Influence of the thermal treatment on the deformation-induced precipitation of a hypoeutectic Al–7 wt% Si casting alloy deformed by high-pressure torsion

A hypoeutectic Al–7 wt% Si alloy was subjected to high-pressure torsion (HPT) at room temperature using an imposed pressure of 6 GPa and torsional straining through five revolutions. Different thermal treatments, prior to the HPT processing, resulted in reducing supersaturated Si concentration in comparison to the as-cast material. Microstructural parameters and microhardness were evaluated in the present work. Processing by HPT produced significant grain refinement with grain sizes of about 200–400 nm. Furthermore, fine deformation-induced Si precipitates from the supersaturated solid solution were observed, which are very useful in retaining the fine microstructure during HPT processing. The microhardness increase was outstanding, with values for processed samples twice higher (84 HV) than that for the as-cast Al–7 wt% Si alloy (44 HV). The results demonstrate that the refining and strengthening of the Al matrix by HPT processing depend on the available supersaturated solid solution during the processing.

Research highlights▶ The objective of this work was to characterize the microstructural refinement and strengthening of an Al–7 wt% Si processed by HPT, considering the influence of the initial Si concentration in supersaturated solid solution. ▶ Severe deformation produced precipitation of fine particles of silicon, which were very useful in retaining fine microstructures (∼200 nm in grain size) during HPT processing. ▶ The refining and strengthening of the Al matrix depend on the available supersaturated solid solution during the HPT processing.