Dendrite Morphology Evolution of Al₆Mn Phase in Suction Casting Al-Mn Alloys

The effects of solute element content and cooling rate on the morphology of Al₆Mn phase in suction casting Al-Mn alloys were investigated by transmission electron microscope, scanning electron microscope, and X-ray diffractometer. Results show that Al₆Mn dendrite morphology with different degrees of development can occur in the microstructure of as-cast Al-Mn alloys. For the Al-4 wt.% Mn alloy, there are small amounts of block Al₆Mn crystals at the center of sample, while we see a block Al₆Mn phase and a feathery Al₆Mn phase in the sample of Al-6 wt.% Mn alloy. Moreover, the block Al₆Mn phases in the Al-8 wt.% Mn alloy disappear, and only snowflake-like Al₆Mn phases play a dominant role in the microstructure. However, with an increase in Mn content to 10 wt.%, more dendritic trunks are formed, and secondary dendrite arms are degraded more seriously due to the formation of an icosahedral quasicrystal in suction casting. In addition to the effect of Mn content on Al₆Mn morphology, with the increase in cooling rate from the center to the edge of samples, the outline diameter of equiaxed dendrite decreases. The evolution of Al₆Mn dendrite morphology and the formation of quasicrystal are further discussed.