Effect of dispersed Al3Sc particles on dynamic recrystallization in P/M 7000 series alloys

Abstract

Al3Sc particles are well known to be a recrystallization inhibitor in Al alloys. In this study, 0.4, 0.8 and 1.5 mass% Sc were added to 7000 series Al alloys in supersaturation by using powder metallurgy. By subjecting rapidly solidified powders of these alloys to a heat treatment at 773 K, Al3Sc particles were precipitated in the matrix. Subsequently, hot extrusion was carried out at 773 K. During extrusion, continuous dynamic recrystallization (DRX) occurred and fine DRX grains with a diameter of 1 mm were formed. The number of DRX grains was the largest in the 1.5Sc alloy. Even though the amount of Sc added in this case was two times larger, the number of DRX grains in 0.4Sc was almost the same as that in 0.8Sc. Since continuous DRX occurs only under conditions where the grain boundary mobility is low, the number of DRX grains is strongly related to the pinning force on the initial grain boundary exerted by Al3Sc particles. The pinning force varied with the diameter and volume fraction of Al3Sc particles. When the pinning force was calculated from the diameter of Al3Sc particles, which was measured by TEM, it was proven that the number of DRX grains was proportional to the calculated pinning force. Except in the case of 0.4Sc, the Al3Sc particle diameter was twice that obtained at the maximum pinning force (dmax). It is possible to promote continuous DRX by decreasing the Al3Sc particle diameter in 0.8 and 1.5Sc alloys. Lowering the heating rate before heat treatment for degassing reduced the critical nucleus size for precipitation of Al3Sc particles as well the diameter of Al3Sc particles. Thus, the pinning force increased in 0.8Sc and 1.5Sc alloys, and the number of DRX grains also increased, as expected.