High cooling rate cells, dendrites, microstructural spacings and microhardness in a directionally solidified Al–Mg–Si alloy

• Reverse dendrites > cells transition occurs for Al3 wt%Mg1 wt%Si alloy with cells prevailing for cooling rates (Ṫ) > 2 K/s.
• Mg2Si and Al–Si–Fe (Mg) IMCs vary differently with Ṫ for cells and dendrites.
• Hardness has opposite evolution with decreasing cellular and dendritic spacings.

Transient unidirectional solidification experiments have been carried out with an Al–3 wt%Mg–1 wt%Si alloy under cooling rates (Ṫ) in the range 0.2–45 K/s. A reverse cells > dendrites transition is shown to occur with the high-cooling rate cellular region associated with Ṫ > 2 K/s and the dendritic region with Ṫ < 0.8 K/s. Experimental growth laws correlating the cellular and dendritic spacings with the cooling rate are proposed. It is shown that the microhardness is directly influenced by both morphologies of the Al-rich matrix and by the relative fractions of Mg2Si and Fe bearing intermetallics that vary differently with the cooling rate.