Microstructures And Properties Of Newly Developed Mg-4Zn-2La-3Y Alloys

Mg-Al system has a good commercial value due to its good castability and high creep resistance.However,the wide application of the Mg-Al system has been greatly limited due to its moderate room-temperature strength or low high-temperature strength.The lightweight advantages of magnesium alloys in some industries such as aerospace and automotive cannot be replaced.So,it is urgent to develop new heat-resistant magnesium alloys.In this paper,Mg-2.3 La was used as the base and Zn was substituted for Al.At the same time,in order to achieve the purpose that Mg-La alloy and rare earth with high solid solubility can be strengthened together,Y is introduced.High strength and heat-resistant die casting,gravity casting and deformation Mg-4Zn-2La-3Y(ZLaW423)magnesium alloys have been prepared.It is of great economic value and theoretical significance to study its microstructure and properties.The results demonstrate that HPDC ZLaW423 alloy owns fine grains and approximately highly interconnected intermetallic phase skeletons,and exhibits ultra-high strength at both room and high temperatures.Interestingly,the eutectic intermetallic skeleton of this alloys is consisted of numerous fine particles,which are mainly consisted of two intermetallic phases,namely W and Mg₁₂RE.Multiple{101}twins and SFs were found in the Mg₁₂RE phase while a few of SFs in the W phase.Additionally,minor long-period stacking ordered phase was observed in the eutectoid phase,and it probably nucleated on the Mg₁₂RE phase following a certain OR as(0002)_14H//(110)_Mg12RE and[1120]_14H//[111]_Mg12RE,or(0002)_14H//(211)_Mg12REand[1120]_14H//[111]_Mg12RE.This special intermetallic skeleton with many interfaces and planar faults can efficiently transfer dislocations across grain boundaries,and this is the key factor for the outstanding mechanical properties of the HPDC ZLaW423 alloy.The creep curves of HPDC and gravity cast ZLaW423 alloy under the conditions of175?-225?/60 MPa-120 MPa only show two stages of slow down and steady creep.Combined with the creep stress index,the stress index of the two alloys under the test creep condition is between 3-6,and the corresponding creep mechanism is dislocation creep.HPDC alloy has no obvious phase transition before and after creep,but the existence of subcrystalline deformation zone proves the creep mechanism of dislocation climbing.The average calculated creep activation energy of the two alloys at the test temperature was about 93 k J/mol and 97k J/mol,respectively,indicating that the creep mechanism of HPDC and gravity cast ZLaW423alloys was dislocation creep accompanied by diffusion creep under the condition of 175?-225?/60 MPa-120 MPa.