Investigation Of Bauschinger Effect Of7A04Aluminum Alloy

Bauschinger Effect is a phenomenon that refers to the metal materials will produce a small amount of plastic deformation after being pre-loaded (the residual strain from1%-4%), if loading in the same direction after unloading, the permanent set stress(elastic limit or yield strength)increases; if reverse loading, the permanent set stress reduces(especially the elastic limit reduced to almost zero).This phenomenon was found by Johann Bauschinger in1881(short for BE). BE of7A04aluminum alloy was tested by Instron1342hydraulic servo fatigue testing machine in this paper, and also have investigated the influence of different experimental conditions on BE, finally, the micro detection means (such as Metallurgical Microscope, XRD, TEM) were used to investigate the mechanism of its production.The test results of BE of7A04aluminum alloy show that:7A04aluminum alloy does exist BE under the condition of reverse deformation after pre-deformation; The tensile elastic recovery amount is about0.30, and the compressed elastic recovery amount is about0.29, it was shown that little difference of the modulus of elasticity between tension and compression, which represented inconspicuous anisotropy. The values of RKI (the ratio of kinematic hardening to isotropic hardening) under different experimental conditions are more than1, which indicates the strong characteristics of cyclic hardening.The history of strain and the strain capacity have great influence on BE of7A04aluminum alloy. The results of analysis shows that:BE under pre-compression was stronger than pre-tension. The BE increased first and then decreased under the change of strain capacity from1%to4%, while the inflection point appeared when the strain capacity achieved to2%. The increase of strain rate leaded BE to get more apparent. BE was getting less obvious as the cyclic increased, but it won’t disappear. The flow stress during the process of cyclic deformation also indicated the characteristics of cyclic hardening.The results of XRD show that the deformation mechanism under tension and compression is dislocation slip. It was found that great III internal stress and little II internal stress appeared during the deformation in the materials. And no twinning was found by metallographic observation. The TEM results show that slip band and dislocation tangling appeared in the crystals after pretension, which could reduce the properties of plastic deformation, so that it has higher yield strength. The dislocations during the process of compression after pre-tension will be redistributed which will make the movable dislocation increase, and less holdback on the reverse motion path will make the capacities of plastic deformation go up, and the yield strength decreased, therefore the BE showed up.