Effect Of Sm And Nd On The Microstructure And Mechanical Properties Of AZ81Magnesium Alloy

Today, with the increasingly serious of global energy crisis and environmentalpollution, the requirement of lightweight materials are becoming more and moreurgent. As the lightest metal structure material, magnesium alloy has become aresearch focus. AZ81magnesium alloy have a slightly lower Al content and tensilestrength at room temperature than that of AZ91magnesium alloy. The creepperformance and strength at high temperature of AZ series magnesium alloy is low.Rare earth alloying is a common and effective method to reinforce magnesiumalloy at present. The effect of Sm and Nd on the microstructure and mechanicalproperties of AZ81magnesium alloy using metallographic microscopy (OM), X-raydiffraction analysis (XRD), scanning electron microscopy (SEM) and transmissionelectron microscopy (TEM) methods, is studied in this thesis. The heat treatmenttechnology is optimized and the strengthening mechanism of Sm and Nd on AZ seriesmagnesium is discussed in this paper.In this paper, AZ81-1.0Sm-0.6Nd alloy performance best in the as-cast tensile testand the microstructure of AZ81-1.0Sm-0.6Nd alloy is analysied and its heat treatmentparameter is optimized to improve properties. And we find that the as-castmicrostructure of alloy are refined significantly and at the same time generate newphase Al2Sm and Al2Nd along the grain boundary after adding Sm and Nd. Bydifferential thermal analysis, the alloy solid solution temperature is determined at410℃, and the optimal solution time is12h which judged from metallographicanalysis. It use hardness test and conductivity measurement to demonstrate the agingtechnology of alloy is aging for16h at200℃.After adding a amount of Sm and Nd into AZ81alloy, the microstructure wasrefined. The further Transmission study found that the punctate and bacilliform β-Mg17Al12phases interlacely distribute. This structure can effectively hinder thedislocation’s base sliding and highly improve strength of alloy at room temperature. The new spherical stable high temperature phases Al2Sm and Al2Nd dispersedlydistribute in the matrix, pin dislocation and hinder the grain boundary slidingeffectively. But the bar phases are composed of Al2Nd and Mg17Al12, which are notconducive to improve the plastic of substrate.At room temperature, when the addition of Sm and Nd reach to2.4%, the alloyhave a tensile strength of283MPa which is10.55%higher than the matrix alloy haveand a highest elongation. Under150℃and175℃, the alloy, content1.6%Sm and Nd,have maximum tensile strength208.8MPa and176.5MPa, respectively, which are10.01%and8.37%higher than the matrix alloy have, respectively.At the condition of50-70MPa,150-200℃, the AZ81-1.0Sm-0.6Nd alloy have abetter creep performance than the matrix alloy with. Especially, under the condition of150℃/50MPa, the steady-state creep rate of AZ81-1.0Sm-0.6Nd is8.82x10-7s-1, Whichis significantly below the steady-state creep rate1.95x10-6s-1that the AZ81alloy have.With the raising of temperature and stress, this advantage decreases, and the effect oftemperature on creep properties is obvious.