Microstructure and Mechanical Properties of Alumina-Dispersed Magnesium Fabricated Using Mechanical Alloying Method

In order to improve the mechanical properties of magnesium, dispersion strengthening with α-alumina particles (pAl2O3) is experimentally investigated as an application of powder metallurgy. The trial process consists of milling, compacting and hot-pressing. The microstructure of hot-pressed discs of magnesium composites were investigated using optical microscopy, scanning electron microscopy, x-ray diffraction (XRD) and electron probe micro analysis. The density, surface hardness, and maximum bending stress for deflection were also measured. All of the mechanically alloyed (MA) prepared powders were composed of only magnesium (Mg) and alumina (Al2O3). Although the particle size of the MA powders varied, the mean values were approximately 80 μm and were approximately the half size of the raw Mg powder. Not only Mg powder, but also pAl2O3 became finer with processing, and the pAl2O3 was almost uniformly dispersed in the Mg powder. In addition, the fine pAl2O3 was almost uniformly dispersed within the Mg of the pAl2O3 dispersion strengthened (ODS) magnesium discs. For all discs, a small quantity of magnesium oxide (MgO) was identified along with Mg and Al2O3. However, in only the 22.7 vol% pAl2O3/Mg disc, an XRD peak assigned to an Al-Mg intermetallic compound (Al12Mg17) was detected, in addition to Mg, Al2O3 and MgO. It is proposed that Al12Mg17 was produced by the solid-state reaction of Mg and Al2O3, and appeared at the interface between the regions of only Mg and regions where pAl2O3 is dispersed in Mg. The density of the discs was above the theoretical density for all pAl2O3 content; the density for the highest pAl2O3 content of 22.7 vol% was approximately 0.8 times greater than that of practical Al alloys. The 22.7 vol% pAl2O3 disc had a maximum hardness value of 280 HV. This value is much higher than that of both pure Mg ingot and AZ91D. The maximum bending stress decreased with an increase in the pAl2O3 content. The reason for this is considered to be that the discs become harder and more brittle, and voids are easier formed in the discs. Therefore, cracks that are generated on the specimen surface propagate easier.