Mechanical properties of age-hardened Al-Zn-Mg alloys

Al-Zn-Mg alloys such as compositions of Al-6%Zn-1%Mg (η-alloy) and Al-6%Zn-3%Mg (T-alloy) were subjected to various ageing treatments and their age-hardening mechanism and factors for controlling grain boundary fractures were investigated by tension tests and transmission electron microscopy.The effect of test temperature on the yield stress was very little in the range between room temperature and 77°K for as-quenched and overaged alloys. However, the effect was appreciable for these alloys containing G. P. zones only at temperatures of about 200°K or lower, or containing intermediate precipitates at below room temperature.The elongation to rupture was decreased with the increase of strength, independent of the width of precipitate free zone (PFZ) in η alloy: while it was dependent on the width of PFZ in T-alloy, since it was observed to be less in smaller width for the same strength.As the results of these tests, the age-hardening mechanism and the important factors for controlling grain boundary fractures in these ternary alloys such as the state of slip band and the width of PFZ were discussed.