Study of Clustering and Precipitation Behavior in a Mg-Zn-Ca Alloy by Atom Probe Tomography

The performance of Mg-Zn-Ca (ZX-series) alloys is strongly influenced by the nano-scale clusters and precipitates formed during heat treatments. To fully understand the clustering behavior of solute elements and the formation of Zn-Ca clusters and precipitates, atom probe tomography (APT) and related analysis were applied to a ZX21 [Mg-2.15Zn-0.97Ca (wt%)] alloy at various heat treatment stages. Following solution heat treatment at 520°C, the supersaturated solid solution matrix composition is Mg-1.23Zn-0.29Ca (wt%), which is different from the bulk composition because of the residual Zn- and Ca-containing second phases. During subsequent aging at 175°C, the nearest neighbor distributions reveal that Zn clustering occurs before Ca, with Ca subsequently joining to form Zn-Ca co-clusters. Alloy hardness increases substantially in the early stage of aging due to the formation of densely distributed Zn-Ca clusters. The peak hardness is achieved after 25 h of aging, corresponding to the evolution of Zn-Ca clusters into the Guinier–Preston (G.P.) zones, which overcomes the decrease in cluster number density and weakening in solid-solution strengthening. Prolonged aging over 25 h leads to a decline in alloy hardness due to the transformation of the G.P. zones into thermodynamically more stable Ca-Mg-Zn precipitates and the decrease in precipitate number density.