The Effect Of Solution Treatment On The Microstructure And Mechenical Properties Of AZ31-xGd Alloys

Magnesium alloy has occupied the focus in automotive and aerospace industries due to low density and high specific intensity, especially in the situation that resources and energies are reducing very quickly at present. AZ31 magnesium alloy, the representative of wrought magnesium alloys, is always given close attention by researchers. However, it is a key problem to impede engineering application of AZ31 magnesium alloy for poor absolute intensity.At present, most researchers take advantage of alloying and heat treatment to improve the property of AZ31 magnesium alloy.The AZ31 magnesium alloys with different mass fraction of Gd element in the paper was smelted and produced in well type resistance furnace under the protect of Ar gas. The microstructure and phase composition of AZ31-xGd（x=4.83, 4.15, 3.52, 3.25） alloys was characterized by OM, SEM,EDS and XRD. At first the effect of 4.83 wt.%Gd on the microstructure of AZ31 magnesium alloy was discussed. Then the as-cast AZG315 alloy was carried out solid solution treatment for different time at 540℃ in order to obtain the law on the morphology evolution of the second phase（Al2Gd）. Meanwhile we put forward the “diffusing and growing up” model, and calculate the diffusion coefficient of Gd element at 540℃. The solid solution treatment process for AZ31-xGd alloys has been determined in this paper based on the microstructure evolution of Al₂Gd phase in AZG315 alloy and the microhardness change of the solutionized AZG315 alloy for different solution time. Finally the evolution discipline on the microstructure and mechanical properties of solutionized AZ31-xGd alloys were discussed in this paper.The following conclusions were reached:1. It is effective to inhibit the formation of Mg₁₇Al₁₂ eutectic phase along boundary by adding 4.83 wt.% Gd to AZ31 alloy.2. The Gd atom and Al atom dissolve into the melt during refinement.Al₂Gd superlattice is formed and grow up by Gd atoms and Al atoms concentrate at the interface between solid and liquid when the matrix grows with dendrite method.3. Al₂Gd phase in as-cast AZG315 alloy is acicular and distributes discontinuously due to the directional diffusion of Gd atoms and Al atoms.4. The morphology changes of Al₂Gd phase will take place under the stimulus of a certain temperature according to the results of forging experiment at room temperature.The diffusion coefficient of Gd atom is 105.06nm²/s at540°C. The acicular Al₂Gd phase will diffuse and thicken in solution process then the thickness will remain the same after solution for 6h.5. The morphology evolution of Al₂Gd phase is coincident with the Ostwald Ripening Theory during solution treatment 540°C.6. The yield strength, ultimate tensile strength and elongation of the solutionized AZG315 alloy is 135 MPa, 244.29 Mpa and 10.93%, respectively,which is increased by 123.95%, 59.89% and 2.34% than that of the solutionized AZ31 alloy.7. The microstructure of Al₂Gd phase is mainly related to Al/Gd atomic ratio when the addition of Gd element is less 4.83 wt.%.8. The acicular Al₂Gd is smaller in as-cast AZ31-3.25 Gd. It will become smaller granule and scatter uniformly after solution treatment which will be good for improving the mechanical properties of alloy.The dynamics model for “diffusing and growing up” has been put forward and the diffusion coefficient of Gd atom in α-Mg matrix at 540°C has been calculated in this paper according to determining the alloys composition and heat treatment process, which provides the basic experiment data for improving the mechanical properties of AZ31 alloy and expanding the application range of rare earth element.