compositional fluctuations composed of Zn-vacancy and Zn-Mg rich regions. Consequently, introduced dislocations bring about strengthening due to cooling strain hardening. The early stage of the decomposition leads to nucleation sites for the latter strengthening phases.