Effect of Modification on the Growth Morphology of Mg2Si in Mg-Si Alloy

　　Effect of Modification on the Growth Morphology of Mg2Si in Mg-Si Alloy
　　In recent years, saving the energy sources and environment pollutionproblems have received more and more attention and effective method to solvethem is to use light-weight and high-effective materials. Magnesium alloys arekind of the lightest structural metal alloys applied in the industrial at present,which are provided with many excellent and electrical properties, such as lowdensity, high specific strength and stiffness, high heat and electrical conductivity,good damping and shock absorption, excellent electromagnetic shieldingcapability and easy processing. In the 21st century when human being put thecontinual development as the first-line problem, saving the resource and innoxiousproduction have become the trends in the world nowadays. Consequently, theunique properties of magnesium alloys accord with the above requirement by thesquare. Therefore, since the 1980s later, magnesium alloys have been becomingone of the hotspot in the research field of materials science.Alloying is the method in common use for the application of magnesium now.Compared to others metals, magnesium Special shape NdFeB magnets has especial physical metallurgyproperties, and so it is narrow that to choose the alloying elements for the alloyingof magnesium. Silicon is one of them. Since a maximum solid solubility of Si intoMg is only 0.003 at.% silicon, Si atom can react with Mg atoms and areprecipitated as an intermetallic compound of Mg2Si. Mg2Si is an effectivestrengthening phase and can improve the high temperature properties. It isnecessary to increase the Mg2Si content for receiving better strength effect.Mg-high Si alloy fabricated by conventional metallurgy method contain coarseMg2Si dendritic crystals (100~1000μm);besides, eutectic Mg2Si phase is theChinese script type particles. The Mg-Si alloys showed very low ductility andstrength. In recent years, it was reported that the mechanical properties of Mg–Sialloys could be improved by the application of advanced processing techniques,such as hot extrusion (HE), rapid solidification (RS), directional solidification (DS)and mechanical alloying (MA). The microstructure and properties of Mg-high Sialloys had a obvious improvement. However, the methods above had the shortagesboth restricted fine of grain or complex technique and the increase of theproduction cost. At the present time, it had been fabricated Mg2Si/Al-Si usingmodifiers such as RE, sodium and Sr. Modification method had got attachimportance to application. At present, the study on the modification of Mg-Sisystem focus on die-casting Mg alloy contain low Si content (commonly<1wt.%),and modified object was main eutectics Mg2Si phase. It is seldom for the study onMg-high Si alloy. So based on the modified characteristic of Mg-Si alloy,combined with existing modified processing on Al-Si and Mg-Si alloy, theexploiture and development of new modification method applied to Mg-Si alloy iscritical for its large-scale applications in industry.Based on the above standpoint and statement, modification and rapidsolidification routes is selected for the modification of Mg-Si alloy, i.e. primaryand eutectic Mg2Si phases were modified by the RE(Sm2O3、Nd2O3、Y2O3、La2O3)and KBF4, and rapid solidification such as spray casting. On the basis of thesework, effect of modification on the primary and eutectic Mg2Si phases(morphology, amount, size and distribution) in the Mg-high Si alloy have beeninvestigated detailedly.
　　The major research efforts of the present study are asfollows:(1) It was found by the modification on Mg-high Si alloy using RE2O3 thatthe modification effect changed as sub modified, modified and overmodified as increasing the RE2O3 content from 0.05% to 0.4%.(a) For four kind of RE2O3, Y2O3 was the best modifier on primaryMg2Si in the Mg-high Si alloy. When the Y2O3 content is 0.2wt.%,primary Mg2Si changed from dendrite(>100μm) to polyhedral shape(15μm) and distributed uniformly in the alloy. Besides, Chinese scripttype eutectic Mg2Si ruptured and changed to point shape.(b) For four kind of RE2O3, Sm2O3 was the best modifier on eutecticsMg2Si in the Mg-high Si alloy. When the Sm2O3 content is 0.2wt.%,most of eutectics Mg2Si changed to point and the size of primaryMg2Si decreased from >100μm to 20-30μm.(c) It was http://www.999magnet.com found that much white acicular phase was present in theMg-high Si alloy modified by RE2O3 and increased with theincreased of RE2O3. As the present of silicon in the melt, Mg, Si andRE were likely to reaction and replace RE from RE2O3.(2) After the composite modification using Y2O3+KBF4 and La2O3+KBF4,the size of primary Mg2Si changed to ~10μm and its morphology waspolyhedron;Most of eutectic Mg2Si changed to point shape and somewere fiber shape.(3) Y2O3 had excellent effect on Mg-high Si alloy as the Si content changedfrom 3% to 9%. For the modification effect on Mg2Si phase, the relationof Y2O3 and Si content was not linear. Y2O3 is effective on the Mg-Al-Sisystem for modification. After Y2O3 addition in the AZ91D-5% Si alloy,the size of primary Mg2Si changed from more than 100μm to 20μm.(4) Solidification rate had great influence on the growth morphology ofMg2Si phase. High solidification rate could restrain the anisotropicgrowth. If solidification rate increased, the size of primary Mg2Si woulddecrease. Besides, the morphology of primary Mg2Si was octahedron.Successful improvement of the microstructure of Mg-high Si alloy play alarge role on the advanced of high temperature properties of Mg alloy. It isexpected that the present research acts as the guidance and promotion.