Effect Of B On The Growth Morphology Of Mg₂Si In Hypereutectic Mg-Si Alloys

In the 21st century, the problems of sources and environment have been the all-important ones in human continuable development, and the development trend of the future industry must be less energy consumed and pollution. Magnesium alloys as the lightest structural material have many excellent properties, such as low density, high specific strength, good heat and electrical conductivity, good damping and shock absorption, excellent electromagnetic shielding capability and easy processing and recycle, so they have been widely applied in the domains of aerospace, automobile, electronic industry and so on. Therefore, Mg-base materials are believed to be the"Green Material"with the most potential in development and application.Pure magnesium itself has many disadvantages, for example, low strength, low hardness and low melting point, so it cannot be used directly. Alloying is one of the most important methods to improve the properties of magnesium, however, compared with other metals, magnesium has a bit special physical metallurgy properties, which narrows the choices of alloying elements for the magnesium alloys. Strengthening effects of alloying elements in magnesium alloys are mainly demonstrate in solid solution strengthening and precipitation strengthening.Among all strengthening mechanisms, only the second-phase strengthening can effectively improve the mechanical properties in both room temperature and high temperature. The effect of second-phase to magnesium matrix varies depending on its morphology, distribution, size and ratio. Because magnesium matrix is very sensitive to stress concentration, big or tough second-phase is inclined to have a shearing effect to the substrate, which may induce crack propagation at the interface of phases, resulting in poorer strength. Based on above analysis, the ideal strengthening state of the second phase should be fine particles well dispersed in the substrate. Accordingly, in the alloying process, the best alloying elements should be those elements that can form compound with magnesium. Because it is ensured that not only can the alloying element be used effectively but also uniform and dispersively distributed second-phases might be formed. Meanwhile, uneven distribution of intensity can be avoided. Since a maximum solid solubility of Si into Mg is only 0.003at.%, Si atoms can react with Mg atoms to precipitate intermetallic compound Mg₂Si in the process of solidification. Mg₂Si is an effective strengthening phase, which can improve the high temperature tensile properties and creep properties. As one of the main alloying elements for Al alloys, Si has drawn a great deal of attentions and intensively studied, however, Mg-Si alloy system has rarely been studied. In order to receive better strengthing effect, it is necessary to increase the Mg₂Si content. Therefore, the study of Mg-high Si alloy is very important.The primary Mg₂Si dendrite is very coarse (100¹000μm) in Mg-high Si alloy fabricated by traditional metallurgy methods, and the Chinese character shape eutectic Mg₂Si network is easy to crack and then become the fracture source. As a result, the tensile strength and elongation of Mg-high Si alloys fabricated by these methods are too low. Grain refining is an important approach to improve theproperties of Mg-high Si alloys. In recent years, many new fabrication techniques are developed such as hot extrusion (HE), rapid solidification (RS), directional solidification (DS) and mechanical alloying (MA). Although the microstructure is refined and the mechanical properties of Mg-high Si alloys fabricated by these new techniques are improved, these methods contribute limitedly in refining Mg₂Si. Moreover, the processes is very complex and hard to control so it is necessary to do further researches.Another effective approach of microstructure refining is increasing the phase nucleation rate or changing the growing manner, for example, changing the anisotropic growth to isotropic growth, namely modification. Modification is usually combined with conventional metallurgy, so the process is simple, economical, and is very popular in the world. In the past few decades, modification has been widely applied to Al-Si alloys, but less work on Mg-Si alloys. At present, the research on Mg-Si alloys mainly concentrate on the die-casting Mg alloys with low Si (Si≤1wt.%), and the modification object is mostly Chinese character shape eutectic Mg₂Si, while the research on the modification of Mg-high Si alloys is so less.Based on the above statement, KBF₄, Mg-B and Mg-B2O3 are selected to modify hypereutectic Mg-Si alloys. Effects of content and different solidification velocity on the primary and eutectic Mg₂Si (morphology, size, amount and distribution) have been investigated detailedly. The major study results are as follows:(1) KBF₄ greatly modify the morphology, size and distribution of primary and eutectic Mg₂Si.(a) With the increase of modifier, primary Mg₂Si changes from sub-modification to full modification, but no over-modification isobserved. Based on our experiments, the best KBF4 content is 2.0-6.0 wt.%.(b) Alloys modified by KBF4 have good character of heredity. When the remelting times are less than 3, the alloys exhibit fully modified microstructures; when the remelting times are 5, the modified effect is still existed, but the amount of primary Mg₂Si is reduced.(c) Alloys modified by KBF4 have good permanence. The full modification structures are still preserved even the alloys are kept for 20, 40, 60, 80 min, respectively. However, when the alloys are kept for 100 min, there is phenomenon of modified invalidation. Based on the results, the holding time should not more than 80 min.(2) The Mg-B modifier could modify the morphology, size and distribution of primary and eutectic Mg₂Si. When the content is increased from 0.2wt.% to 1.0wt.%, the size of primary Mg₂Si decreased from ⁵0μm to 10μm, and there is no over modification.(3) The Mg-B2O3 modifier could modify the morphology, size and distribution of primary and eutectic Mg₂Si. When the content is increased from 0.1wt.% to 1.0wt.%, the size of primary Mg₂Si decreased from ³5μm to 10μm, and there is no over modification.(4) In the condition of gravity casting, with the increase of solidification rate, the size of primary Mg₂Si decrease in both unmodified and modified alloys. However, the solidification rate cannot modify the morphology of primary Mg₂Si in the unmodified alloys; but it is contrary when modified by KBF4, Mg-B and Mg-B2O3.(5) Most of the primary Mg₂Si modified by KBF4, Mg-B and Mg-B2O3 are octahedrons, both perfect and defective octahedrons. The mechanism ofmodification is because of the adsorption and poisoning of B.Light materials with improved room temperature properties and high temperature creep properties is crucial for aero industry, automobile industry etc. As a result, successfully refining large dendritic primary Mg₂Si and Chinese character shape eutectic Mg₂Si structure in the Mg-high Si alloys is significative to improve tensile properties and creep properties at high temperature. Hopefully, present essay may act as a guidance and promotion to the research and development of Mg - high Si alloys.