Effects of Mn-dispersoids on the fatigue mechanism in an Al–Zn–Mg alloy

Using the tensile stage installed in the scanning electron microscope of model JEOL JSM 840A, two different aluminum alloys have been fatigued and surface slip traces studied in-situ. For observation of the internal dislocation structure after cyclic deformation, transmission electron microscopy (TEM) is used. The two different alloys examined are 7xxx series alloys, whose chemical compositions are Al–4.6Zn–2.0Mg–0.75Mn–0.15Zr (or Al–0.75wt.%Mn alloy), and Al–4.0Zn–2.6Mg–0.15Zr. From the results of the in-situ fatigue tests, one may see that the Al–0.75wt.%Mn alloy exhibits homogeneously distributed deformation, whereas the non-bearing Mn alloy shows the typical coarse slip steps inhomogeneously distributed. Analyzing the characters of the dislocations that are interacting with Mn-dispersoids in the Al–0.75wt.%Mn alloy, Mn-dispersoids are found to block the motion of the dislocation slip and to make the dislocations cross-slip. In contrast, the alloy without Mn exhibits planar slip on a single active slip plane. From these observations, it is concluded that the Mn-dispersoids in the Mn containing alloy are responsible for the uniform deformation through cross-slip, for retarding fatigue damage accumulation, and enhancing fatigue life.