Impact of Mn on the precipitate structure and creep resistance of Ca containing magnesium alloys

摘要

The reason for the enhanced creep resistance of Ca-modified magnesium alloys is investigated based on their microstructure and precipitate morphology. It is known from literature that the creep resistance of commercial AZ91 is improved by the addition of Calcium through the formation of a skeleton-like intermetallic phase (IP) at the grain boundaries. Compared to AZ91 + Ca, the alloy MRI230D, which is a Mg-Al-Ca-Sn based alloy with similar Ca content, shows a significantly better creep resistance. This is probably caused by small precipitates in the alpha-Mg grain interior that were found in earlier TEM investigations of as-cast MRI230D alloy. For this work, we have investigated the evolution of the precipitate structure from as-cast to crept conditions via atom probe tomography (APT). Based on the APT data, an intensive cluster analysis for all deformation states was performed using Voronoi cluster search algorithm. Cluster analysis revealed that Ca atoms show an especially high grade of clustering in as-cast conditions. This changes in the crept states, where co-clustering of Ca and Mn atoms was observed. Further analysis revealed that the number density of clusters is generally higher in MR1230D than in AXZ931 and clusters tend to be smaller and enriched with Mn. All of this led to the conclusion that a minimum amount of Mn is necessary for a stabilization of the clusters in the Mg matrix, leading to an improvement of the creep resistance.