FORMATION OF METASTABLE AND EQUILIBRIUM PHASES DURING MECHANICAL ALLOYING OF AL AND MG POWDERS

Formation of metastable and equilibrium phases during mechanical alloying by high-energy ball milling of mixtures of Al and Mg powders was studied using X-ray diffractometry (XRD) and differential scanning calorimetry (DSC). Different phases were formed, depending on the overall composition of the starting mixture. A powder mixture of nominal composition 40 at. pct Mg gradually becomes converted into a metastable supersaturated Al(Mg) face-centered cubic (fcc) solid solution having approximately 23 at. pct Mg in solution. Powder mixtures of nominal composition of 60 or 80 at. pct Mg gradually transform into the equilibrium gamma phase during mechanical alloying, but for the composition of 80 at. pct Mg, some unalloyed Mg is left. Mechanical alloying is comparable to rapid solidification in producing metastable phases in the Al-Mg system, except that mechanical alloying is likely to leave some residual unalloyed elements. There is no indication of the formation of the other equilibrium phase, the beta phase, present in the phase diagram. The reason why the beta phase does not form is thought to be related to the complex structure and a very large unit cell associated with this phase. However, the beta phase is obtained if the mechanical alloyed powders are heat-treated at higher temperatures.