Modification Of Mg₂Si In Mg-Si-Sn Alloys With P

The development and application of the magnesium alloys have been limited in the high temperature (～150℃) due to its low tensile strength and creep resistance. As a result of adding silicon to the magnesium alloys, the fluidity of molten metal can be improved, and therefore, it can be cast to form complex shapes and thin castings. Since a maximum solid solubility of Si into Mg is only 0.003 at. %, Si atom can react with Mg atoms and are precipitated as an intermetallic compound of Mg₂Si. The intermetallic compound Mg₂Si exhibits a high melting temperature (1085℃), high hardness (4.5×109N?m-2), low density (1.99×103kg?m-3), a reasonably high elastic modulus (120GPa), a low thermal expansion coefficient (7.5×10-6K-1) and it can improve the high temperature properties.Under traditional casting conditions, the Mg-high Si alloys show the low strength and low ductility because of the coarse dendrite Mg₂Si and brittle eutectic microstructure in alloys. Microstructural refinement is an aproach to improve the mechanical properties of alloys, so refine the primary and eutectic Mg₂Si in the Mg-Si alloys has become hot issue in the field. The modification is an effective and inexpensive method; the primary Mg₂Si present polyhedron and the eutectic Mg₂Si present very fine fibre shape in the modified Mg-Si alloys. Improvement of microstructure in the modified Mg-Si alloys result in the improvement in the mechanical properties.In recent years, the modification has been used widely in the Al-Si alloys. Such as P modified the primary Si, the salt, Sr, Al-Ti, or Al-B modified the eutectic Si; RE modified the primary or the eutectic Si and so on. Moreover, there are compound modifiers and the treating technology direct for the hypereutectic Al-Si alloys; they can modify the primary and eutectic Si at the same time.Modification of P element is the most effective and steady modified method, which have applied widely in research and production of the primary and eutectic Si in the Al-Si alloys. However, only little research is focused on the Mg-Si alloys. Unfortunately, the P modifier still exist many shortcomings, such as dramatic reaction and even bring harmful elements, resulting in a great deal of smoke, which will pollute the environment. Besides, it is easy to produce slag, which will corrupt the liner of the stove. Although the P element can be introduced into the melt by a Cu-P master alloy, Cu addition can greatly reduce the corrosion resistance of the Mg alloy. Therefore, to promote the development of Mg-Si series alloys, it is of theoretical and practical values to develop a new P-contained modifier and investigate the modification mechanism.Besides, control the cooling rate, the superheating treatment and isothermal heating treatment are also effective ways to refine the particles and improve the mechanical properties of alloys. Howerve, the above mentioned technology has used in the Mg-Si alloys so rare. The effect of modification, the injection experiment and partial remelting isothermal holding on the microstructure of Mg-Si-Sn alloys have been investigated in this study. The conclusions are as follows:(1) Si content has a great influence on the morphology and the size of the Mg₂Si. When the Si content is 3 wt. %, the primary Mg₂Si shows bloky shape; when the content is 3-4wt. %, the primary Mg₂Si indicates bloky and equiaxed dentritic shape; when the content is 4-5wt. %, the primary Mg₂Si shows equiaxed and dentritic shape; when the Si content is more than 5wt. %, the primary Mg₂Si represents dentritic one; moreover, the size increases with the Si content increasing.(2) P modifier changes the the morphology of primary Mg₂Si in the Mg-5Si-1Sn alloys effectively, which makes it changed from the dendritic shape to the fine polyhedron one. The size of the primary Mg₂Si decreased with the increase of the P content. When the P content reaches 0.2wt. %, the size decreases to～14μm. The mechanism of P modification is heterogeneous and the nucleus may be Ca3(PO4)2.(3) The Sn-P modifier addition has an obvious modification effect in both the gravity casting and the spay casting. With the cooling rate increasing from gravity casting to spay-casting, the primary Mg₂Si turned from the coarse dendrite and polyhedron to the finer polyhedron respectively in the unmodified and P-modified Mg-5Si-1Sn alloy. In addition, the Chinese script eutectic Mg₂Si phase turns thiner.(4) The primary Mg₂Si of Mg-4Si-4Al-1Zn alloy is considerably refined when the superheating temperature increases from 750℃to 900℃, and the distribution is uniform. When the temperature achieves 900℃, the size of the primary Mg₂Si decreases from～39μm to～10μm, but the morphology keeps unchanged. In addition, the coarse Chinese script Mg₂Si turns to fined fiber and fined Chinese script one, the mechanism is the elimination of hereditary.(5) With the increase of isothermal treatment temperature, the morphological evolutions of primary and eutectic Mg₂Si in the Mg-5Si-1Sn alloy are as follows: coarse dendrite (?) dendrite (?) dendrite broken off or passivation (?)spherical + nearly spherical, and Chinese script eutectic Mg₂Si (?) granule + rod-shape (?) granule + rod-shape (?) Chinese script. The morphological evolutions of Mg₂Si in Mg-5Si-1Sn alloy modified by 0.2%P are as follows: polyhedron (?) polyhedron (?) polyhedron+abundant circle shape (?) passivation + abundant circle shape + broken off, and the evolution of the eutectic Mg₂Si is similar to the one in the unmodified alloy. So the partial remelting can only make the unmodified-Mg-Si-Sn alloys obtain the spherical primary phase. Being a solid solution of P in Mg₂Si crystal, the stability of Mg₂Si becomes weaken, resulting in formation of circle primary Mg₂Si in the P-modified alloys. Moreove, the secondary remelting can make the unmodified Mg-5Si-1Sn alloy obtain the disired microstructures, i.e. the coexistence of nearly spherical primary phase, the granule and the rod-shape eutectic phases.