Effect of dispersed Al₃Zr and Al₃(Sc, Zr) particles in 7000 series aluminum alloys on microstructure formation during hot extrusion

It is well known that Al₃Zr and Al₃(Sc, Zr) particles precipitate in the matrix by adding Zr and Sc to aluminum alloys and these particles inhibit a static recrystallization by pinning the grain boundary. In this study, 1.33%Zr, 0.4%Sc-0.8%Zr and 0.8%Sc-0.4%Zr (in mass%) were added to 7000 series aluminum alloys in supersaturation by using powder metallurgy. By applying rapidly solidified powders of these alloys to a heat treatment at 773 K, Al₃Zr and Al₃(Sc, Zr) particles were precipitated in high density. 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 μm were formed. Continuous DRX is generated since the particles pin the grain boundaries and the mobility of the grain boundary lowers. So, the number of DRX grains are strongly related to the pinning force calculated by the average diameter of Al₃Zr and Al ₃(Sc_0.5, Zr_0.5) particles precipitated in 1.33%Zr added alloy and 0.4%Sc-0.8%Zr alloy, respectively. Though, in 0.8%Sc-0.4%Zr added alloy, since Al₃(Sc_0.5, Zr _0.5) and Al₃(Sq_0.86, Zr_0.14) particles were precipitated concurrently, the diameter of particles exhibits a bimodal distribution and shows a widely dispersed distribution. As a result, the pinning force can't be accurately calculated by using the average diameter of particles and the number of DRX grains are weakly related to the calculated pinning force in 0.8%Sc-0.4%Zr added alloy.