Researches On The Solidification Characteristic And Mechanical Properties Of Mg-Zn-Gd-based Magnesium Alloy

Mg-Zn-Gd alloys are a series of the most promising heat resistant magnesium alloys due to their high room temperature mechanical properties and excellent creep resistant at elevated temperature,which attracts high attentions in the field of astronavigation,military and automotive industries.Fruitful works have been done to improve the strength of Mg-Zn-Gd alloys recently.However,the understanding on the solidification microstructure and the relationships between microstructure and mechanical properties is still unclear.In this work,solidification behaviors of Mg-Zn-Gd alloys were investigated by directional solidification experiments and free solidification experiments.First,different compositions of Mg-Gd and Mg-Zn-Gd alloys were designed.The effects of Gd concentration and solidification paramenters?G,v?on the directional solidified microstructure,as well as mechanical properties were studied.Then,the solidification path of Mg-Zn-Gd ternary casting alloys was investigated by free solidification experiments and numerical analysis.Finally,Mg-5.5Zn-2.0Gd-0.6Zr?wt%?magnesium alloy with low Gd content was developed.The microstructure and mechanical properties under different heat treatments were discussed.The main research results are as follow:?1?A modified Yokoyama model was developed.With this model,the self-diffusion coefficients of some pure metals,i.e.Mg,Zn,Gd,Al,Si,et al.have been calculated.It was found that the calculated self-diffusion coefficients were more close to the experimental results than other ones.On this basis,a prediction model for liquid diffusion coefficients in the multi-component melts was established.The predicted diffusion coefficients of solutes Si,Mg,Cu,Fe and Ni in Al-Si-Mg,Al-Cu-Mg and Al-Fe-Ni alloys showed in favorable agreement with the experimental ones.Thus,the diffusion coefficients of solutes Zn and Gd in Mg-5.5Zn-2.0Gd?wt%?melt were predicted.?2?The evolution of directional solidified microstructure for Mg-xGd?x=1.38,2.35,4.38 wt%?alloys were investigatied.Mg-1.38 Gd and Mg-2.35Gd?wt%?alloys exhibited a typical cellular structure in the wide range of growth rate?10²00?m/s?and there is without dendrites near the calculated criterion growth rate for cellular-dendrite transition?vc-d?by Kurz-Fisher model.For Mg-4.38Gd?wt%?experimental alloy with higher Gd content of,dendritic structures were observed when v=100²00?m/s.Using non-linear fitting method,the relationships between cellular spacing???and growth parameters?G,v?of Mg-xGd alloys were established at G=20³0K/mm and v=10²00?m/s.These measurement values of the cellular spacing were in good agreement with the ones calculated by Trivedi model.?3?The directional solidification structure and microsegregation of Mg-x Gd alloys were determined,which are reasonablly fitted to the results predicted by Scheil model.Tensile test showed that the directional solidified Mg-2.35 Gd alloy exhibited higher strength than the non-directional sample under the same cooling rate.The tensile strength of the directionally solidified experimental alloy was improved while the corresponding elongation decreases with the increase of growth rate and temperature gradient.?4?The evolution of directional solidified microstructure and the morphologies of the second phase for Mg-5.5Zn-xGd?x=0,0.8,2.0,4.0 wt%?alloys were firstly investigatied under different solidification paramenters.It is found the approximate criterion growth rate for cellular to dendritic transition?vc-d?for Mg-5.5Zn-xGd alloy decreased with the incease of Gd content.The relationships between the dendritic arm spacing??1,?2?and v for Mg-5.5Zn-xGd alloys were established using nonlinear fitting method.The values ?1 and ?2 decreases exponentially with the increase of v and the exponent values were found to be close to theory values of 1/4 and 1/3,respectively.?5?Room termperature mechanical properties of Mg-5.5Zn-xGd?x=0,0.8,2.0,4.0 wt%?alloys directionally solidified at different solidification paramenters were investigatied.The tensile test showed that the ultimate tensile strength?UTS?increased with the increase of growth rate for a certain composition of Mg-5.5Zn-xGd alloy.For a certain growth rate,UTS first increased from 0 wt% to 2.0 wt%,then decreased with the further increase of Gd content.The direcational solidied Mg-5.5Zn-2.0Gd experimental alloy showed the maximum ultimate tensile strength.?6?The solidification path of Mg-5.5Zn-2.0Gd?wt%?ternary casting alloy was investigated by experiments and numerical analysis.Experimental results showed that at lower cooling rate??0.75K/s?,? + W?Mg₃Zn₃Gd₂?eutectic will be formed first,while at higher cooling rate??7.71K/s?,??Mg?+ I?Mg₃Zn₆Gd?eutectic will be formed first.A numerical model for predicting solidification path of the primary phase in multi-component alloy with considering the effects of solute diffusion in liquid phase and the cooling rate was developed.It was found that the numerical results were in favorable agreement with the experimental ones.Furthermore,for Mg-Zn-Gd alloy,higher Zn-content and higher cooling rate will promote the formation of I?Mg₃Zn₆Gd?phase.However,higher Gd-content and the lower cooling rate is favorable for the formation of W?Mg₃Zn₃Gd₂?phase.In addition,the tensile test of Mg-5.5Zn-2.0Gd experimental alloy showed that the ultimate tensile strength?UTS?,the yield strength?YS?and elongation were all improved with the increases of cooling rate.Meanwhile,the fracture morphologies changed from intergranular fracture characteristic to transcrystalline rupture characteristic.It was found that the improved strength of experimental alloys was mainly attributed to grain refining strengthening and the unique icosahedral structure of quasicrystal I?Mg₃Zn₆Gd?phase formed at high cooling rate could also further improve the mechanical properties.?7?Mg-5.5Zn-2.0Gd-0.6Zr?wt%?with moderate Gd content was designed and developed.It was found that the as-cast alloy was composed of ??Mg?matrix,interdendritic ??Mg?+ W?Mg₃Zn₃Gd₂?eutectic,icosahedral quasicrystalline I?Mg₃Zn₆Gd?phase and Mg3 Gd particles within ??Mg?matrix.After solution treatment at 505 oC for 16 h,most of ??Mg?+ W?Mg₃Zn₃Gd₂?eutectic dissolved into ??Mg?matrix,but a new phase,Zn2Zr3,precipitated within ??Mg?matrix.The appropriate aging treatment was done at 220 oC for 16 h,after which some dispersive fine eutectics re-precipitated along the grain boundary.Furthermore,rod-like ?1? and plate-like ?2? formed within ??Mg?matrix.The ultimate tensile strength,yield strength and elongation of the experimental alloy in the T6 state at room temperature were 280 MPa,175 MPa and 7.5%,respectively.According to strengthening contribution calculation,it is revealed that both the second phase strengthening and the solid solution strengthening played important roles in improving the mechanical properties after T6 treatment.