Study On Aging Precipitation Behavior Of Mg-9Li-xAl (x=3,6) Alloys

With the development of the urgent demand for lightweight materials and devices inmodern industrial field, Mg-Li alloys have been displaying a series of advantages such as lowdensity, high specific stiffness, high specific strength, good damping ability, excellentelectromagnetic shielding ability and easily recovery, which make them stand outconspicuously in the fields of automotive, medical equipment, IT industry and other fields.However, the existing commercial alloys of Mg-Li-Al, due to its poor stability performance,is easy to be soften at room temperature, which greatly limits the extensive application ofMg-Li alloys. Therefore, it is really necessary to study the aging behavior of Mg-Li-Al alloys.In this work, Mg-9Li-xAl (x=3,6) alloys were prepared by vacuum induction meltingmethod. Microstructure and phase composition were systematically studied in the process ofsolid solution and aging treatment by optical microscope (OM), scanning electron microscope(SEM)(with EDS), X-ray diffraction (XRD) and transmission electron microscope (TEM), aswell as hardness tests. Main conclusions are as follows:1、As-cast Mg-9Li-3Al and Mg-9Li-6Al alloys are mainly composed of eutecticstructure (α-Mg+β-Li), and black granular AlLi phases with particles diameter less than2μmin beta phase. Besides that, in LA96alloys, eutectic structure (α-Mg+AlLi) exists in alphaphase as a platelike morphology, and some white particles of MgLi2Al phases appearsimultaneously in beta phase. With the increase of Al content, hardness increases andmicrohardness of beta phase is higher than alpha phase.2、AlLi in alpha and beta phase is completely dissolved into the matrix after solidsolution treatment, while the MgLi2Al phase could not be completely dissolved into thematrix. Compared with air cooling, water cooling is more beneficial for formingsupersaturated solid solution in solid solution treatment. The optimized solid solutiontemperatures for LA93and LA96alloys are350℃and400℃, respectively, and temperatureholding for1h, water cooling. In the process of solid solution, lattice constants, a and cvalues in α-Mg phase, and c value in β-Li phase, decrease first and then slightly increase.3、LA93and LA96alloys attain peak hardness in solid solution for1h, which are75.45Hv and81.3Hv, respectively. In the process of solid solution, AlLi phase that is dissolvedinto the matrix can creat solid solution strengthening, and the precipitation strengthening becomes weaken due to the reduction of AlLi phase. The effect of these two mechanismsmake the microhardness of studied alloys increases first and then decreases.4、In the process of aging at room temperature, AlLi phase precipitates from LA93andLA96alloys. With the extension of room temperature aging time, more AlLi precipitates. Inartificial aging process, during an incubation period of aging in long time and lowtemperature or short time and high temperature, MgLi2Al generates in the alloys. Thisphenomenon is called reversible hardness. In the early two-stage aging process, MgLi2Alphase appeares in LA93and LA96alloys, with prolonging of aging time, MgLi2Al phasedecomposites into AlLi phase. Finally, over-aging phenomenon occures.5、In room temperature aging, the microhardness of Mg-9Li-xAl alloys has an upwardtrend with the increase of aging time; In artificial aging process, hardness of LA93and LA96alloys declines, when the temperature is higher than100℃, the new generationof MgLi2Alcauses the hardness gradually increasing. In the initial two-stage aging process, the emergingMgLi2Al phase makes the two alloys reach hardness peak, which is86.0Hv and92.4Hv,respectively; Whereafter, along with over aging phenomenon happening, hardness decreasesby17.44%and20.16%, respectively.