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

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 66 MPa after adding 0.10 wt.% Sc and 0.10 wt.% Zr, and improved by 96 MPa after adding 0.25 wt.% Sc and 0.10 wt.% Zr, respectively. Introduction of 0.10 wt.% Zr to Al-Zn-Mg alloy, the grain refinement effect of scandium did not occur by adding only 0.10 wt.% Sc, but appeared with an increase in scandium content to 0.25 wt.%. In the presence of 0.10 wt.% Zr, antirecrystallized effect of scandium appeared at 0.10 wt.% 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 eta' 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 Al-3(Sc, Zr) particles. (C) 2012 Elsevier B.V. All rights reserved.