Effect Of Precipitates On Twinning And De-twinning Behavior Of Pre-twinned Magnesium Alloys

Magnesium alloys have been more and more attractive in industrial applications due to their excellent properties, such as high specific strength, high specific stiffness, high electromagnetic shielding performance and easy to be recycled. However, comparing to aluminium alloys, the strength of magnesium alloys is relatively low. As a consequence, the industrial application of magnesium alloys has been limited. So the study of the hardening methed in magneium is essential. And the precipition hardening is an important way to improve the strength of the commercial magniusm alloys.The materials we used in present study were the most common-used commercial magniusm alloys, AZ61 and ZK60. And the effect of pre-compression and aging treatment on the following deformation along ED(extrusion direction) were studied. The deformation behavior and the precipitation behavior were studied by optical microscopy(OM), electron backscatter diffraction(EBSD), X-ray diffraction(XRD) and scanning electron microscopy(SEM). And the precipitates inside a {10-12} extension twin of ZK60 alloy were observed using transmission electron microscopy(TEM). The main results are as follows:Three morphologies of precipitates of ZK60 were observed in the {10-12} extension twin after aging,(0001)twin plates, [11-20]twin laths and [0001]twin rods. involving formation of both Mg4Zn7 and Mg Zn2. The Mg4Zn7 phases have two variant orientations of the [010] zone axis, [010]Mg4Zn7 ‖ [0001]twin and(-201)Mg4Zn7 ‖(-12-10)twin or(-201)Mg4Zn7 ‖(10-10)twin, while the Mg Zn2 phases have an orientation of [-12-10]Mg Zn2 ‖ [0001]twin and(10-12)Mg Zn2 ‖(10-10)twin.For the recompression process of AZ61 alloy, the hardening effect of the precipitates on twin expansion cannot counteract the softening effect caused by recovery causing the decrease in the compression yield strength, while tesion yield strength increased causing by he preferential nucleation of precipitates within the preexistent {10-12} twins at the initial stage of aging. Unlike the AZ61 alloy, both of the compression and tension yield strength of ZK60 were extremely increasd after aging treatment. Different precipitation behaviors and precipitate morphologies of these two alloys may cause the different changing trends of yield strength. Otherwise, the tension-compression yield asymmetry along ED of both AZ61 and ZK60 alloys were improved after aging.The aged samples of both AZ61 and ZK60 alloys had two stage with different slope in B hardening stage during recompression, named B1 and B2. And the slope of B1 was obviously larger than that of B2. Although the changing trends of B1 for these two alloys were different, the slopes of B2 were not changed after aging. So we can get the conclusion that the precipitates produced by aging only had effect on the initial stage of hardening stage B. For the tension process, the aging made the hardening stage B flatter in AZ61, while the stage B of ZK60 was more obvious after short aging(3 h).