Research On Damping Capacity And Mechanical Property Of Mg-Li Alloys

With the progress of the cause of aerospace techniques, transportation and3Cproduction, gradually, the weight-alleviation and vibration/strepitus-reduction of alloyshave coming to the centre of the research field. Magnesium alloy has been widelyconcerned for its low density and fine damping capacity. But the contradiction of thestrength and damping restricts the practical application of magnesium alloys. The Mg-Lialloy has the minimum density of all the alloys so far, and the addition of Li also makesits crystal structure changed. The emerge of the body-centered cubic structure changesthe damping mechanism of Mg-Li alloy, which makes the alloy not subject to therestriction of strength and damping, and even has a broad application prospect. But thestudy of its damping mechanism and change rules has been ignored, and the recentlyresearch of the high damping capacity is just stay in qualitative understanding. In orderto solve this problem, the optical microscope (OM), X-ray diffract meter(XRD),scanning electron microscope(SEM), tensile properties test, amplitude-damping test andtemperature-damping test are used to analyze the various properties of the Mg-Li alloy,and the damping mechanism and performance rules have been systematic studied. Afteralloying the binary alloy, its influence and law of the alloying element has beenanalyzed in order to set the stage for the further practical application of Mg-Li alloy.To begin with the analysis of Mg-xLi alloy, this study has conducted theperformance testing of Mg-xLi alloy of diverse phase composition, and reached thefollowing conclusion: addition of Li element could effectively transform the crystalstructure and phase composition of the alloy, as well as refine the grain. With thecontent of Li increased, the critical strain amplitude and damping performance would beenhanced with it. And the appearance of the β-Li phase in the alloy has substantiallyadvanced the yield strength, tensile strength and elongation of the alloy, among whichthe alloy of two-phase coexistence performed the maximum strength and stabledamping capacity. While the damping mechanism of Mg-Li alloy can no longer beexplained by the dislocation model of Granato-Lücke, a new superposition of dampingmechanism has emerged. Meanwhile, a dislocated damping P1-peak of the alloy arose inthe low-temperature region; and so did a sliding Grain boundary P2-peak inhigh-temperature region. Both of them can be concluded that the grain boundary slidingand the dislocation pinning can account for the enhancement of damping capacity of the alloy.Afterwards, this study conducted further analysis of the well-performedtwo-phase-coexistence-state out of all three phases. By researching the alloy of variousLi content under the state of two-phase-coexistence, following conclusion can be drew:the strength of two-phase-alloy changes slightly along with the content variation of Lielement, once the content was at8.5wt%, the α-Mg phase in the alloy emerged twodifferent grain shape, which turned out to be an advancement of damping capacity andmechanical property of the alloy. This have further validated the damping mechanism inMg-Li alloy, and this damping mechanism can be deemed as a new type one which wasengendered by the mutual superposition of three kinds of mechanism: dislocationpinning in low-temperature region, and sliding of Grain boundary in high-temperatureregion. Moreover, under low-temperature dislocation pinning were dominant factors,and accordingly, sliding of Grain boundary and dislocation pinning effected obviouslyunder high-temperature.Ultimately, this study used Al element to conduct the alloying treatment of Mg-Lialloy, by analyzing the performances of the ternary alloy (Mg-xLi-Al), we came tofollowing conclusion: the addition of Al element lead to the concomitant appearance ofboth β-Li phase and the second phase of MgAlLi2in grain boundary. The appearance ofthe second phase has enhanced the damping capacity of the alloy, and with the secondphase strengthened, the yield strength and the tensile strength of the alloy have beenadvanced. But at the same time, this treatment could hamper the elongation. After thetreatment of alloying, the ternary alloy (Mg-xLi-Al) at the state oftwo-phase-coexistence performed the highest strength and optimal damping capacity,and the equilibrium between mechanical property and damping capacity wassuccessfully realized.