Effect Of Aging Treatment On Microstructure And Mechanical Properties Of AZ31 Magnesium Alloy Containing Gd

Due to their high specific strength,low density,good cutting performance and casting performance,the demand of magnesium alloy has been increased in the automotive and aerospace industries.As a representative of wrought magnesium alloys,AZ31 magnesium alloys has been closely attention to by researchers.However,the absolute low strength of AZ31 magnesium alloy is a crucial problem in engineering applications.Now alloying and heat treatment are effective measures to raise the performance of AZ31 magnesium alloy.In this experiment,five groups of AZ31 magnesium alloys with different Gd contents were prepared by smelting in an electric resistance furnace under the protection of Ar gas,followed by solid solution and aging treatment.The microstructure and mechanical properties of as-cast,T4?500°C,16h?and T6?200°C,32h?AZ31-x Gd alloys?x=0,3.25,3.52,4.15,4.83?were characterized by means of OM,SEM,EDS,Vickers hardness and room temperature tensile test.First,the grain size of as-cast and T4 AZ31 magnesium alloy with different Gd content was observed.The second phase of the alloy was observed and the Vickers hardness of the AZ31-xGd alloy was measured.Then,the age hardness of AZ31-xGd alloy was tested,and the best T6 process was obtained.The evolution of the morphology of the second phase of AZ31-3.52Gd alloy during aging treatment was studied.The model of the second phase"stacking growth"was proposed and the diffusion rate of Al atoms in the?-Mg matrix was calculated.The following conclusions were reached:1.The addition of rare earth Gd resulted in grain refinement significant,when the rare earth Gd content is 3.52wt.%,the grain size of the as-cast decreases from 110?m to 44?m.After adding Gd to AZ31 magnesium alloy,Al₂Gd phase is preferentially formed,which inhibits the formation of Mg₁₇Al₁₂phase.With the increase of Gd content,the second phase increases obviously.When the Gd content of rare earth was 3.52wt.%,the distribution of the second phase was the most homogeneous.2.The Vickers hardness of the as-cast AZ31-xGd alloy showstrend that increases first and then decreases with the increase of Gd content.When the addition amount of Gd is 3.25wt.%,The hardness reaches the maximum.After the AZ31-xGd alloy was solution treated at 500?for 16 hours,the?-Mg₁₇Al₁₂phase and Al₂Gd compounds almost dissolved into the matrix completely,with only a small amount of them distribute on the?-Mg matrix and grain boundaries uniformly.The hardness of AZ31-xGd alloy samples increased after solution treatment.3.With the increase of aging time,the hardness of the alloy increases obviously,and the hardness reaches the maximum when the aging time is 32h.During the aging process,the morphology of Al₂Gd phase changes from granular to short rod to block,the uniform distribution of short rod Al₂Gd phase has the effect of precipitation strengthening and dispersion strengthening.Therefore,the change of Al₂Gd phase morphology can improve the mechanical properties of magnesium alloy significantly.The optimum T6 treatment process was determined as solution at 500?for 16h,aging at 200?for 32h.4.During the aging process,In the aging process,the precipitation of Al₂Gd phase conforms to the planar source diffusion of Al.When the aging time is 32h,the diffusion rate of Al atoms in the?-Mg matrix is Dv:31.34nm²/s.5.With the increase of Gd content,the yield strength,ultimate tensile strength and elongation all increased first and then decreased.The maximum yield strength,ultimate tensile strength and elongation of the alloy reached205MPa,101MPa and 18.5%respectively when the amount of Gd was3.52wt.%.The fine Al₂Gd particles in the bottom of dimples have the effect of strengthening the mechanical properties.Therefore,the mechanical properties of the alloy are improved.