Effect of milling time on microstructures and hardness of nanocrystalline Al-7Si-0.3Mg alloy powders

Al–7Si–0.3 Mg alloy powders produced by mechanical crushing were subjected by high energy ball milling in different times, the optical microscopy(OM), X-ray diffraction analysis(XRD), transmission electron microscopy(TEM), and microhardness test were used to investigate the effect of milling time on the microstructures and hardness of Al–7 Si–0.3 Mg alloy powders. The results show that, the sizes of eutectic Si particles significantly reduce from the micrometer level to the submicrometer level by high energy ball milling, the particle shape changes from the polyhedron to the circular, and the stacking faults appear in the Si particles. With increasing the milling time to 60 h, the average particle size of the alloy powders is reduced from 134 μm to 22 μm, the grain size of the Al(Si, Mg) matrix is reduced from 438 nm to 23 nm, and the microhardness of the powders is increased from HV 93 to HV 289. The increase of hardness is mainly attributed to the grain refinement caused by ball milling, which results in a strong fine grain strengthening effect. The refinement of silicon particles during ball milling and the increase in solid solubility of Mg and Si atoms in the matrix caused by ball milling are also contributed to the hardness increase.