Influence of microstructural stability on the creep mechanism of Al-7Â wt% Si alloy processed by equal channel angular pressing

A Na-modified, as-cast Al-7 wt% Si alloy was processed by equal channel angular pressing (ECAP) up to 8 passes by route A at ambient temperature using a 90° square section die, obtaining improved strength, ductility and work of fracture. From the first pass, porosity is removed, the eutectic constituent is refined and the eutectic silicon particles are partially redistributed. Additionally, a fine and homogeneous strain-induced silicon precipitation occurs in the supersaturated solid solution retained in the casting. These fine precipitates assist in grain refinement, resulting in a 250 nm grain size after one pass and 210 nm after 8 passes. This microstructure cannot sustain grain boundary sliding because it coarsens rapidly even at the lowest testing temperatures. Deformation at high temperatures gives values of n of about 8 and values of the activation energy corresponding to the self-diffusion of aluminum, 142 kJ/mol, which can be rationalized by a constant substructure slip creep mechanism. These values are influenced by the presence and evolution of the fine intradendritic silicon precipitates. Coarsening of these precipitates with time and temperature increases their interparticle distance causing variations in experimental n and Q values.