Microstructural evolution and microhardness change of Al-7wt%Si-0.3wt%Mg alloy granules/powder particles during high energy ball milling

Al-7wt%Si-0.3wt%Mg alloy granules produced by mechanical crushing of machining chips were subjected to high energy ball milling. It was found that ball milling significantly reduced the sizes of eutectic Si particles from micrometer level to submicrometer level, changed their morphology from being faceted into being rounded, and caused formation of stacking faults in them. Ball milling also resulted in a drastic change of the Al(Si,Mg) matrix by increasing its Si content from 0.16 to 1.81 at%, and reducing its average grain size to 45 nm. These microstructural changes of the powder particles lead to a dramatic increase of the microhardness of the powder particles to 187 HV, which is more than twice that of the original granules. Analysis of the contributions of various strengthening mechanisms shows that the major contribution to the microhardness increase comes from grain refinement.