In the previous research, using in-situ tensile stage installed in scanning electron microscope, surface slip trace of Al-4.6Zn-2.0Mg-0.75Mn-0.15Zr alloy and the commercial Al-4.0Zn-2.6Mg-0.15Zr alloy was observed during fatigue. According to the experimental results, coarse slip band formed by localized deformation was observed in the commercial Al alloy that does not contain Mn, whereas in the case of Mn-bearing alloy deformation was dispersed all over the specimen surface and to form very fine and evenly distributed slip bands. Also, using the dislocation trace analysis through transmission electron microscope, it was known that homogeneous deformation Mn-containing Al alloy is due to dislocation cross-slip induced by the interaction between dislocation and Mn-dispersoid.
In this present paper, the interaction between dislocation and Mn-dispersoid and distribution of dislocations are observed under the various fatigue condition. Especially, studies emphasize on the cross-slip of dislocation at Mn-dispersoid. To see the stress mode on the interaction, low cycle fatigue was carried out in fully reversed stress and repeated tensile stress condition. From these experiments, it is observed that dislocation structure in fully reversed stress fatigue is more complex and non-directional compared with that in repeated tensile stress fatigue. Varying the strain amplitude($Δε_α$=± 0.5, ±1.5, ±3%) in low cycle fatigue, the behavior of dislocation at Mn-dispersoid is observed also. Under the condition of Δεa=±0.5%, most of dislocations are fixed at Mn-dispersoid and only a part of attached to Mn-dispersoid are cross-slipped, whereas in $Δε_α$=±1.5% and ±3%, dislocations are not fixed at Mn-dispersoid and most dislocations are cross-slipped by the interaction with Mn-dispersoid. From these results, it can be concluded that although the difference in dislocation structure with stress mode the dislocation cross-slip at Mn-dispersoid are not affected by stress mode and it is needed the critical stress to occur the dislocation cross-slip at Mn-dispersoid.
