Morphologies of Al4Sr Intermetallic Phase and Its Modification Property upon A356 Alloys

In general, the modification performance of Al-Sr master alloys is primarily dependent upon the morphologies and sizes of the Al4Sr intermetallic phase. In this paper, the crystal structure, morphologies, sizes, hardness and elastic modulus of Al4Sr in Al-Sr master alloys prepared from variant processes were studied by means of optical metallurgical microscope, X-ray diffraction (XRD), scanning electron microscopy (SEM), and nanoindentation system. The results revealed that the microstructures and modification performance of the Al4Sr phase were related to the preparation processes. That is to say, when a “direct reaction-hot extrusion” process was used, the Al4Sr phase exhibited a homogeneous distribution in the Al matrix with small size and roundish shapes, which ensured the Al-Sr master alloy wire advantages involving high recovery, good reproducibility, no delitescence of modification, no corrosion on equipments, and good workability. However, in the case of the traditional “direct reaction” process, the Al4Sr phase was in large size with shapes of rectangular stripe and plates, which limited the Sr content increasing due to the brittleness of the Al-Sr alloy It was also found that the morphology and size of the Al4Sr phases changed during heat treatment at high temperature up to 600 °C.