The mechanical behaviors of high strength aluminum alloys containing 5 - 5.3Zn, 2.2-2.5Mg, 1.4-1.6Cu, and 0.2-0.3Cr in weight percent have been investigated as a function of manganese contents in the range of 0.068, 0.97 weight percent.
A role of adding manganese in Al-Zn-Mg-Cu alloy is to form 0.03 ~ 0.25㎛ fine Mn-rich dispersoids whose structure and composition have not been clearly identified and to increase strength owing to dispersion hardening particles without sacrificing ductility contributed by homogenizing slip distribution which leads to inhibiting planar slip and reducing local strain and/or stress concentration.
In this study, the distribution of dispersoids was observed in use of TEM and the density of Mn-dispersoids was increased with increasing manganese contents without changing the morphology and the size of the dispersoids. And the observation of dislocation arrangements after 10 percent fatigue cycling showed that the degree of slip homogenization was increased as the density of Mn-dispersoids was increased, while Mn-free alloy showed inhomogeneous planar slip.
Tensile test results showed increase in strength, but ductility was significantly decreased in Mn-bearing alloys. This can be explained by the detrimental effect of coarse inclusions formed by Fe impurity in Mn-bearing alloys compared to Mn-free alloy.
In the tear tests of the alloys containg dispersoids fractured surface showed microdimples which are the characteristic feature by void initiation and coalescence. Toughness was not decreased linearly with increasing Mn-dispersoid density. So it can be concluded that toughness was dependent on not only void formation due to local stress concentration on second phase particles but strain homogenization by dispersoids So optimum toughness can be gained in moderate dispersoid density level.
Fatigue cracks were initiated by inclusion/matrix debonding or on broken inclusions by strain concentration at the surface of specimen and propagated through matrix. So inclusion which mainly constisted of Fe and Si must be avoided to improve fatigue property as well as tensile properties.
In the low cycle fatigue(LCF) 1.0wt.%Mn-bearing alloy had the best iterative stress resistance by dispersion hardening effect and slip homogenization due to Mn-dispersoids.
