Effects Of Strontium And Calcium Additions On The Microstructures And Mechanical Properties Of As-cast AZ91 Magnesium Alloy

In recent years, the application of Magnesium alloys as the light structural materials has been expanding in the automotive and electronics industries due to their low intensity, high specific strength and stiffness, excellent machinability. As a typical cast magnesium alloy in the Mg-Al system, AZ91 offers a combination of castability and corrosion resistance, as well as low cost. However, the newly developed Mg alloys based on AZ91 have the disadvantages of limited strength, high cost, low ductility, hot shortness and sticking tendency, which hinder their applications in industries.In the present work, microstructures, mechanical properties, solidification characteristics as well as alloying mechanisms of AZ91 based alloys with alkaline earth additions of Ca and Sr have been investigated by optical microscope (OM), scanning electron microscope (SEM), X-ray difractometer(XRD), diferential thermal analysis (DTA), etc.In the AZ91 based alloys with additions of Sr, the crystal lattice constant ofα-Mg increased due to the solution of Strontium. With the amount of strontium addition exceeding 0.1wt.%, the intermetallic compound Al4Sr was observed in the grain boundaries with the rob shaped or blocky morphology. The melting temperature of theβeutectic phase hardly increased remarkably after adding Sr. Failure of the AZ91 alloys with Sr addition was brittle through quasi-claevage fracture, microcracks formed in theβ/Mg interface and eutectic structures and finally expanded along the fragile grain boundaries. The suitable Sr addition (0.1～0.2 wt.%) to AZ91 lead to the decrease of volume and size ofβ-phase as a result of superssing theβ-phase growth. The distribution of Al4Sr might pin the grain boundaries and prevent grain-boundary slipping, thereby strengthening the alloy.A certain percentage of Sr addition to AZ91 containing 1.3 wt.%Ca formed Al4Sr,Mg2Ca and Al2Ca precipitation, most of which distributed in and near theβphase, few in the matrix. The mechanism of fracture was quasi-cleavage, with a mixture of cleavage and ductile dimple. The melting temperature of theβeutectic phase increased by more than 30℃after Ca and Sr were added to AZ91, indicating a positive effect of the Ca additions to thermal stability of the alloy. But the melting temperature can hardly increase by further increasing the quantity of Sr added. The proper Sr addition to AZ91 containing Ca might refine the dendritic structure, suppress the tendency of divorce eutectic, postpone crack initiation and propagation resulting from pinning effects of the intermetallic particles. Consequently, the reinforced AZ91 containing Ca and Sr with improved microstructure and mechanical properities can be obtained.