بررسي اثر عناصر بهساز و سرعت انجماد بر مورفولوژي و خواص مكانيكي كامپوزيت ريختگي Mg-Mg2Si توليد شده به روش در جاي غير واكنشگر

چكيده انگليسي :

An Investigation on Effect of Chemical Modifiers and Cooling Rate on Morphology and Mechanical Properties of Cast In Situ Non reactive Mg Mg2Si Composite Negin Maleki Email address n maleki@ma iut ac ir Date of submission on April 7 2012 Department of Materials Engineering Isfahan University of Technology Isfahan 84156 83111 Iran Degree M Sc Language Farsi Supervisors name Mahmood Meratian Associate Professor meratian@cc iut ac ir Masoud Panjepour Assitant Professsor panjepour@cc iut ac ir Abstract The Mg 5 Si is a hypereutectic alloy that the formation of Mg2Si phase during solidification fell it in to the category of non reactive in situ metal matrix composites However good properties of this composite is dramatically influenced by the tendency of Mg2Si phase to form coarse Chinese script or dendrite shape under normal cast condition which results in the low ductility and strength of the composite and restricts its application In view of the above an attempt has been made to refine microstructure of the in situ composite of Mg 5Si by chemical thermal and chemical thermal methods To fabricate Mg Mg2Si composite industrially pure Mg ingot and Si were used as starting materials The melting process was carried out in a low carbon steel crucible by using an electric resistance furnace under protective argon atmosphere which also stirs the melt To fabricate modified samples different amounts of Bismuth and Zinc 0 5 to 1 5 mass fraction were added in to the Mg Si melt as chemical modifiers The effect of chemical modifiers content on microstructure of Mg2Si Mg composite was investigated by optical and scanning electron microscopy followed by EDX analysis and X Ray diffraction Quantitative image analyze resulted that by addition of 1 Bi or Zn the following changes in microstructure would happen 1 Decreasing the average size of Mg2Si from more than 100 m to about 20 m or less 2 Changing the morphology of Mg2Si from dendritic shape to polyhedral shape 3 More even distribution of Mg2Si in the matrix and reducing of clustered polyhedral Mg2Si However when the modifier content exceeded 1 it is found that many primary Mg2Si particles became coarse again and over modification had occurred which is a complicated and unknown phenomena in Mg Si and Al Mg Si systems The effect of cooling rate on modifying microstructure was investigated by the use of a directional solidification system Structural characterizations of directed solidified Mg Mg2Si Mg Mg2Si 1 Bi and Mg Mg2Si 1 Zn samples were carried out on four thermal sections of each fabricated The Results showed that an increase in cooling rate cause an almost same modification effect on samples with chemical modifiers Although the structure of Bismuth and Zinc modified samples effected in a good way by higher cooling rates there were no evident of modified Mg2Si particles in samples with almost low cooling rates This may happen by segregation of dense modifiers due to longer solidification time or weakening effect of low heat transfer on chemical modification mechanism It also worth to mention that samples which were affected by both zinc chemical modification and thermal modification exhibited finer and fiber shape eutectic instead of former flake like In the next step hardness wear resistance compression and tensile strength of selected modified and unmodified samples were investigated The results showed that modified structures have better mechanical and wear properties than unmodified samples However the amount of this enhancement changes from minor to major amounts due to the nature of each test Best results were belonged to tensile examination results were modified structure has shown 11 enhancement in ultimate tensile strength Keywords In situ Mg Mg2Si composite chemical modification Thermal modification Directional solidification Mechanical properties Wear resistance

ملكي، نگين

بررسي اثر عناصر بهساز و سرعت انجماد بر مورفولوژي و خواص مكانيكي كامپوزيت ريختگي Mg-Mg2Si توليد شده به روش در جاي غير واكنشگر

كامپوزيت در جاي Mg/Mg2Si , بهسازي شيميايي , بهسازي تبريدي , انجماد جهت دار , مقاومت به سايش