Effect of isothermal ageing on the Zn-Al alloys creep characteristics

The aim of this study is to correlate the creep parameters of aged Zn-Al samples with the microstructure developed during heat treatment, and to point out the practical interest of such microstructure modifications. Two different isothermal ageings were carried out at 100°C and at 250°C for different periods of times (from 30 min. to 8 weeks). During those ageings, complex precipitations occur. The decomposition of the initial supersaturated solid solution begins with a cellular reaction leading to cfc and hcp lamellar. The cfc phase decomposes into transition phases. The ageing temperature (100 or 250°C) influences the decomposition stages. Creep testson aged samples reveal the influence of the microstructure developed during the heat treatment at 100 or 250°C. An improvement of the creep properties by one order of magnitude can be observed when the sample exhibits the appropriate structure.