Microstructures And Properties Of NiAl-xSc And Ni-(50-x)Al-xSc Alloys

NiAl-xSc(x?33.3 at%)alloys and Ni-(50-x)Al-xSc(x?15 at%)alloys were prepared by using vacuum arc smelting furnace with a water-cooled copper mold.The microstructures and phases of the alloys were analyzed by optical microscope(OM),scanning electron microscope(SEM),X-ray diffraction(XRD)and energy spectrum(EDS).The thermal parameters of the alloys were measured by differential scanning calorimeter(DSC),and the growth interface type and solidification process of the alloys were analyzed.The mechanical properties of the alloys were tested and analyzed.The solidification structures of NiAl-xSc(x?20)alloys are NiAl(Sc)+(NiAl(Sc)+NiAlSc)+AlNi₂Sc;The solidification structures of NiAl-25Sc alloy is(NiAl(Sc)+NiAlSc)+AlNi₂Sc.The solidification sequence of NiAl-xSc(x?25)alloys is NiAl(Sc)?(NiAl(Sc)+NiAlSc)?AlNi₂Sc.The solidification structures of NiAl-33.3Sc alloy is NiAlSc+(NiAl(Sc)+AlNi₂Sc)+Sc(unmelted).The solidification sequence of NiAl-33.3Sc alloy is NiAl(Sc)?NiAlSc?AlNi₂Sc.Ni-(50-x)Al-xSc(x?15)alloys are mainly composed of NiAl(Sc)+AlNi₂Sc,of which Ni-35Al-15Sc alloy has a small amount of(AlNi₂Sc+Ni-16.93Al-21.53Sc)eutectic analogs structure.The solidification sequence of Ni-(50-x)Al-xSc(x?15)alloys is NiAl(Sc)?AlNi₂Sc.Among them,Ni-35Al-15Sc alloy finally solidifies to form(AlNi₂Sc Ni-16.93Al-21.53Sc)eutectic analogs structure.After homogenizing heat treatment at 980?×120 h,in the NiAl-xSc(x?25)alloys,the NiAl(Sc)in the eutectic analogs structure is spheroidized.In the NiAl-33.3Sc alloy,the original(NiAl(Sc)+AlNi₂Sc)is spheroidized.The BCT dendrite growth model theory shows that the undercooling gradient of the solidified alloy sample pointing toward the center decreases and the undercooling is dominated by the component undercooling?Tc.The morphology of the solidified microstructure of NiAl(Sc)dendrites from the bottom of the sample to the center changed from granular to coarse dendrites.The melting enthalpy of NiAlSc in the alloy is 12 845.76 J·mol^-1,and Jackson factor?=0.4<2,that means NiAlSc grows in a continuous manner and a coarse solid/liquid interface.The(NiAl(Sc)+NiAlSc)lamellar eutectic structure of NiAl-xSc alloys have regular eutectic morphology.The melting enthalpy of AlNi₂Sc in the alloy is 7 542.59J·mol^-1,and Jackson factor?=0.2<2,that means AlNi₂Sc grows in a continuous manner and a coarse solid/liquid interface.The hardness and elastic modulus of NiAl are 225.4 HV and 208.3 GPa,respectively.The addition of Sc atoms increases the hardness of NiAl(Sc),but decreases the elastic modulus.In the solidified NiAl-xSc alloys,the largest hardness of the NiAl(Sc)is 853.4HV,and the minimum elastic modulus of the NiAl(Sc)is 169.5 GPa.After the homogenization heat treatment,the hardness of NiAl(Sc)and eutectic analogs structure are decreased,and the elastic modulus of the NiAl(Sc)is decreased.In the solidified Ni-(50-x)Al-xSc alloy,the largest hardness of the NiAl(Sc)is 641.7HV,and the minimum elastic modulus of the NiAl(Sc)is 171.8 GPa.After the homogenization heat treatment,the hardness of NiAl(Sc)and AlNi₂Sc in the alloy are increased,and the elastic modulus of the NiAl(Sc)is decreased.The intermediate phases such as NiAlSc and AlNi₂Sc formed with addition of large amounts of Sc,which reduced the compressive strengths of the solidified NiAl-xSc alloys and Ni-(50-x)Al-xSc alloys.All fractures of alloys are brittle fractures and show river-pattern cleavage sections.