Low cycle fatigue properties of Al 7039 under various aging condition were investigated by examining the microstructure and the fracture surface. Five different aging conditions were chosen; three of them were of single aging, while the other two were of double aging.
For the specimen under one of the two double aging conditions, namely, 16 hrs at 77℃, and 4 hrs at 160℃, the fatigue life appeared to be well agreering with that predicted by Tomkins for ductile metal, while for those under the other four aging conditions, the fatigue life seemed rather in agreement with that for high strength metal by the same author. The result was analyzed by microscopic examinations through T.E.M. and S.E.M., and they conformed the fact that the fatigue life is dependent on the size of precipitates ($MgZn_2$), and large second phase particles ($CrAl_7$) inadvertantly introduced while alloying.
