The effects of heat treatment on the microstructure and mechanical properties of Mg-3Sn-1Mn alloy were preliminarily investigated by using optical and electron microscopy,X-ray diffraction（XRD） analysis,and tensile and creep test.The results indicate that the heat treatment has an obvious effect on the microstructure and mechanical properties of the Mg-3Sn-1Mn alloy.After the solid solution treatment at 420 ℃,a majority of the Mg2Sn phases in the alloy are dissolved into the matrix.However,after the further aging treatment at 250 ℃,lots of fine Mg2Sn phases in the aged alloy are precipitated at the grain boundaries and within the grains.As a result,the tensile and creep properties of the aged alloy are significantly improved.The mechanism for the higher tensile and creep properties of the aged alloy is related to the dispersive distribution of the Mg2Sn phase in the--Mg matrix.
Mg-14Li-1Al （LA141）, LA141-0.3Y, LA141-0.3Sr, and LA141-0.3Y-0.3Sr alloys were prepared in an induction furnace in the argon atmosphere. The microstructures of these alloys were investigated through scanning electron microscope （SEM）, X-ray diffractometer （XRD） and energy dispersive spectrometer （EDS）. The results show that yttrium and/or strontium additions produce a strong grain refining effect in LA141 alloy. The mean grain sizes of the alloys with addition of Y and/or Sr are reduced remarkably from 600 to 500, 260, 230 μm, respectively. Al 2 Y, Al 4 Sr and Mg 17 Sr 2 phases with different morphologies are verified and exist inside the grain or at the grain boundaries, thus possibly act as heterogeneous nucleation sites and pin up grain boundaries, which restrain the grain growth.
The as-cast microstructure and Sr-containing phases in the AZ31 magnesium alloys with different Sr contents （0%, 0.3%, 2.5% and 5.0%, mass fraction） were investigated. The results indicate that after adding Sr to the AZ31 magnesium alloy, the dendrite/grain size is decreased, and with the Sr content increasing from 0 to 5.0%, the dendrite becomes finer, the dendrite morphology becomes more passive and the distribution of alloying phases at dendrite/grain boundary is dispersed. Furthermore, the morphology of the β-Mg 17 Al 12 phase in the alloy with addition of 0.3%Sr changes from continuously irregular strip-like shape to discontinuously irregular strip-like shape and/or fine granule-like shape. At the same time, some lamella-like eutectic phases are found in the alloys with additions of 2.5% Sr and 5.0% Sr, and the lamella spacing in the alloy with addition of 5.0% Sr is finer. Adding high Sr content to the AZ31 alloy can bring the new ternary eutectic and/or divorced eutectic phase of Mg 11 Al 5 Zn 4 in the alloy, and the Mg 17 Sr 2 and Mg 2 Sr phases are formed in the alloys with additions of 2.5% Sr and 5.0% Sr.
The phase constituent evolution of Mg-Zn-Y-Zr alloys with the mole ratio of Y to Zn both in the as-cast and as-annealed states at the Mg-rich corner was investigated by XRD and SEM/EDS analysis and was further explained from the ternary phase diagram calculation. The results show that the formation of the secondary phases in Mg-Zn-Y-Zr alloys firmly depends on the mole ratio of Y to Zn, and X （Mg 12 YZn）-phase, W （Mg 3 Y 2 Zn 3 ）-phase and I （Mg 3 YZn 6 ）-phase come out in sequence as the ratio of Y to Zn decreases. The mole ratios of Y to Zn with the corresponding phase constituent are suggested quantitatively as follows： the phase constituent is α-Mg ＋ I when the mole ratio of Y to Zn is about 0.164; α-Mg ＋ I ＋W when the mole ratio of Y to Zn is in the range of 0.164 0.33;α-Mg ＋W when the mole ratio of Y to Zn is about 0.33; α-Mg ＋W＋X when the mole ratio of Y to Zn is in the range of 0.33 1.32; and α-Mg ＋X when the mole ratio of Y to Zn is about 1.32. The results also offer a guideline for alloy selection and alloy design in Mg-Zn-Y-Zr system.
The as-cast microstructure,tensile and creep properties of Mg-3Sn-1Ca and Mg-3Sn-2Ca magnesium alloys were investigated and compared by using optical microscopy and scanning electron microscopy,X-ray diffraction analysis and tensile tests. The results indicate that the as-cast microstructures of Mg-3Sn-1Ca and Mg-3Sn-2Ca alloys are different.The former is mainly composed ofα-Mg,eutectic CaMgSn and solid state precipitation of Mg 2 Sn,whereas the latter is mainly composed ofα-Mg, primary CaMgSn,eutectic CaMgSn and Mg2Ca phases.As a result,the two alloys obtain different tensile and creep properties. Mg-3Sn-1Ca alloy shows relatively higher ultimate tensile strength and elongation at room temperature and 150℃than Mg-3Sn-2Ca alloy,however,the yield strengths of Mg-3Sn-1Ca alloy at room temperature and 150℃are relatively low.In addition,the creep properties of Mg-3Sn-1Ca alloy at 150℃and 70 MPa for 100 h are obviously lower than those of the Mg-3Sn-2Ca alloy.
The as-cast microstructure, mechanical properties and casting fluidity of ZA84 alloy containing TiC were investigated. The experimental results indicate that adding 0.5wt%TiC to ZA84 alloy can refine the as-cast microstructure, and do not cause the formation of any new phase. After 0.5wt%TiC was added to the ZA84 alloy, the morphology of ternary phases on the grain boundaries changed from coarse quasi-continuous net to fine disconnected net, and the distribution of ternary phases became dispersive and homogeneous. At the same time, the tensile properties of ZA84＋0.5TiC alloy at room temperature were comparable to those of AZ91D alloy, and were higher than those of ZA84 alloy. At 150 ℃, the tensile and creep properties of ZA84＋0.5TiC alloy were also higher than those of ZA84 and AZ91D alloys. In addition, compared with the AZ91D alloy, the casting fluidity of ZA84＋0.5TiC alloy was slightly poor, but better than that of ZA84 alloy. The reason could be related to the effect of TiC on the solidification temperature range of ZA84 alloy.
To reduce the surface cracks of extrusion rod for AZ31 magnesium caused by nonhomogeneous metal flow in extrusion process, 3D computer finite element （FE） simulations of extruding a wrought magnesium alloy AZ31 into rods have been performed and the results have been verified in extrusion experiments under identical conditions. The tendency to generate the dead zone is decreased by employing the die angle 60° at the cone-shaped die comparing with the die angle 180°. The surface additional tensile stresses of the rod at the die exit are decreased greatly so that the surface cracks caused are avoided by using the die angle 60°. The extrusion die with die angle 180° would increase the higher temperature rise and possibility of crack formation on the rod surface that caused by die angle 60° and temperature rise decrease tensile strength of the AZ31 rod. The experimental results show that die angle 180° could cause continuous cracks on the surface of the extruded rod. The extrusion force required is reduced approximately 15 ton by employing the die angle 60°. Theoretical results obtained by the DeformTM-3D simulation agreed well with the experiments. The obtained results provide the fundamental and also practical guidelines for the design and correction of dies to produce magnesium rod with good surface quality.
The influence of impurity content on the microstructure and mechanical properties of ZK60 magnesium alloys was investigated by optical microscopy,scanning electron microscopy and tensile test.ZK60 alloys were prepared by changing holding time of alloy melt during semi-continuous casting in order to control the content of impurity elements.The alloy with lower purity content is found to have less second precipitates and larger grain size in the as-cast state.However,in the as-extruded state,reducing impurities brings about a decrease in grain size and an increase in yield strength from 244 MPa to 268 MPa,while the elongations in the as-extruded alloys with different contents of impurities are almost the same.After T5 treatment,impurity content is found to have more obvious effect on the yield strength of ZK60 alloy.The yield strength of ZK60-45 alloys with low impurity content is increased up to 295 MPa after T5 treatment.
The influence of impurities on damping capacities of ZK60 magnesium alloys in the as-cast,as-extruded and T4-treated states was investigated by dynamically mechanical analyzer at room temperature.Granato and Lucke dislocation pinning model was employed to explain damping properties of the alloys.It is found that reducing impurity content can decrease the amount of second-phase particles,increase grain size and improve damping capacity of the as-cast alloy slightly.The as-extruded alloy with lower impurity content is found to possess obviously higher damping capacity in the relatively high strain region than that with higher impurity concentration,which appears to originate mainly from different dislocation characteristics.The variation tendency of damping property with change of impurity content after solution-treatment is also similar to that in the as-extruded and as-cast states. Meanwhile,the purification of the alloy results in an evident improvement in tensile yield strength in the as-extruded state.
