Effect of extrusion ratio on microstructure evolution and mechanical properties of ultra-high strength Al-Zn-Mg-Cu-Zr-Sc alloy

Ultra-high strength Al-10.8Zn-2.6 Mg-Cu-Zr-Sc alloy rods with three different extrusion ratios (9,16,25) were prepared by extrusion process. The effects of extrusion ratio on microstructure evolution, recrystallization behavior, texture characteristics and mechanical properties of the alloy were studied. With the increase of extrusion ratio, the grain structure of the alloy gradually changes from the uneven distribution of wide and narrow bands with extrusion ratio of 9 to the alternating distribution of wide banded grains and fine grain zones with extrusion ratio of 16, and finally presents a uniform wide banded structure at extrusion ratio of 25. The Tphase (AlZnMgCu) particles are effectively broken with the increase of extrusion ratio. In terms of recrystallization behavior, all samples are dominated by dynamic recovery (DRV) mechanism, and dynamic recrystallization (DRX) occurs in local areas. The samples with extrusion ratio of 9 are prone to continuous dynamic recrystallization (CDRX), while the samples with extrusion ratio of 16 and 25 are mainly discontinuous dynamic recrystallization (DDRX), and there are recrystallized grains caused by particle excitation nucleation mechanism. Texture analysis shows that all samples are dominated by copper texture, S texture and brass texture. With the increase of extrusion ratio, the intensity of Bass texture increases, while the intensity of Cube texture increases first and then decreases. In terms of mechanical properties, with the increase of extrusion ratio, the tensile strength and elongation of the alloy increased first and then decreased. The sample with extrusion ratio of 16 showed the best performance, and the tensile strength and elongation were more than 590 MPa and 10 %, respectively. In the peak-aged state, the mechanical properties of the alloy continue to increase with the increase of the extrusion ratio. The sample with an extrusion ratio of 25 exhibits the highest tensile strength (836 MPa) and yield strength (835 MPa). This is mainly due to the strong plastic deformation during the extrusion process, which refines the grains, breaks up the T-phase particles, and promotes the precipitation of higher volume fraction of precipitation strengthening phase.