Intermetallic Phase Stability And Its Effect On Properties Of Mg-Al-Ce Alloy

Nowadays,more and more attention has been paid to energy and environmental problems.The task of energy saving and emission reduction is becoming more and more arduous,and the demand for environment-friendly materials is becoming increasingly urgent.As the lightest metal structural material,the magnesium alloy has attracted more and more attention due to its excellent performances of high specific strength,high specific stiffness and easy recyclability.In the trend of energy saving and emission reduction,magnesium alloys show great application value and have been using in the aerospace field,automotive industry and 3C products to some extent.However,the poor high-temperature performance of common magnesium alloys greatly limits its further application on key parts of automobile,which is one of the most promising fields.Mg-Al-RE(AE)heat-resistant alloys have a wide application prospect in automobile and other related fields.The thermal stability of the main strengthening phase Al11RE3 is very important for the high-temperature properties of AE alloys.However,until now this key scientific issue remains controversial.This work was based on the above engineering and science background.The high-pressure die-cast(HPDC)magnesium-aluminum-rare earth alloy containing single rare earth cerium(Ce)was designed on the basis of the HPDC AE44 magnesium alloy with relatively excellent heat-resistant performance.The microstructural and strengthening phase thermal stability,mechanical properties and corrosion resistance of Mg-4Al-4Ce-0.2Mn(AlCe44)alloy and the modified alloys Mg-4Al-4Ce-0.7Si-0.2Mn(AlCe44Si)and Mg-6Al-3Ce-0.2Mn(AlCe63)were investigated.The microstructure of AlCe44 was mainly composed of a-Mg matrix and the intermetallic phases distributed at the grain boundary.The intermetallic phases mainly include about 5.034 wt.%acicular/lamellar Al11Ce3 phase and trace granular Al2Ce phase(-0.149 wt.%).The strengthening phases exhibited good thermal stability.The intermetallic phases were still stable without morphology and structure changing after heat treatment at 200? for 2000 hours or at 300? for 800 hours.Under more serious condition of 400? for above 50 hours,although the acicular/lamellar morphology of Al11Ce3 changed to granular morphology,the phase structure would not change.On the other hand,besides temperature,the stress is also a contributing factor to microstructural instability,and the microstructure near grain boundaries would become loose and part phase transition from Al11Ce3 to Al2Ce would be happen when the test condition was 70 MPa and just 200? for 100 hours.The tensile properties of HPDC AlCe44 could keep well until 250?.The ultimate tensile strength(UTS),tensile yield strength(TYS)and elongation are 110.4 MPa,102.5 MPa and 15.1%,respectively at room temperature and UTS and TYS are higher than 100 MPa at 250?.On the other hand,heat treatment could affect the properties of the alloy.With the increase of heat treatment temperature and time,the tensile strength of heat-treated samples would decrease gradually at room temperature.In addition,with the increase of heat treatment temperature time,the corrosion resistance of AlCe44 increased to some extent.The microstructure and the intermetallic phases of two kinds of modified alloys were roughly same as these of AlCe44 alloy.While there was a certain amount of Mg2Si in AlCe44Si,and there were 3.057 wt.%Al11Ce3 and 1.306 wt.%Al2Ce in AlCe63.After annealed at 400? for 200 hours,part of the acicular phase in AlCe44Si did not change,while in the same conditions,all the acicular phase have been transformed into granular morghology in the AlCe44 alloy.The edge of acicular intermetallic phase became coarsening in the alloy AlCe63 while no change happened in AlCe44 after annealed at 300? for 800 hours.Therefore,the preliminary research showed that the intermetallic phase stability of AlCe44Si alloy was better than that of AlCe44 alloy,suggesting that addition of Si could further improve the thermal stability of AlCe44 alloy,while the stability of intermetallic phase of AlCe63 alloy was worse than that of AlCe44 alloy.