Influence of the supersaturated silicon solid solution concentration on the effectiveness of severe plastic deformation processing in Al-7 wt.% Si casting alloy

A comparative study of room temperature severe plastic deformation (SPD) of a hypoeutectic Al-7 wt.% Si casting alloy by high pressure torsion (HPT) and equal channel angular pressing (ECAP) has been performed. Microstructural parameters and microhardness were evaluated in the present work. Three different initial Si solid solution contents have been considered: as cast (C sample, 1.6 wt.% Si), annealed and quenched (Q sample, 1.2 wt.% Si) and annealed and furnace cooled (S sample, 0.7 wt.% Si). The samples processed by ECAP have smaller average Si particle sizes (0.9-1.7 μm), than those for samples processed by HPT (2.4-4.4 μm). The initial supersaturated Si solid solution is the major factor affecting the microstructure and the mechanical properties of the material. Fine deformation-induced Si precipitates from the supersaturated solid solution were responsible of the large grain refinement obtained by both SPD processing methods, which was considerably higher than that reported for pure aluminium. Q samples, processed by both SPD methods, containing an intermediate concentration of Si in solid solution, show the highest hardness due to the finest and most homogeneous microstructure. The finest and homogeneous grain size was ∼0.2 μm for the HPTed and ∼0.4 μm for the ECAPed Q samples.