Effect Of Substitution Of Cu,Co,Ni For Zn On Structure And Mechanical Properties Of LPSO Dominant Mg–2Y–2Zn Alloys

Due to its low density,high specific strength and rigidity,magnesium alloy has good electrical and thermal conductivity,and has many advantages such as excellent damping,shock absorption and electromagnetic shielding.It is widely used in aerospace,automotive,and 3C fields,and has great research value and application prospects.However,the mechanical properties are poor in plasticity at room temperature and the absolute strength is low,which restricts the wide application of magnesium alloys.In recent years,Mg-RE-Zn magnesium alloys with long-period stacking structure(LPSO phase)as the reinforcing phase have received more and more attention due to their excellent mechanical properties.However,the influence of LPSO phase types,morphology,volume fraction and distribution on the mechanical properties of the alloy is not clear.In this paper,based on the Mg-Y-Zn system,the effect of replacing Zn with three alloying elements of Cu,Co,and Ni on the microstructure and mechanical properties of long-period magnesium alloys has been studied to further improve the strength of magnesium alloys and expand the application of magnesium alloys.The main findings are as follows:The gradual replacement of Zn by Cu element on the influence of long-period magnesium alloy Mg-Y-Zn reveals that the as-cast state of Mg₉₆Y₂Zn₂,Mg₉₆Y₂Zn₁Cu₁and Mg₉₆Y₂Cu₂is mainly composed of?-Mg phase,LPSO phase and intermetallic compound.With the replacement of Cu,the morphology of the LPSO phase in the as-cast alloy gradually changed from bulk to lamellar and the volume fraction gradually increased.After solution treatment,due to the poor thermal stability of the LPSO phase formed by the Mg-Y-Cu system,the volume fraction of the LPSO phase gradually decreases.The yield strength,tensile strength and elongation of Mg₉₆Y₂Cu₂alloy after extrusion are 305 MPa,446 MPa and 5.3%.The excellent mechanical properties are mainly due to the increase of lamellar LPSO phases and grain refinement.The effect of replacing Cu with Co and Ni on the Mg-Y-Zn-Cu alloy was studied,and it was found that the as-cast,solid solution and extruded states of Mg₉₆Y₂Zn₁Cu_0.5Co_0.5and Mg₉₆Y₂Zn₁Cu_0.5Ni_0.5alloys all consist of?-Mg phase,LPSO phase and intermetallic compound phase.The introduction of Co element makes the alloy contain more Mg YCo₄phase.After solution treatment at 500?,it was found that the addition of Co and Ni increased the thermal stability of the LPSO phase in the alloy,and the volume fraction of the LPSO phase increased slightly.The two elements have different effects on the mechanical properties of the extruded alloy.The addition of Co makes the plasticity of the alloy increase,while the addition of Ni makes the strength of the alloy increase significantly.This is mainly related to the volume fraction of the LPSO phase and the grain size of the extruded alloy.The effect of replacing Cu with Co and Ni on Mg-Y-Cu alloy is studied.Mg₉₆Y₂Cu₁Co₁and Mg₉₆Y₂Cu₁Ni₁alloys are composed of?-Mg phase,LPSO phase and intermetallic compound phase in the as-cast,solid solution and extruded states.In Mg₉₆Y₂Cu₁Co₁alloy,it is found that Co element with Y and Mg forms a large amount of Mg YCo₄phase.The volume fraction of LPSO phase in the alloy is the smallest,indicating that the addition of Co element is not conducive to the formation of LPSO phase in the alloy.The Mg₉₆Y₂Cu₁Ni₁alloy has the largest volume fraction of LPSO phase.It can be seen that Ni element is easier to form LPSO phase than Cu,Co,and Zn.After solution treatment at 500?,the LPSO phase formed by Mg₉₆Y₂Cu₁Co₁and Mg₉₆Y₂Cu₁Ni₁alloys showed strong thermal stability.In the TEM observations of extruded Mg₉₆Y₂Cu₂,Mg₉₆Y₂Cu₁Co₁and Mg₉₆Y₂Cu₁Ni₁,it can be seen that Mg₉₆Y₂Cu₂alloy contains a large number of dense lamellar LPSO phases,Mg₉₆Y₂Cu₁Co₁alloy contains bulk LPSO phases and very few lamellar LPSO phases,and Mg₉₆Y₂Cu₁Ni₁alloy contains bulk LPSO phases and lamellar layers.The tensile strength of the three long-period magnesium alloys Mg₉₆Y₂Cu₂,Mg₉₆Y₂Cu₁Co₁and Mg₉₆Y₂Cu₁Ni₁at room temperature are446 MPa,363 MPa and 426 MPa,respectively.The difference in mechanical properties is mainly due to the difference in LPSO phase morphology and grain size.