EFFECT OF SCANDIUM ON THE DECOMPOSITION OF ALUMINUM ZINC-ALLOYS

The effect of scandium on the hardness, structure and heat evolution of an aluminium-40% zinc alloy was investigated. It was found that contrary to its action in other aluminium-based alloys, scandium does not raise the maximum hardness during an ageing treatment. This may be explained by insufficient scandium redissolution at the rather low temperature necessary for achieving a solid solution of zinc in aluminium. Differential scanning calorimetry studies show a similar heat evolution in both alloys upon linear heating, starting with the reversion of modulated structures. An exothermal effect appears only in the scandium-bearing alloy due to increased heterogeneous precipitation of alpha(r)' and beta on Al3Sc particles. Above 200-degrees-C, the dissolution of the alpha(r)' and beta phases starts, followed by the monotectoid reaction.