3-D morphology and growth mechanism of primary Al6Mn intermetallic compound in directionally solidified Al-3at.%Mn alloy

3-D morphology and growth mechanism of primary Al6Mn intermetallic compound have been investigated in directionally solidified Al-3at.%Mn alloy at a low growth rate of 1 μm/s. A faceted growth of primary Al6Mn with strong anisotropy has been observed. Its 3-D morphology exhibits elongated, hollow and distorted polyhedron with concave surfaces. This mainly depends on the crystal structure and growth conditions. Al6Mn crystals tend to form regular octahedron bounded by eight {110} planes according to the crystal structure. The nonuniform transport of heat and solute during directional solidification increases the disparity in growth rates of crystal planes and strengthens the growth anisotropy. The growth of primary Al6Mn is controlled by a screw-dislocation mechanism and terminates when nucleation of a new crystal occurs which is preferential on the concave surface. The discontinuous growth and nucleation of Al6Mn crystal lead to a final stacking structure.