Evolution of precipitation during non-isothermal ageing of an Mg-Ca-Zn alloy with high Ca content

In this work, the precipitation behaviour of an Mg-Ca-Zn alloy with high content of Ca relative to Zn was studied. Differential scanning calorimetry (DSC) was combined with transmission electron microscopy (TEM) and hardness measurement to examine the precipitate evolution in this alloy. A non-isothermal age-hardening heat treatment was performed to allow for a direct comparison to DSC results. Thermodynamic analysis aided in identifying ageing heat treatments to further de-convolute the overlapping DSC data. Results suggested the precipitation events followed sequential evolution towards the formation of two types of equilibrium phases (i.e. Mg2Ca and Mg6Ca2Zn3). Early stage decomposition of the solid solution had no observable effect on hardness, but was followed by the formation of hardening GP zones. Fine basal plates, as well as large coarse basal plates of Mg2Ca-type formed on further ageing. Contrast due to fine unidentified nanoscale precipitates was also observed in the over-aged state. These precipitates tended to disappear, while blocky Mg2Ca equilibrium phase precipitates formed as the heat treatment progressed to the highest temperature studied (i.e. 300 degrees C). However, the equilibrium Mg(6)Ca(2)Zn(3)phase was not observed when this final test temperature was achieved at the end of the non-isothermal heat treatment process. (C) 2012 Elsevier B.V. All rights reserved.