Identification and description of intermetallic compounds in Mg–Si–Sr cast and heat-treated alloys

Abstract

The ternary Mg–Si–Sr system is experimentally investigated in this work. Twelve compositions located in the Mg rich corner of the ternary system are processed. The microstructure and phase content of the different alloys are analysed in the as-cast condition and after additional heat treatments. Apart from the Mg matrix, various intermetallic phases are observed. The binary compounds Mg2Si and Mg17Sr2 are identified, as well as, two ternary phases MgSiSr and MgSi2Sr.The morphology and distribution of the intermetallic phases is discussed with particular interest on the ternary intermetallics MgSiSr and MgSi2Sr for which only scarce information is available in literature. The phase stability and the stability of the intermetallic phases in these alloys are discussed. The Mg-rich side of the Mg–Si–Sr phase diagram is constructed based on descriptions of the binary phase diagrams Mg–Si, Mg–Ca and Ca–Si from literature and assuming complete solubility (i.e. a line compound) between the ternary phase MgSiSr and the binary phase Sr2Si. It is also assumed that MgSi2Sr is a stoichiometric compound. These assumptions were made based on the chemical similarities between Ca and Sr and between Si and Sn, and the similarity in crystal structures between the two studied ternary compounds of the Mg–Si–Sr diagram and the ternary compounds present in the Ca–Mg–Si and Mg–Sn–Sr system. A good agreement is found between the constructed phase diagram and the experimental results.