Slow and rapid cooling of Al-Cu-Si ultrafine eutectic composites: Interplay of cooling rate and microstructure in mechanical properties

Ternary Al-15 wt% Cu-7 wt% Si and Al-22 wt% Cu-7 wt% Si alloy specimens were generated by transient directional solidification (DS) and rapid solidification (RS) techniques. The microstructures are constituted by an alpha-Al dendritic matrix surrounded by two eutectic, that is, a binary eutectic (Si + alpha-Al) and a bimodal eutectic, consisting of cellular-type binary eutectic colonies (alpha-Al + Al2Cu) in a ternary eutectic matrix consisting of alpha-Al + Al2Cu + Si. The bimodal eutectic exists at cooling rates from 0.5 to 250 K/s. The secondary dendritic spacing, lambda(2), of the DS samples varied from 5 to 20 mu m and from 10 to 18 mu m for both examined alloys. The lambda(2) from 2.7 to 4.0 mu m characterized the RS samples. Mechanical properties have been determined for various samples related to different dendritic spacing values. Based on the evaluation of the rapidly solidified microstructures, it was possible to assess the cooling rates.