Effect Of Microstructure On Corrosion,Ignition Proof And Fatigue Behavior Of Mg-RE Alloys

The application of magnesium alloys in the fields of industry and people’s livelihoods is restricted by the problems of flammability,corrosion,and poor fatigue properties.It is of great significance to reveal the internal mechanism of these problems and propose solutions to broaden the application range of magnesium alloys.In this dissertation,the commercial WE43 alloy is taken as the research object.It is determined that precipitatate is the controlling factor of corrosion rate by studying the influence of different microstructures on corrosion behavior.The mechanism that the second phase can effectively improve the corrosion resistance of magnesium alloy is further analyzed by preparing Mg-RE alloys containing a high volume fraction second phase.WE43 alloy belongs to Mg-RE alloy with medium rare earth content.Its ignition-proof temperature is only 644℃,which is difficult to meet the requirement of melting and thermomechanical processing.In this dissertation,the mechanism of the second phase with a high volume fraction on ignition-proof behavior and the formation mechanism of the dense high-temperature oxide film are systematically studied by preparing MgRE alloys with a high volume fraction second phase.The research on the fatigue behavior of WE43 alloy is insufficient,especially the effect of microstructure on dislocation behavior is not clear during cyclic deformation.The low cycle fatigue behavior of WE43 alloys with different microstructures is systematically studied.Finally,in this dissertation,hot isostatic pressing(HIP)technology is conducted on WE43 alloy,and its service performance is improved by changing the microstructure.The mechanism of HIP improving the mechanical properties is analyzed,and the effectiveness of HIP in improving the service properties of magnesium alloys is demonstrated.The effect of microstructure on the corrosion behavior of WE43 alloy in 3.5 wt.%NaCl aqueous solution was studied.The different microstructures were obtained by hot rolling.The corrosion behavior was evaluated by immersion test and electrochemical test.The results showed that compared with grain refinement,basal texture,and deformation twin,the precipitate,promoting the formation of dense corrosion product film,was the rate controlling step of corrosion behavior.The mechanism of the second phase with a high volume fraction on the corrosion behavior of Mg-RE alloy was studied.The results showed that the higher the volume fraction of the second phase was,the better the corrosion resistance of Mg-RE alloy was.The Volta potential difference between the second phase Mg5Gd and Mg matrix was about 53 mv,so the Mg5Gd formed a sacrificial anode and played a cathodic protection role.A dense three-layer protective corrosion film was formed on aged WE43 alloy in 3.5 wt.%NaCl solution,and the film is discussed in detail by direct imaging.The threelayer corrosion product film is composed of RE2O3,MgO layer doped with RE2O3 and Mg(OH)2 layer from inside to outside.The RE2O3 has the preferential orientation with(211)plane and[231]direction parallel to(002)plane and[110]direction of MgO,respectively.Thus,the rare earth oxide particles can fill the gap of porous MgO layer,and improve the compactness of the film.The inner RE2O3 layer and mixed layer of MgO and RE2O3 play an important role in the formation of the protective film.The effect of the second phase with high volume fraction on the ignition-proof behavior of Mg-RE alloys was studied by thermogravimetric analysis.The microstructure of the oxide film was characterized by transmission electron microscopy.The results showed that the second phase with high volume fraction can promote the formation of dense oxide film on the melt surface,thus improving the ignition-proof performance.The oxide film was divided into three layers.The outermost layer was Gd2O3 and a small amount of ZrO2,the middle layer was MgO and the bottom layer was the cubic phases(Gd,Zr)H2.The closely arranged cubic phases played an important role in the compactness of the film.The low-cycle fatigue behavior of solutionized(T4)and aged(T6)WE43 magnesium alloys was studied at room temperature.The plastic strain amplitude decreased with the increasing cycle number in T4 alloy,which was due to the dense persistent slip bands and dynamic precipitates hindering<a>dislocation slip.In contrast,the plastic strain amplitude increased gradually in T6 alloy,which was attributed to the enhanced activation of pyramidal slip.The hindering effect of precipitation on dislocation slip and twin growth provided the critical resolved shear stress(CRSS)necessary for the activation of pyramidal<c+a>.The effect of hot isostatic pressing(HIP)on the microstructure and mechanical properties of WE43 alloy was investigated.The evolution of microstructure such as shrinkage porosity and precipitation free zones(PFZ)formed around grain boundary was characterized in detail.After HIP treatment,the density of the alloy increased,the distribution of alloying elements was more uniform,and the width of the PFZ decreased.The room-temperature tensile properties and low-cycle fatigue life were significantly improved,and the crack propagation mode changed from intergranular fracture to intergranular and transgranular mixed fracture.