Spinodal decomposition in Al/Zn alloys

The effect of spinodal decomposition on the mechanical behaviour of Al/Zn alloys was studied over the range of 30 to 60 wt % zinc. Two solution treatment temperatures, 365 and 435° C, were used; extensive ageing studies were carried out at 22 and 100° C, and limited tests were made on samples aged at other temperatures, 0, 55, and 200° C. The yield and tensile strengths were significantly increased by spinodal transformation, but ductility was seriously impaired. The tensile fracture was intergranular, with one exception, and was related to grain-boundary precipitation and a narrow denuded zone.Calculated yield strengths based on Cahn's analysis did not agree with those derived from the correct dislocation model. The wrong model did give fortuitous agreement. Spinodal hardening appears to offer a promising new hardening mechanism in aluminium alloys if the particular composition and treatment can be found to eliminate the serious lack of ductility.