Microstructure homogeneity of milled aluminum A356-Si3N4 metal matrix composite powders

A metal matrix composite was produced by co-milling an A356 aluminum alloy powder obtained by rotating electrode off-equilibrium solidification, with different mass fractions (10, 20 and 30%) of Si3N4. The structural and microstructural modifications occurring during the milling were investigated with X-ray powder diffraction (XRPD). Whole powder pattern modeling (WPPM) of the XRPD reveals the inhomogeneous nature of the material in terms of silicon content and allows the crystallite size distribution and dislocation content to be followed in detail for all phases present in the powder. Neither microscopy nor the traditional Scherrer equation can reveal such a detailed picture in this case. Short milling times are sufficient to homogenize the microstructure and to obtain nanoscale crystallites. Long milling times are advantageous to increase the dislocation density that might be favorable for subsequent sintering.