The Investigation Of Microstructure,Mechanical And Corrosion Properties Of Extruded Mg-Dy-Ni Alloy Sheets

The development and application of magnesium?Mg?alloys have been restricted due to their rapid loss of strength at elevated temperature and poor corrosion resistance.The long period stacking ordered structure?LPSO?phase possess excellent mechanical and physical properties due to its unique atomic stacking arrangement.So,the addition of LPSO phase into?-Mg matrix could significantly improve the high temperature mechanical properties of the alloy,which has a wide prospect in application.However,as a new Mg/LPSO heat-resistant magnesium alloy,there are still many essential scientific issues to be solved.For example,the influence of the structure type of LPSO phase on the high temperature mechanical properties and the strengthening mechanism of the alloy are still not clear.Also,the effect of the structure type of LPSO phase on the corrosion properties of the alloy,and corresponding the corrosion mechanism needs to be further clarified.The solution of these scientific problems lays a data foundation for the design and development of new heat-resistant magnesium alloys.Combined with the above two scientific problems,the extruded Mg-Dy-Ni alloy sheets containing LPSO phase was prepared by hot extrusion process.The microstructure evolution of the alloy during ageing process was systematically investigated.The relationship between microstructure and mechanical properties at high temperature was revealed,and the strengthening mechanism of different LPSO phases in the alloy was elucidated.On this basis,the corrosion properties alloy with different LPSO phase were studied,and the corrosion behavior and mechanism of LPSO phase alloys were discussed.The experimental results are as follows:The as-extruded alloy is mainly composed of?-Mg,Mg₂Dy phase,lamellar18R-LPSO phase distributed along extrusion direction and small amounts of fine stripe 14H-LPSO phases in the grain interior.After ageing at 250?,the lamellar18R-LPSO phase in the extruded alloy gradually change into blocky one,and the volume fraction of the blocky 18R-LPSO phase increased gradually with increasing ageing time.A large number of 14H-LPSO phases were precipitated in the grain of the extruded alloy.In addition,the average grain size of the extruded alloy also slightly increased after a long time ageing.Interestingly,the extruded alloy exhibited two ageing peaks aged for 54 h and108 h at 250?,whose peak hardness values are 90 HV and 91 HV increasing by 10%as compared with that?82 HV?of as-extruded alloy.The first ageing peak was due to a high number density of blocky 18R-LPSO phases in conjunction with the precipitation of underaged 14H-LPSO phase.The second ageing peak mainly arise from the precipitation of high volume fraction of 14H-LPSO phases in the grain interior,thus the strengthening role of precipitation strengthening mechanism were enhanced.The tensile testing results show that the extruded alloy aged for 108 h exhibits the highest tensile yield strength and ultimate tensile strength at temperatures ranges from RT to 300?,and the extruded alloy aged for 54 h indicates the highest elongation to failure at 300?.The high tensile strengths of the alloy are mainly attributed to grain refinement and precipitation strengthening of 14H-LPSO phase and dispersion strengthening of blocky 18R-LPSO phase.Compared with that in the extruded Mg-Dy-Zn and extruded Mg-Dy-Cu alloys,the lamellar 18R-LPSO phase has a higher volume fraction and distributes more dispersively in the extruded Mg-Dy-Ni alloy.The corrosion rate of the extruded Mg-Dy-Ni alloy measured by weight loss method is higher(5.28×10^-3g/cm²/h)and the corrosion current measured by polarization is larger(2.54e^-3A)than that of the other two alloys.The electrochemical impedance spectroscopy shows that the high frequency capacitive arc radius of the alloy is smaller.The charge transfer resistance in the solution after corrosion is only 0.91?/cm²,thus the corrosion resistance of the alloy is lower than that of the other two alloys.The main corrosion type of the extruded Mg-Dy-Ni alloy is filiform corrosion,because the typical corrosion filament exists at the interface of a-Mg/18R-LPSO,and is consistent with that of extrusion direction.The high corrosion rate of the alloy is attributed to the formation of micro-couples promoted by 18R-LPSO phase with high volume fraction and dispersed distribution,and the filamentary corrosion morphology further provides a channel for the penetration of the corrosion solution.In addition,the lower hydrogen evolution overpotential of Ni itself makes the alloy prone to hydrogenation and improves the corrosion rate of alloy.Additionally,the smaller grain size is more easy to accelerate the rate of hydrogen release of alloy during corrosion.The corrosion product of three alloys is porous Mg?OH?₂.It is difficult to prevent the corrosion solution from penetrating into the matrix.Simultaneously,the hydrogen produced by the reaction also causes the exfoliation of the corrosion product,which further accelerates the corrosion of the alloy.