Impact toughness of an ultrafine-grained Al-11mass%Si alloy processed by rotary-die equal-channel angular pressing

By means of rotary-die equal-channel angular pressing (RD-ECAP) a brittle casting aluminum alloy (Al-11mass%Si alloy) was transformed into a tougher material. The absorbed energy of the alloy, as measured by Charpy impact testing, increased markedly with increasing RD-ECAP passes. The maximum value of over 10 J/cm2 was obtained in the sample processed by RD-ECAP for 32 passes at 623 K, which is 10 times the value of as-cast alloy of 0.9 J/cm2. The increased impact toughness of this alloy is related to breakage of the large aluminum dendrites and interdendritic networks of eutectic silicon in the first several passes, modified grain or grain fragment boundaries, ultrafine grains or grain fragments (about 300 nm), the production of fine particles (smaller than 1 μm) and a homogenized microstructure by multipass RD-ECAP. As a result of RD-ECAP, the silicon particles were spread out more evenly. The effect of RD-ECAP processing temperature on impact toughness is minor for fewer than 12 RD-ECAP passes; however, for more than 12 passes, higher temperatures are more effective. For this Al-11mass%Si alloy, the best RD-ECAP temperature is 623 K.