The Electron Microscopy Investigation Of The Precipitated Phases In Mg-Al And Mg-Re-Zn Alloys

Among lightweight and high-strength magnesium-based alloys, magnesium-aluminum alloys, especially the Mg-9.0wt.%Al-1.0wt.%Zn (AZ91) alloy, have been showing increasingly broad application aspects due to their various advantages in castability, mechanical strength and ductility. Now, much attention in the material researches has been paid to the alloys, and especially investigations of the microstructures and mechanism for improving their strength and toughness have been carried out. In this thesis, the precipitation process, the precipitates’crystallography and the age-hardening mechanism for several typical magnesium-based alloys have been investigated using X-ray diffraction (XRD) technology, scanning electron microscopy (SEM) and transmission electron microscopy (TEM).The aging process of the solution-treated Mg-9.0wt.%Al-1.0wt.%Zn-4.0wt.%Sn alloy was carried out and observed in-situ on a transmission electron microscope (TEM) with a heating-stage for the specimen (403K for1hour). Either the G.P. zone or the transition phase was not observed during the aging process. Besides, the results show that the number of the continuous precipitates per unit volume (Nv=4.92×1010/mm3) for the above specimen is larger than that in AZ91(Nv=2.7×109/mm3) under the same condition. Meanwhile, the number of the continuous precipitated γ-Mg17Al12particles per unit volume in the Mg-9.0wt.%Al-1.0wt.%Zn-4.0wt.%Sn and AZ91alloy aged at523K for48hours under traditional aging condition (heated in a muffle furnace) had been measured and compared, and the similar conclusion was achieved. Therefore, a small amount addition of Sn to Mg-9.0wt.%Al-1.0wt.%Zn (AZ91) alloy will accelerate the continuous precipitation of the γ-Mg17Al12phase, and will devote to improving the age-hardening effect for the Mg-9.0wt.%Al-1.0wt.%Zn-4.0wt.%Sn alloy.A new crystallographic orientation relationship (OR) between continuous precipitated γ-Mg17Al12particles and a-Mg matrix has been determined as (0001)α//(321)γ,[1210]α//[133]γ in the as-cast Mg-9.0wt.%Al-1.0wt.%Zn-4.0wt.%Sn alloy which was solution-treated at693K for24hours and then aged at523K for48hours in a traditional aging condition (heated in a muffle furnace). Besides, another new crystallographic OR between the y-Mg17Al12and the a-Mg matrix has been observed as (0001)α//(121)γ,[1210]α//[111]γ in a{1012}α type twin region in the same above alloy. The long axis of the continuous precipitated γ-Mg17Al12particles with this latter OR in this {1012}α type twin area is along the [0001]α direction, which will be as obstacles to the dislocation movements on the basal (0001)α plane of the a-Mg matrix. Consequently, this will contribute to the improvement of the mechanical strength of the alloy.The microstructure investigation of rapidly solidified ribbons of Mg-9.0wt%Al-1.0wt%Zn-4.0wt%Sn alloy shows that the microstructure is constituted by α-Mg, γ-Mg17Al12and β-Mg2Sn phases. Compared to as-cast condition, the grain size with several hundred microns was significantly reduced to several hundred nanometers in rapidly solidified condition. According to the Hall-Petch relationship σs=σ0+kd-1/2and with a large value of k to the magnesium based alloys, the toughness of the magnesium based alloy will be improved greatly when the grain size is decreased as above. It is the first time to identify that, the OR between α-Mg phase and the y-Mg17Al12phase which exists at the grain boundaries is the Burgers OR, and the OR between α-Mg phase and a γ-Mg17Al12particle which exists in a grain is also the Burgers OR. In addition, the OR between α-Mg phase and a spherical particle of β-Mg2Sn phase distributed diffusely in α-Mg grains is in form of (0001)α//(110)β,[1210]α//[112]β, which is different from all the ORs reported in the as-cast Mg-Sn alloys.In the solution-treated as-cast Mg-9.05wt.%Gd-2.85wt.%Zn alloy (at773K for16hours), the14H LPSO structure have been observed. By the proposed atomic structure model for the14H LPSO structure and kinetic theory, the electron diffraction patterns have been calculated, and the simulated have shown good agreement with the experimental results. The aging process of the solution-treated as-cast Mg-9.05wt.%Gd-2.85wt.%Zn alloy (at773K for16hours) was in-situ observed in a TEM with a heating-stage maintained at the temperature523K for the specimen. The experimental results show that the precipitation sequence at the early aging stage is that: supersaturated solid solution (SSS)â†'G.P. zone or γ" phaseâ†'γ’ phase. In an as-cast Mg-9.05wt.%Gd-2.85wt.%Zn alloy, which have been solution treated at773K for16hours and then aged at623K for0.5hour in the muffle furnace, the14H and18R type long period stacking ordered (LPSO) structures have been observed by TEM methods.