Effect of dispersed Al₃Sc particles in P/M 7000 series aluminum alloys on continuous dynamic recrystallization during hot extrusion

Powder metallurgy was used to supersaturate a 7000 series of aluminum alloys with 0.4, 0.8 and 1.5 mass% Sc. Al₃Sc particles were precipitated in high density upon applied heat treatment at 773 K. During subsequent hot extrusion, continuous dynamic recrystallization (DRX) occurred and fine DRX grains with a diameter of 1 μm formed. The 1.5 mass%Sc alloy achieved the greatest number of DRX grains. Interestingly, the number of DRX grains observed for the 0.8 mass % Sc alloy was almost the same as that observed for the 0.4 mass% Sc alloy, although the amount of Sc added was two times larger. Continuous DRX occurs only under conditions in which the grain boundary mobility is low; therefore, the number of DRX grains is strongly related to the pinning force on the initial grain boundary exerted by Al₃Sc particles. When the pinning force was calculated from the diameter of Al ₃Sc particles (measured by TEM), the number of DRX grains was determined to be proportional to the calculated pinning force. However, in the case of 0.8 and 1.5 Sc alloys, the Al₃Sc particle diameter was twice that obtained at the maximum pinning force (d_max). Thus, it is possible to promote continuous DRX by decreasing the Al₃Sc particle diameter in 0.8 and 1.5 Sc alloys. Lowering the rate of heating prior to heat treatment for degassing was found to reduce the critical nucleus size for precipitation of Al₃Sc particles as well the diameter of Al ₃Sc particles. Thus, the pinning force increased in 0.8 Sc and 1.5 Sc alloys, and the number of DRX grains also increased, as expected.