Study On Microstructure And Its Effect On The Performance Of Heat Resistant Rare Earth-Magnesium Alloy

Magnesium and rare earth resources are abundant in our country,which provides unique advantages in researching and developing magnesium alloys containing rare earth.The development of automotive heat-resistant magnesium alloys will be the key to the development of magnesium industry in twenty-first century.In this dissertation,the goal is to develop heat-resistant magnesium for automotive transmission components.The study focuses on the performance of alloy materials,but also takes into account the cost of materials.It is a key research task for the microstructure stability and its effect on the performance of heat resistant magnesium alloy containing rare earth.Through researching the effect of pure rare earth and RE on the microstructures,mechanical properties and corrosion properties of as-cast Mg-Al-based alloy,the mechanisms of action and strengthening mechanisms of rare earth in the alloys were explored.The mircrostructures,mechanical properties and the strengthening mechanisms of second phase in Mg-12Ymm-4Zn(Ymm=Y-rich misch metal,wt.%)extruded alloy were explored.Mg-4A1-4RE(La:Ce:Pr:Nd =23:55:6:16,wt.%)magnesium alloys were prepared successfully by the high-pressure die-casting technique.The formability,microstructure,strengthening phase,tensile properties,mechanical properties as well as corrosion resistance were investigated systematically.The results show as follows.This series of alloys have good casting properties and formability,refined and uniform microstructure,no obvious casting defects.All of these indicate that the alloys have excellent performance of the die-casting.The grain size of the alloys is about 10?m.There exist two different strengthening phases in the alloys,i.e.Al2RE and Al11RE3,with the latter being the main strengthening phases in the alloys containing high RE content relatively.The outstanding microstructure of the HPDC Mg-4A1-4RE alloy is the“orderly stack" of the the Al11RE3 intermetallic compounds formed at grain boundaries.The quantitative phase analysis was performed by Rietveld refinement.The main strengthening phase of AE44 alloy is lamellar/acicular Al11RE3,coupled with some Al2RE particles.Phase concentrations of Al11RE3 and Al2RE are 5.73 wt.%and 0.36 wt.%,respectively.The main strengthening phase Al11RE3 is not thermal stability when heating temperature at 400? up to 5000h.The Al-RE phase morphology evolves from lamellar/acicular to short-rod and finally to polygonal particle.The distribution of Al-RE phases becomes loose gradually,and the Al-RE phases no longer only concentrate at the grain boundaries.Some Al11RE3 intermetallics would be transformed to Al2RE in the test condition(5000 h).Phase concentrations of Al11RE3 and Al2RE are 4.46 wt.%and 0.96 wt.%,respectively,when heating temperature at 400? up to 5000 h.The mechanical properties of the studied alloy can keep well until 200?.The ultimate tensile strength(UTS),tensile yield strength(TYS)and elongation(?)are over 252 MPa,146 MPa,11.4%respectively,at room temperature(RT)and the ultimate tensile strength(UTS),tensile yield strength(TYS)and elongation(?)are 112 MPa,106 MPa,25.1%at 200?.The cold-chamber HPDC AE44 alloy has a relatively high corrosion-resistance property due to the lamellar/acicular Al11RE3 clustered at grain boundaries as the corrosion barrier.After heat treatment,the corrosion performance degrades due to the changes of the microstructure,as well as morphology and distribution of Al-RE phases.Mg-xAl-yLa(x=4,8;y=2,5,8)series alloys were prepared by metal mould casting method and their microstructures,thermostability,mechanical properties and corrosion properties were investigated.AlLa45 alloy is mainly composed of ?-Mg and Al11La3 phases.AlLa42 and AlLa88 alloys are composed of a-Mg??-Mg17Al12 and Al11La3 phases.Results showed that,AlLa45 alloy exhibited the best comprehensive mechanical properties at room temperature and high temperature in their own alloy systems.The enhancement could be attributed to grain refining strengthening,secondary phase strengthening induced by Al11La3 second phase particles.Mg-4Al-xPr(x=2,5)series alloys were prepared by metal mould casting method and their microstructures,thermostability,mechanical properties and corrosion properties were investigated.Adding Pr element to Mg-4Al alloy led to the Mg17Al12 phase that has poor thermal stability and tends to coarsen rapidly at elevated temperatures was vanished.The content of Al11Pr3 and Al2Pr strengthening phases are increased significantly with the increase of the rare earth(Pr).Al11Pr3 phase is sensitive to temperature.The main strengthening phase Al11Pr3 is not thermal stability when heating temperature at 400? up to 5000 h.The Al11Pr3 phase morphology evolves from lamellar/acicular to polygonal particle AlbPr phase.AlPr45 alloy exhibited the best comprehensive mechanical properties with room and high temperatures.The enhancement could be attributed to fine crystal reinforcing and grain refining strengthening.Mg-4Al-xRE(x=2,5)series alloys were prepared by metal mould casting method and their microstructures,thermostability,mechanical properties and corrosion properties were investigated.AE42 and AE45 alloys are mainly composed of a-Mg,a large amount of Al11RE3 phases and a small amount of Al2RE.The lamellar/acicular Al11RE3 clustered at grain boundary.The main strengthening phase Al11RE3 is not a thermal stability phase when heating temperature at 400? up to 5000 h.The Al11RE3 phase morphology evolves from lamellar/acicular to polygonal particle Al2Pr phase.AE45 alloy exhibited the best comprehensive mechanical properties with room and high temperatures.The enhancement could be attributed to the lamellar/acicular Al11RE3 clustered at grain boundary and grain refining strengthening,secondary phase strengthening induced by Al11Pr3 second phase particles.A new Mg-12Ymm-4Zn(Ymm=Y-rich misch metal,wt.%)extruded alloy was prepared by water-cooled mold casting and hot extrusion,and its ultimate tensile strength(UTS)and tensile yield strength(TYS)could reach 314 MPa and 231 MPa at 300?,which were slightly lower than those at room temperature(338 MPa and 278 MPa).So far there is rare report on such high strength of Mg alloys at 300?,and it has been confirmed that introducing long period stacking ordered(LPSO)phase with high volume fraction and nano-spaced stacking faults(SFs)is an effective way to develop extruded Mg alloys with excellent high-temperature strength.