Microstructure, mechanical properties and fracture behavior of in-situ Al-5Mg-Al4Sr composites

In this investigation, in-situ Al-5Mg-XAl4Sr (X=0, 2.5, 5 and 7.5 wt.%) composites have been developed by the addition of Al-10Sr master alloy in Al-5Mg alloy through gravity casting method. During solidification, in situ Al4Sr particles are formed, and the morphology of the Al4Sr particle is needle or rod-like. The average alpha-Al grain size is decreased, but the sphericity of the alpha-Al grain and the volume fraction and size (length) of Al4Sr particles increase with an increase in Sr concentration in the alloy. As a result, the bulk hardness and tensile strength are increased, but the ductility is increased up to 5 wt.% of Al4Sr addition and then decreased at 7.5 wt.% of Al4Sr added composite. The overall fracture surface exhibits the mixed mode of fracture (ductile and brittle), and the proportion of the brittle mode of fracture is increased with the increase in Al4Sr concentration in the composite. Finally, a good combination of strength (UTS=371 MPa) and ductility (% elongation=16.6) is achieved in Al-5Mg-5 wt.% Al4Sr composite.