A study was done on the peripheral-recrystallized structures formed in Al-Zn-Mg-Cu-Zr alloy materials during extrusion and on their quenching sensitivity. It was found that the peripheral-recrystallized structures were thicker at the head and tail sections of the extrusion than at the other sections. The further downwards towards the tail, the greater the thickness of the structures became. The peripheral-recrystallized structures had a remarkable quenching sensitivity and under some heat treatment conditions, attained only half of the hardness of the fiber structure. Even with the same fiber structure, the extrusion (material) tail had a higher quenching sensitivity than that of the head portion. It is thought that the drop in strength that is encountered frequently in the proximity of the tail portion of the extrusion during actual production of this type of alloy may occur under the same mechanism as controls the above-described phenomenon, and that the phenomenon of the sharpening of sensitivity to quenching is caused by the change of Al3Zr from a precipitate coherent with the matrix to a precipitate incoherent with the matrix, resulting from the strain during extrusion. The incoherent Al3Zr precipitate is thought to become an eta-phase precipitation site. This zone of sharp quenching sensitivity responds to the surface or to the tail, which latter undergoes a large plastic strain during extrusion. In the section undergoing plastic strain, the quenching sensitivity is especially acute, and if an adequate quench rate cannot be achieved during solution treatment, such a high quenching sensitivity can be a cause of an insufficient increase in hardness after ageing. Most billet homogenizing treatments on this alloy are done at temperatures of about 450-degrees-C-480-degrees-C. However, it is shown that when a lower temperature of 400-degrees-C is selected as a homogenizing treatment parameter, the sharp sensitivity to quenching can be restrained.
