Analysis Of Solidification Path And Mechanical Properties Of Mg-2Ni-(4-x)Gd-xY-0.5Zr(x=0,1,2,3,4wt.%) Alloys

Due to the unique structure and great strengthening effect,Mg-Zn-RE alloys containing LPSO phase have been the research focus of magnesium alloys.Ni element can replace Zn element in Mg-Zn-RE alloys to achieve the same strengthening effect,so Mg-Ni-RE alloys have become a research hot-spot after Mg-Zn-RE alloys.At present,researches on the solidification microstructure and mechanical properties of Mg-Ni-RE alloys with single rare earth element have some achievement.However,there are fewer researches on Mg-Ni-RE alloys with two rare earth elements.In order to enrich the database of Mg-Ni-RE alloys and further explore the effect of phases types on the mechanical properties of the alloys,Mg-Ni-RE alloys were designed with Y and Gd as rare earth elements,and set the Ni/RE ratio of 0.5,at the same time,the total mass fraction of Gd and Y remained the same.Solidification path and mechanical properties of Mg-Ni-Gd-Y quaternary alloys were studied.In this paper,phase diagram analysis and experimental verification were used to study the solidification path and predict microstructure of the alloys.Based on Thermo-Calc software and its TCMG4 database,the isothermal section phase diagrams of Mg-Gd-Y ternary alloys were calculated.Combined with the published experimental phase diagrams of Mg-Ni-Gd and Mg-Ni-Y ternary alloys,the possible precipitated phases in the Mg-Ni-Gd-Y alloys were predicted.Based on the solidification cooling curves of the alloys’collected by double thermo-couples differential thermal analysis device,the solidification fraction,the phase precipitation temperature and phase transformation type were determined,and then the solidification path of the alloys was analyzed based on the data collection.The microstructure morphology of as-cast and solution-state alloys were observed by metallographic microscope,scanning electron microscope and transmission electron microscope.Tensile and compressive mechanical properties and hardness of alloys in as-cast and solution-state were measured by universal testing machine and Vickers microhardness tester.The results showed that there are three precipitated phases in as-cast alloys,which areα-Mg matrix,18R-LPSO phase and binary phase Mg₅Gd or Mg₂₄Y₅.After solution treated at450℃×10 hours,the grid-like LPSO phase was discontinuous,and the structrue transformed from 18R to 14H,at the same time a new binary phase Mg₂Ni precipitated in the location of disolved LPSO phase.After solution treated at 500℃for 10 hours,Mg₅Gd/Mg₂₄Y₅phase was unstable and decomposed,and onlyα-Mg matrix,LPSO phase and Mg₂Ni phase were left in the alloys.The tensile and compressive properties of same alloys in as-cast state are better than those in solid solution state.As the LPSO phase in the as-cast alloy is continuous,which plays an important role of second phase strengthening and hinders dislocation movement.In the microstructure of alloys at solid solution state,the precipitated phase is discontinuous,which reduced the second phase strengthening effect and deteriorates the alloys’mechanical properties.At the same state,Mg-2Ni-4Gd-0.5Zr alloy has the best tensile properties,with tensile strength and elongation reaching 117MPa and 3.70%,respectively.As-cast Mg-2Ni-4Y-0.5Zr alloy exhibits the best compressive properties,with compressive stress reaching 257MPa under the same strain.At the same state,Mg-2Ni-4Y-0.5Zr alloy has the best hardness,its as-cast state hardness is 61.2HV,and Mg-2Ni-4Y-0.5Zr alloy has the worst hardness,its as-cast state hardness is 50.0HV,indicating that the alloys’hardness is related to the amount of precipitation of the second phase.However,in the alloys with the same state,because of the increase of solution temperature,the amount of the second phase decreases,so that the effect on hardness is weakened,but the effect of solution strengthening on hardness is enhanced,thus the change of state does not cause the regularity of the change of alloys’hardness.