Effect Of Gd And Surface Coating On Microstructure,Mechanical Properties And Corrosion Resistance Of Mg-Li Alloy

Magnesium-lithium alloys exhibit an attractive combination of low density and high strength/weight ratio,making them ideal for lightweight engineering applications.However,the high chemical activity of mg-Li alloys greatly limits their applications,especially in the automotive and aerospace industries,where exposure to harsh operating conditions is unavoidable.Therefore,the development of effective corrosion protection systems for mg-Li alloys is a very important issue for their industrial applications.In the paper,the microstructure and properties of extruded Mg-8Li-3Al-x Gd（x=0,0.6,1.2 and 1.8 wt.%）and Mg-9Li-1Zn alloys were systematically studied by means of microstructure and properties testing methods.It provides some theoretical support for the subsequent research and development of high strength and high corrosion resistance magnesium lithium alloy.The surface microstructure,mechanical properties and corrosion resistance of the extruded Mg-8Li-3Al-x Gd（x=0,0.6,1.2 and 1.8 wt.%）alloys show that the Mg-8Li-3Al matrix alloy is mainly composed ofα-Mg,β-Li and Al Li phases.After Gd element was added to LA83 matrix alloy,α-Mg phase was obviously refined,and Al₂Gd particles were formed in the alloy.This is mainly due to the good crystallographic orientation between（111）_Al2Gd//（0001）_α-Mg in Mg-8Li-3Al-x Gd（x=0.6,1.2 and 1.8 wt.%）alloys.The results illustrate that the average lattice mismatches of（111）_Al2Gd//（0001）_α-Mg,（111）_Al2Gd//（112?0）_α-Mg,and（111）_Al2Gd//（011?0）_α-Mg are 12.96%,4.37%,and 9.63%,respectively.During the solidification process,Al₂Gd particles can be used as an effective heterogeneous nucleation substrate forα-Mg phase,which makes it a heterogeneous nucleation matrix and hinders the movement of grain boundaries and dislocation as well as grain rotation,thus refiningα-Mg phase.In the extruded Mg-8Li-3Al-x Gd alloy（x=0,0.6,1.2 and1.8 wt.%）,the maximum tensile strength（UTS）of LA83-1.2Gd alloy is 278 MPa,which is about 35%higher than that of LA83 matrix alloy（206 MPa）.The main reason affecting the strength of the extruded Mg-8Li 3Al-x Gd（x=0,0.6,1.2 and 1.8wt.%）alloy is that the Al₂Gd particles formed in the alloy after the addition of Gd element strengthen the fine grain ofα-Mg phase and the second phase of the alloy.The corrosion resistance of extruded Mg-8Li-3Al-x Gd alloys（x=0,0.6,1.2 and1.8 wt.%）was tested by Using Princeton P4000 electrochemical workstation.The comprehensive electrochemical results show that the corrosion resistance of LA83-x Gd（x=0.6,1.2 and 1.8 wt.%）alloys is improved after Gd element is added to LA83 alloy,and the corrosion potential(E_corr)of LA83-1.2 Gd alloy,-1.51(V_SCE),is the highest compared with other alloys.In addition,the corrosion current density(I_corr)of LA83-1.2Gd alloy is 26.25μA/cm2,which is 64%lower than that of LA83 alloy.The corrosion resistance of LA83 alloy decreases with the addition of rare earth element Gd,which is mainly due to the formation of three kinds of micro-couples at the interfaces of Al Li/α-Mg,β-Li/α-Mg and Al Li/β-Li respectively.With the addition of Gd element in the alloy,many Al₂Gd phases will be preferred to be generated,which will consume a large amount of Al element in the alloy,leading to the reduction of Al Li phase content in the main cathode or even almost replaced by Al₂Gd phase.Therefore,Al₂Gd phase becomes the main cathode phase in the process of micro-galvanic corrosion.In addition,the potential difference of Al Li/α-Mg,Al2Gd/α-Mg andβ-Li/α-Mg was found to be 340 m V,250 m V and 135 m V,respectively.It can be seen that the potential difference of Al₂Gd/α-Mg is smaller than that of Al Li/α-Mg,and the intensity of the micro-galvanic corrosion process is significantly reduced by the Al₂Gd phase as the main cathode meeting.Therefore,adding rare earth Gd into LA83 alloy can improve the corrosion resistance of the alloy.The effects of single-layer and double-layer coatings on the microstructure and corrosion resistance of Mg-9Li-1Zn（LZ91 for short）alloy were investigated by Means of Fourier transform infrared spectroscopy（FIRT）,scanning electron microscopy（SEM）and electrochemical testing.The LZ91 matrix alloy without surface coating is mainly composed ofα-Mg andβ-Li phases.Mg（OH）₂ protective film can be formed on the surface of the alloy after anodic oxidation coating.A mixture of tetraethoxysilane（TEOS）andγ-amino propyltriethoxysilane（APTES）with different volume ratios（silane solution）was coated on the surface of anodized coating alloy.The results show that the corrosion current density of LZ91 matrix alloy can be effectively reduced by anodic oxidation coating and anodic oxidation/silane double coating.Either a single anodized coating or a double layer coating is achieved by depositing a barrier layer to prevent the material from contact with a corrosive environment.However,because the protective film formed on the surface of anodic oxidation coating is stacked layer by layer in the electroplating process,many holes will be formed on the surface.The role of silane solution is to cover the holes of anodic oxidation coating,so as to better protect the alloy and make its surface not easy to be corroded.Adding different corrosion inhibitors on the surface of anodic oxidation coating will have different effects on the corrosion resistance of alloy.The mixed solution of Na OH and ethylenediamine tetraacetic acid disodium（EDTA-2Na）is a kind of corrosion inhibitor with alkaline p H value.The mixed solution of Na OH and EDTA does not improve the corrosion resistance of the alloy after a single layer of anodic oxidation coating.Adding sodium silicate solution on the surface of the single-layer coating alloy can effectively improve the corrosion resistance of the alloy,because Mg（OH）₂ on the surface of the single-layer anodized coating will react with sodium silicate,and also form a transparent silicate coating on the surface,thus improving the corrosion resistance of the alloy.