Influences Of Heat Treatments And Rare-earth Elements On Microstructure And Mechanical Properties Of AM50 Magnesium Alloy

Magnesium and its alloys are the desirable materials in many industries. Because of its apparent advantages, such as the high strength-mass ratio, low density and good workability, they are the very subjects of present researches in materials engineering. In addition, some severe problems like environmental pollution and natural resources make it urgent for the massive applications of magnesium alloys. On this topic, due to the ultimate goal of automobile lightening, development for Mg alloys with superior properties is widely studied. In a word, the improvement over the present Mg alloys and research for new types is of marked importance.The AM50 Mg alloy has played a very important role in auto industries. However, because of its low strength and other disadvantages, its application is limited. In order to increase its application, many researches concentrate on improving the mechanical properties of AM50 alloy. They, in a word, are dependent of the microstructural characteristics of the alloy. For alloys in the Mg-Al system, the Mg17Al12 intermetallics (βphase) and the average grain size in the microstructure determine general mechanical properties of alloys. Additionally, the shape, distribution and nature of theβphase is a key factor to affect the properties of alloys. Many researches therefore try to modify the microstructural features, to achieve higher properties, which, in general, indicates that adding rare-earth elements is a very efficient way. As a result, in the present study, the effects of rare-earth additions (Ce, Y) on the microstructure and mechanical properties of AM50 alloy has been investigated, using the method of adding the element Ce and Y into the alloy.Here, Ce and Y additions, with the range from 0 to 1.0 wt.%, were used, along with varied processes including solution and quenching. On the one hand, for the microstructures of alloys, phase characterization, average grain size and shape and distribution of phases and precipitates were investigated, using XRD and microscopy. One the other hand, for the mechanical properties, general mechanical properties, such as ultimate tensile strength, yield strength and elongation, were studied. Finally, the relationship between microstructural features and mechanical properties as well as the theoretical background was probed.For the preparation of the rare-earth included AM50 alloy, elemental Ce and Y additions were added into the metal liquid, with various contents of 0,0.3,0.45,and 1.0 wt.% for each addition, respectively, and then the liquid was solidified. Each specimen was divided into three groups. The first one is solidified at normal cooling speed, as the as-cast sample. Another group was quenched during solidification. The final group was soluted after the preparation for the initial ingots. Microstructural examinations and mechanical tests were carried out for all samples but the as-quenched ones, since they were prepared for the comparison for microstructural features and therefore free of the mechanical tests.Due to the addition of Y into AM50 alloy, many variations in microstructure and mechanical properties were caused. For the as-cast microstructure, AM50 comprisedα-Mg as the matrix and theβphase in discontinuous network at grain boundaries. It should be noted that affected by the Y addition, spot-like Al2Y precipitate formed at the grain boundaries. In general, the microstructure was refined and the content ofβphase wasdrastically decreased, which resulted from the increase in the Y content in the alloy. When the Y content increased up to 0.3 wt.%, only very small amount ofβphase was observed in the microstructure. Therefore, when the Y content was further increased, the variation inβphase was limited. The size and density of the precipitate were however increased. When the Y content reached 1.0 wt.%, the precipitate density arrived reached the highest level in this work. Solution could induce recrystallization in the microstructure. Meanwhile, it promoted theβphase to dissolve into the matrix. It appeared that there was no network-like phase in the microstructure. Solution slightly influenced the precipitate. For instance, the precipitate size was decreased, and furthermore the overall microstructure was homogenized. Quenching refined the microstructure as well. As a result, no phase or precipitate in complex shape was observed in the microstructure. Bothβphase and Al2Y exhibited the spot-like appreance. As Y content increased, the precipitate density was increased, which is similar to variation of the above process. However, average size of the precipitate was decreased. The reinforcement effect of Y addition led to the significant improvement in mechanical properties of AM50 alloy. With increasing the Y content, the amount ofβphase was decreased. In addition, the density of Al2Y was increased, which explained for the improvement in the mechanical properties. Additionally, solution would further decrease the amount ofβphase and then improve the mechanical properties, whereas very high amount of Y content generated some negative effects on the properties. When Y content increased up to 1.0 wt.%, the amount ofβphase was very small, at which point, Y content would consume the Al content in the matrix, to form the precipitate. In this regard, the overall mechanical properties could be weakened.The role of elemental Ce is limilar with that of Y, when the amount of addition is small. Al11Ce3 intermetallics at small size formed at the grain boundaries. Meanwhile, theβ-phase network was broken, with the slight variation in its volume fraction. As 0.45 wt.% Ce addition was used, more the needle-like Al11Ce3 precipitate was generated, along with larger size. When Ce addition reached 1.0 wt.%, the amount of Al11Ce3 precipitate was further increased, when the precipitate exhibited the cluster appearance. Solution had little effect on the precipitate, which is different from that on Al2Y. Ce addition and process made their contribution to the improvement for the mechanical properties of AM50 alloy as well. In comparison with Y addition, there was some difference. For the AM50 alloy containing higher amount of Ce addition, a lot of cluster-like Al11Ce3 would lead to the crack generation and then crack propagation. It definitely had the negative role in the mechanical properties. When the Ce addition further increased from 0.45 wt.%, the ultimate tensile strength, yield strength and elongation were decreased.it indicates that the current amount of Al11Ce3 had the negative role more than the positive one. As observed from the results, the elongation is decreased in advance, which demonstrated that the detrimental effects precedentially affected the ductility of the alloy. As the addition was increased, the alloy strengthes were decreased as well, at which point, the detrimental effects of Ce addition were strong. One can thereby anticipate that as the Ce addition is further increased, the mechanical properties would be severely affected.