Effect of pre-aging and Al addition on age-hardening and microstructure in Mg-6 wt% Zn alloys

The age-hardening response and microstructural variation Mg-Zn and Mg-Zn-Al alloys were examined by hardness test, transmission electron microscopy (TEM) and three-dimensional atom probe. The samples were prepared by hot-extrusion after casting. The two-step aged samples exhibit enhanced age-hardening response at an earlier stage compared to the single-aged ones. TEM observations exhibited that the peak aged Mg-Zn samples had two kinds of precipitates: one was a rod along the c-axis of the matrix phase; the other was a plate lying on the basal plane. The Mg-Zn-Al samples had rods and cuboidal precipitates. After two-step aging, the microstructure becomes finer for both the Mg-Zn and Mg-Zn-Al alloys. The rodlike precipitates were dominant in the peak aged Mg-Zn alloy, while the comparable number of rods and cuboidal precipitates were present in the Mg-Zn-Al alloy. Atom probe analyses for the samples pre-aged at 70 degrees C clearly showed the formation of Zn-rich zones. (C) 2008 Elsevier B.V. All rights reserved.