EFFECTS OF Sr ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF Mg–9Al–1Si–0.3Zn ALLOY

Abstract

Mg-Al-Si alloys (AS series alloys) with a high content of silicon performed low ductility and strengths because of large amount of coarse Chinese scripts Mg2Si particles distributed in the matrix. Trace elements, such as Ca, P, Sr and RE were selected to modify the morphology of Mg2Si particles. Among these, Sr showed remarkable modification effect, however, the modification mechanism was not clarified yet. In this work, the effects of Sr on the morphology of Mg2Si in Mg-9Al-1Si-0.3Zn (mass fraction, %) alloy and the mechanical properties were investigated, and the modification mechanism was explained by comparing Mg2Si morphologies. The Mg-9Al-1Si-0.3Zn alloy was composed of primary hopper Mg2Si particles of 40 µm in size, coarse Chinese scripts Mg2Si of 50—80 µm, island-like Mg17Al12 and the �-Mg matrix. 3D hopper crystals and Chinese scripts Mg2Si particles were extracted by the electrochemical method. Both morphologies reveal the corner effect which leads to the preferential growth orientation of edges and corners of the octahedron Mg2Si particle, respectively. When Sr was added into the alloy, the growth rates of both edges and corners of the Mg2Si particles were depressed significantly due to the adsorption of Sr on the surface of Mg2Si particles. When Sr addition went up to 0.32%, Mg2Si phase was fully modified to small polygonal particles of 5—20 µm. Moreover, the grain size of the alloy decreased from 210 µm to 160 µm with Sr increasing, which is in accord with the GRF (growth restriction factor) mechanism. The increasing of mechanical properties is mainly attributed to the refinement of Mg2Si phase.