Faceted–nonfaceted growth transition and 3-D morphological evolution of primary Al₆Mn microcrystals in directionally solidified Al–3 at.% Mn alloy

A comprehensive understanding of the growth pattern of intermetallic compounds (IMCs) during solidification is critical to both the crystal-growth theory and its property optimization. In this article, growth pattern and three-dimensional (3D) morphology of primary Al6Mn IMC were investigated in directionally solidified Al-3 at.% Mn alloy at a wide range of growth rates. A transition from faceted (<60 µm/s) to nonfaceted growth (>100 µm/s) was observed with increasing growth rates. Correspondingly, 3D morphologies of primary Al6Mn change from a solid polyhedron to a hollow structure, and then to a dendrite. This kind of change is associated with the competitive growths of different crystal planes determined by the crystallographic anisotropy and growth kinetics of Al6Mn. A growth model based on atomic cluster attachment is proposed to reveal the growth transition, and a growth-rate ratio between different crystal planes is used to appropriately reveal the formation mechanism of different morphologies at low rates.