Effect of Mg/Si ratio on the microstructure and hardness–conductivity relationship of ultrafine-grained Al–Mg–Si alloys

Ultrafine-grained Al–Mg–Si alloys with four different Mg/Si ratios (=0.75, 1.10, 1.48, and 1.94) were prepared by using extrusion and cold-drawing. The microstructural evolution during aging treatment was examined by using transmission electron microscopy. Hardness and electrical conductivity of the alloys were, respectively, measured before and after aging treatment. Experimental results showed that β^″ precipitates were dispersed within the grain interior in all the four aged alloys. Although the precipitate size was insensitive to the Mg/Si ratio, the precipitate number density as well as the inter-precipitate spacing was greatly dependent on the Mg/Si ratio. The optimized Mg/Si ratio within present work was 1.48, which led to the densest precipitation, and resulted in the highest hardness and simultaneously greatest conductivity. These findings are rationalized by considering the precipitation behaviors in the ultrafine-grained length scale and their effect on the hardness–conductivity relationship. A strengthening model was proposed for the ultrafine-grained Al–Mg–Si alloys, which accounted for the multiple contributions from grain size, dislocations, solid solution atoms, and precipitates to the hardness. The calculations were in good agreement with the experimental results. Similarly, the effect of multiple microstructural features on the electrical conductivity was also quantitatively described by adopting a model. The contribution of precipitation to both the hardness and conductivity is well demonstrated for the ultrafine-grained Al–Mg–Si alloys, which will be helpful for material design of advanced ultrafine-grained Al alloys with enhanced performance.