Microstructural characterisation and thermal stability of a metastable Mg-8.6 wt.% Zr alloy produced by physical vapour deposition

Abstract

Abstract The thermal stability of a Mg-8.6 wt.% Zr alloy processed by physical vapour deposition has been studied using differential scanning calorimetry and transmission electron microscopy. The alloy, in the as-deposited condition, is a solid solution of Zr in the Mg matrix. The microstructure is characterised by elongated grains oriented with their [0001] direction parallel to the growth direction. After DSC experiments, three exothermic transformations have been observed. The first one takes place above the deposition temperature and is related to the recovery phenomena due to stress relaxation generated during the growth process. The second transformation corresponds to the breakdown of the solid solution by the precipitation of Zr in the Mg matrix. This reaction takes place in two stages: (i) Zr plates are formed in the basal plane and (ii) these initial precipitates grow along the [0001] direction. An epitaxial relationship between those Zr precipitates and the Mg matrix of [0001]Zr || [0001]Mg and [1120]Zr || [1120]Mg has been obtained. The third exothermic transformation appears above 720 K and is caused by the strong oxidation of the deposit.