Effect of temperature on synthesis and properties of aluminum-magnesium mechanical alloys

The synthesis of an Al_0.7Mg_0.3 mechanical alloy was studied using a planetary mill. Several distinct temperature regimes of mechanical alloying were achieved using milling jars equipped with finned heat sinks and an external air conditioner installed to cool the entire milling chamber. Wireless temperature sensors were attached to the milling jars to monitor the process temperature. Intermediate and final products were collected and were analyzed by electron microscopy and X-ray diffraction. The temperature history of the milling jars exhibited two peaks during mechanical alloying. The first peak occurred when particles of the starting powders deformed to produce flakes. The second peak was observed when the flakes agglomerated and re-fragmented forming layered composites that served as precursors for the mechanical alloy. The temperature of milling affected the magnesium solubility of the produced Al-Mg mechanical alloys. Decreasing the milling temperature from ∼70-80 °C to 20-30 °C resulted in an increase of the dissolved Mg concentration in Al from 2-3 at.% to ∼25 at.% for the Al _0.7Mg_0.3 composition. The formation of intermetallic phases was favored at higher milling temperatures, where high solubilities cannot be achieved.