Study On The Damping Properties Of As-cast Magnesium Alloys

In order to eliminate unwanted vibrations and acoustic noise in various industryfields, the applications of suitable high damping magnesium alloys in structures areconsidered as an effective way. The purpose of the present investigation is to provide abetter understanding of high damping Mg-Cu-Mn, Mg-Si and Mg-Ni series alloys. Themain conclusions of the thesis can be listed as follow:First of all, a high damping Mg-1%Mn based alloy with good corrosion resistanceand adequate strength was prepared by studying the properties of Mg-Mn alloys. Theaddition of Cu can remarkably reduce the grain size of Mg-1%Mn alloy, and precipitatespherical secondary Mg₂Cu in the matrix grains as well as thick film-shaped divorcedMg₂Cu eutectics at grain boundaries. The increasing precipitations on grain boundariesdeteriorate the mechanical properties and promote the occurrence of inter-granular fractureof the alloy. With a low strain amplitude, increasing Cu content do not increase the weakpinning points to dislocations and hardly changes the damping capacities. While at highstrain amplitude the internal friction values decrease with the smaller grain size andincreasing amount of precipitates.Secondly, the microstructure characterization, mechanical properties and dampingproperties of Mg-x%Si and Mg-9%Si-x%Si alloys were studied. The addition of Si canrefine the microstructure of pure Mg and improve the tensile properties. Under a lowstrain amplitude, the solute of Si atoms could reduce the damping value. The precipitationof Mg₂Si phase increase the dislocation density and improve the internal friction to somedegree. In high strain range, the damping values decrease gradually with grain refinementand increasing Mg₂Si particles. The morphology of Mg₂Si phases in Mg-9Al-x%Si alloysis found to be a function of the Si content. The Mg₂Si particles with up to1.3%Si have aChinese script shape, and the dendritic Mg₂Si crystals are precipitated predominantlyinside the grains of2.3%Si-containing alloy. The morphology and volume of Mg₂Siparticles as well as the distribution of Mg17Al12eutectic have a significant effect on thedamping capacities of Mg-Al-Si alloys. In low strain amplitude, the dislocation densityincreases due to the formation of Mg₂Si phases improving the damping property. Whilst in the high strain region, dispersed Mg17Al12precipitates and coarse Mg₂Si dendrites lead toa decrease of internal friction values.In the end, the effects of Cu content on microstructure, mechanical properties anddamping capacities of Mg-3%Ni alloy were investigated. The addition of Cu canremarkably refine the dimension-Mg dendrites of Mg-3%Ni alloy and precipitatelamellar-shaped （-Mg+Mg₂Cu） eutectics, which improves the mechanical properties ofMg-3%Ni-x%Cu alloys when Cu content is less than1.5%. In low strain range, with theincrease of Cu content, the damping capacities of Mg-3%Ni-x%Cu alloys hardly change,while in high strain amplitude the internal friction decreases gradually with grainrefinement and increasing eutectics.