Effects of Sc and Zr microalloying additions on the microstructure and mechanical properties of new Al–Zn–Mg alloys

Abstract

Effects of Sc and Zr microalloying additions on the microstructure and mechanical properties of new Al–Zn–Mg alloys alloyed with a small amount of copper were investigated comparatively by tensile tests and microscopy methods. Compared with the strength of peak-aged Al–Zn–Mg alloy, the yield strength increased by 66MPa after adding 0.10wt.% Sc and 0.10wt.% Zr, and improved by 96MPa after adding 0.25wt.% Sc and 0.10wt.% Zr, respectively. Introduction of 0.10wt.% Zr to Al–Zn–Mg alloy, the grain refinement effect of scandium did not occur by adding only 0.10wt.% Sc, but appeared with an increase in scandium content to 0.25wt.%. In the presence of 0.10wt.% Zr, antirecrystallized effect of scandium appeared at 0.10wt.% scandium concentration, and enhanced with an increase in the content of scandium. Main aging precipitates were two kinds of GP zones in under-aged alloys, and η′ phases in peak-aged alloys, respectively. Small additions of Sc and Zr did not retard or suppress the formation of aging precipitates in Al–Zn–Mg–Sc–Zr alloys. The strengthening effect from aging precipitates was much stronger than from Sc and Zr microalloying additions in Al–Zn–Mg–Sc–Zr alloys. In addition, the most important strengthening mechanisms of Sc and Zr microalloying in aged Al–Zn–Mg alloys was Orowan strengthening of secondary Al3(Sc, Zr) particles.