Properties And Brittle Fracture Mechanism Of Co-Sc Intermetallic Compound

In this paper,the microstructure and properties of Co-Sc intermetallics were studied by rapid quenching method and first-principle.The stability,physical properties and mechanical properties of Co-Sc intermetallics were analyzed by the first-principles.The effectof point defects and grain boundaries being the properties of intermetallics were further analyzed.The microstructures and phase compositions of Co-Sc compound were analyzed by OM,SEM,EDS and XRD.The results show that the microstructures of Co-50 at%Sc alloy were composed of B2-CoSc and（Co₂Sc+CoSc）_eutectic.The HV hardness of the cast alloy is 856.The plasticity of the cast alloy is poor.After heat treatment,the granular Co₂Sc was uniformly distributed on the B2-CoSc matrix.Single-phase B2-CoSc intermetallicswas not obtained during the experiment.The plasticity of the cast alloy after heat treatment is poor.The first-principles calculation results show that the formation enthalpies of CoSc,Co₂Sc and CoSc₂ are-0.54 eV,-0.51 eV and-0.36 eV,respectively.The binding energies of CoSc,Co₂Sc and CoSc₂ are-6.08 eV,-6.07 eV and-5.56 eV,respectively.Therefore,the stability of CoSc is the best.The calculated elastic constants of CoSc,CoSc₂ and Co₂Sc are 2.7GPa,59.3 GPa and 172.1 GPa,respectively,tell that Co₂Sc is the strongest alloy to overcome volumatic deformation.CoSc is the most stiffness and the worst plasticity one which is revealed byγcalculation that are 0.85,0.34 and0.32 for CoSc,CoSc₂ and Co₂Sc,respectively.The calculated B/G of CoSc,CoSc₂and Co₂Sc are 0.04,2.84 and 2.46.Therefore,CoSc is brittle material,and CoSc₂ and Co₂Sc are plastic materials.The point defect calculation results show that the formation heats of single vacancy defect at Co site,average double vacancies defect at Co site,three vacancies defect at Co site and single vacancy defect at Sc site,average double vacancies defect at Sc site,three vacancies defects at Sc siteare-6.78 eV,-2.655 eV,-1.25 eV,-4.172eV,-0.089 eV and 1.057 eV,respectively.The vacancy defect at Co site compound is more formation than the vacancy defect at Sc site compound.The formation energies calculation results show that the vacancy defect at Co site compound is easier formation than the vacancy defect at Sc site compound,in the same way.The bonding energies of anti-site defects calculation results show that the bonding energies of anti-site defects at Co site compound is smaller than the bonding energies of anti-site defects at Sc site compound,anti-site defects at Co site compound is easier formation than the anti-site defects at Sc site compound.The stable of single vacancy defect at Co site is best.Therefore,the vacancy defect at Co site and anti-site defects at Co site can exist steadily.The interface binding energy of CoSc（110）/Co₂Sc（221）is 5.4.4 eV.Therefore,the CoSc（110）/Co₂Sc（221）interface can be stable.The binding energy of CoSc（110）/Co₂Sc（221）is digger than that of CoSc（110）/Co₂Sc（110）,one which is revealed by the binding energy that the heterogeneous interface reduces the stability of the interface.The main reasons the brittle fracture of Co-Sc intermetallicsare as follows by experiment and the first-principles calculationmetolds:1)B2-CoSc compound is brittle material.2)anti-site defects at Co site reduces bounding force of CoSc compound.3)The heterogeneous interface increases the interfacial energy.4)The dispersed shrinkage in the alloy exist inevitablily.