Effects of Melt Superheating on the Microstructure and Tensile Properties of a Ternary Al-15 Wt Pct Si-1.5 Wt Pct Mg Alloy

Unsteady solidified microstructures of the Al-15 wt pct Si-1.5 wt pct Mg alloy under two degrees of melt superheating are examined: 4 and 21 pct above the alloy liquidus temperature. The dendritic array and the eutectic mixture have been investigated, which are affected not only by the solidification kinetics but also by the melt superheating. The directional solidification experiments permitted that aligned Al-rich dendrites could be formed for both high- and low-melt superheatings. These dendrites appear as ‘islands’ isolated from each other and dispersed within the α-Al+Si+Mg₂Si eutectic matrix. Such configuration arises in samples under the entire range of examined cooling rates, i.e., from 0.5 to 50 K/s. Directionally solidified samples having different α-Al dendrite interphase spacings have been characterized and subjected to tensile tests. Such microstructural spacings translate the effectiveness in blocking the dislocations motion during loading, which is promoted mainly by the dendritic/eutectic boundaries. This mechanism is especially operative since the applied tensile load is perpendicular to the dendritic growth path. As such, the evolution of tensile properties as a function of these spacings was assessed. Both elongation (δ) and the ultimate strength (σ_u) are enhanced nearly by 40 pct due to the reduction in the dendrite interphase spacing. The highest properties (σ_u: ~ 310 MPa and δ: ~ 7 pct) are associated with a microstructure formed by aligned dendrites with 70 μm in spacing embedded in a eutectic mixture. Three factors appear to contribute to the higher strength values observed for the alloy processed under lower-melt superheating, i.e., higher fraction of dendrites, solid solution strengthening of Mg in α-Al, and finer eutectic Si spacing.