Research On Microstructure Modification And Creep Performance Of Mg₂Si/Mg-Sn Composite

Nowadays, the Magnesium matrix composites have very broad application prospects inlots of fields such as aerospace and military, as its high specific strength and stiffness. In thisthesis, the Mg₂Si/Mg-Sn composites are prepared by the melt casting, which the reinforcingphase is Mg₂Si particle and the matrix is Mg-Sn alloy. The cast morphology can be observedby OM in order to analyses the growth of Mg₂Si. Besides, to refine the Mg₂Si crystal andenhance the creep capability, the modification by Ca and La is necessary. Moreover, thecomposites creep behavior can be deduced in combination of the creep testing data and thetheoretical calculation. Lastly, fracture characters of tensile samples can be observed by SEM.The results show that the coarse dendritic and petaloid Mg₂Si crystal is showed when thepercentage of Mg₂Si is over13.7%, moreover, the Mg₂Si can be modified by adding little Snelement, the dendritic Mg₂Si crystal is recovered adding over Sn contents.Since13.7%Mg₂Si/Mg-5%Sn composites are modified by Ca and La, the size of Mg₂Sicrystal modified by Ca-La can refined from116μm to18μm, of which the modified effectionis much better than Ca or La alone. The morphology of Mg₂Si crystal is strongly optimized toregular polygonal. After analyzing, the Ca atom can poison the Mg₂Si crystal growth, whichenrich around the crystal edge. Through altering the addition of Ca-La and the proportion ofCa and La atom, adding1%Ca-La, the equivalent Ca and La content, can modified Mg₂Sicrystal most ideally, while the content of Ca-La is over1%, the Mg₂Si crystal grow to largescale again.The results of creep testing for Mg₂Si/Mg-Sn composites show that the creeptemperature and stress are the important factors of creep performance, the composite creepcapability get worse with the enhance of theser factors. Additionally, the increasing of Mg₂Siand Mg₂Sn contents can improve the composite creep performance, while the ability ofdeformation get worse. According to creep theory, the creep deformation of Mg₂Si/Mg-Sncomposites is mainly controlled by dislocation climbing, the grain boundary slippage and thereinforced phase as the accessory creep mechanism. By comparison of the unmodified composites, the modified composites have the higher creep resistance and deformation abilitybecause of the refined Mg₂Si crystal and the high temperature-resistant compound exist.Taking the sample fracture into analysis after creep testing, the results are thatMg₂Si/Mg-Sn composites fracture mechanism is quasi-cleavage fracture with the low contentof Mg₂Si. O type and V type cavities can be seen clearly around the fracture. The character ofcleavage fracture is obvious with the increasing Mg₂Si content. The amount of cavities can befounded around the sharp-angled of dendritic Mg₂Si crystal,which appears the second flawand the tear ridge becomes short and tortuous. The fracture mechanism of compositestransmits to the mixture of quasi-cleavage and dimple fracture,which modified by Ca-La. Theamount of cavities decrease while the flaw is extended through the grain boundary.