| Magnesium alloys have great potential for application in structure parts in automobiles, computers, airplanes, spaceflight and national defence due to their excellent propoties such as low density, high specific strength, superior damping characteristics, good electromagnetic shielding performance, etc.The most commonly used magnesium alloys are those based on the Mg-Al system. Principally, AZ91 is the most widely used commercial magnesium alloy. The study will concentrate on this kind of alloy. Concerning the Mg-Sn alloys have lots of advantages, such as good shielding performance and mechanical strength, stable microstructure in elevated temperature, and low price, we developed a new kind of magnesium alloy-Mg-9wt%Al-lwt%Zn-0.2wt%Mn-4wt%Sn. Transmission electron microscopy (TEM), optical microscopy(OM), X-ray diffraction(XRD), scanning electron microscopy with energy dispersive spectrometer(SEM/EDS) were employed to characterize the crystallography of continuous precipitations of these two kind of magnesium alloys, including microstructure, phase constitution, dimension, and orientation relationships. This study would help to provide a scientific basis for the development and applications of magnesium alloys.Firstly, die-cast AZ91 and Mg-Al-Zn-Sn were solution treated.Then An in-situ observation of the precipitating process of continuous precipitations in these two kinds of magnesium alloys, were carried out with a transmission electron microscope equipped with a heating stage maintained. The results showed that the size of the continuous precipitates in both alloys grew with the aging time, the figure turned regular and the shape became vivider than before. Most of continuous precipitations were lath-shaped lying along the (0001)αof the magnesium matrix phase. No G.P.zones were observed during the process of aging.Secondly, AZ91 specimen which aged at 200℃for 8h was observed in the transmission electron microscope. We surveyed the 3-D size as overall information by utilizing various samples along various directions. The statistical results showed that the 1st-D(along[1010]αor length),2nd-D(along [0001]αor width) and 3rd-D([1210]α or thickness) sizes are concentrated on 750±300 nm,45±35 nm and 200±100 nm, respectively. By using the selected-area electron diffraction (SAED)method with the tilting technique, the reported orientation relationships and morphologies of the continuous precipitates have been reconfirmed as follows. The predominant fraction of continuous precipitates is thin lath, with the primary habit plane parallel to the basal plane of the matrix (0001)α, and possesses the Burgers OR. We had also observed the Pitsch-Schrader OR among these thin lath-shaped precipitates. Another type, comprising only a small fraction, has the primary growth direction perpendicular to the basal plane (0001)a and has the Crawley OR. The Porter OR precipitates have the primary growth direction lying at an angle of about 16°to the normal of the (0001)a which is even less common. We had observed continuous precipitate particles with the Potter OR in an AZ91 alloy using HRTEM. However, precipitate particles with the Gjonnes-Ostmoe OR were not found in our experiment. In this study, a new orientation relationship of continuous precipitates was observed in the AZ91 alloy. By using the SAED method with the tilting technique, it was confirmed that the new orientation relationship is in the form of [0001]α//[131]γ, (1120)α//(303)γ. We had also calculated six stereographic projections for the reported orientation relationships. And the Crawley OR can be obtained from Pitsch-Schrader OR by the interpretation of O-Lattice theory and Invariant-Line theory.Rapid solidification is non-equilibrium solidification, which can effectively fine grains and secondary phases, increase the solid solubility of alloy elements in matrix, improve phase constitution and form new metastable phases and so on. Based on these adavantages, we made die-case Mg-Al-Zn-Sn specimen by conventional solidification and thin ribbon specimen by rapid solification. By using DTA and DSC, we confirmed the solution temperature (400℃) of the Mg-Al-Zn-Sn alloy and by using OM, and we found that the grains were fined because of addition of Sn. The constitution of the alloy includedα-Mg,γ-Mg17Al12 and Mg2Sn based on the results of XRD and EDS. Die-cast Mg-Al-Zn-Sn alloy specimen was aged at two different temperature,130℃and 250℃. No Mg2Sn phase was found in the low temperature aging specimen, but we confirmed two different orientation relationships of Mg2Sn phase at the high temperature aging specimen. Transmission electron microscopy on the ribbon specimen with 0.2nm thickness revealed lots of ball-shaped continuous precipitations on the Mg-matrix which were not observed in the conventional solidification die-cast specimen. Beside, we investigated that the ball-shaped continuous precipitates are three-element compound with Mg, Al and Sn by EDS method. Both y-Mg17Al12 phase and Mg2Sn phase were observed in the 0.3 mm thickness ribbon specimen, which were confirmed by HRTEM and simulation. |
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