Balance Optimization Of Strength And Damping Capacity Of Mg-Zr Series Magnesium Alloys

In this thesis, the present widely used high damping MgZr magnesium alloys were chosen to form systems (MgZr,MgZnZr,MgZnZrY) through adding alloying element into them. The variation of strength and damping capacity of these alloy systems after general deformation and heat-treatment were systematically investigated by optical metallographic (OM), X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM), mechanical tensile properties test, dynamic mechanical analysis (DMA).In the first part of the thesis, the background and significance of a recent progress of damping magnesium alloys was critically overviewed, which includes the token, test approach and damping forming mechanism. Based on investigation actuality, the objective and significance of this thesis were represented.In present study, the hot-extrusion deformation of conventional high damping Mg-Zr magnesium alloys was carried out by using a various processing parameter of temperature T and strain rateε, and the effect of deformation temperature and strain rate on microstructure, strength and damping capacities of Mg-Zr alloys have been investigated. By fitting , the quantitative relationship between extruded Zener-Hollomon( Z =εexp(Q/RT)) parameter and grain size has been builded:d=376.6 - 13.6ln Z. The quantitative relationship between extruded parameter and the yield strength can expressed asσ=100.9+296.5(376.6 - 13.6ln(εexp (Q/RT)))^-1/2. The quantitative relationship between extruded parameter and the damping capacity can expressed as Q_h^-1 = A(376.6 - 13.6 ln(εexp(Q/RT)))³ /εexp(-C₂/ε) .The effect of series deformation processes (first extrusion, second extrusion and forging) and heat treatments on MgZnZr magnesium alloys was studied. We found that the mechanical properties of alloys after deformation process or heat treatment changed little, but the damping properties have obvious change for MgZnZr magnesium alloys in these different processes technology. The damping capacity of alloys treated by forging was increased more, and the damping properties in the forging state are more than twice than in the extrusion state. The damping properties have obvious increase for MgZnZr magnesium alloys in three different heat treatment technology, and the damping capacity of alloys treated by solution are increased more. The influences of processes technology on damping properties can be explained by the G-L theory. The high strength extruded MgZnZrY alloy was chosen to study the influence of Yttrium addition on microstructure, strength and damping capacity. In the light of present literature reports, Yttrium played significant role of the grain refinement, and the coexisting of Y, Zn atom will introduce long period structure and quasicrystalline phase. When the ratio of Y/Zn is 0.05-0.2, quasicrystalline phase(I phase-Mg3Y6Zn) formed. When the ratio of Y/Zn more than 0.5, long period structure ( X phase-Mg12YZn-LPS) formed. The W phase formed on a large scale of 0.05-0.5. These phases have both contribution on mechanical properties and damping capacities, and the contributions from big to small are: W+LPS>LPS>W+I>W. Also found in the end that in the best performance of the alloys, when W and LPS phases exist, the new phase relationships between W phase and Mg matrix formed([001]W//[011—0]α,(110)W//(0001)α).