First-Principles Study On W(Y)-B-C Hard Materials And Hard Rare Earth Oxygen Sulfides

By utilizing computers,more and more materials with excellent properties have been founded.In this background,we researched the W-B-C ternary system,YB₂C and hard rare earth oxygen sulfide with the theme of“First-principles Study on W?Y?-B-C Hard Materials and Hard Rare Earth Oxygen Sulfides”.The main research contents and conclusions are as follows:It is known that transition metal tungsten could easily form strong covalent bonds with boron and carbon atoms.In order to find new hard materials,we researched the W-B-C ternary system.Firstly USPEX software was utilized to search W-B-C ternary compounds.We got 8 metastable ternary structures:W₂B₂C?R3m?,W₂B₂C?R3m?,W₃B₂C₂?R 3 m?,W₃B₂C₂?R3m?,W₄B₂C₃?Cm?,W₄B₂C?Pm?,W₄B₃C?Amm2?andW₂B₃C?P6m2?.The Vickers hardness of W₂B₂C?R3m?and W₃B₂C₂?R3m?are as high as 36.75 and 37.92GPa respectively which is very close to that of superhard materials??40GPa?.The Vickers hardness of W₃B₂C₂?R3m?,W₄B₂C₃?Cm?and W3B6C?P6m2?are 32.26,31.71 and 27.32GPa respectively which are much higher many common structural materials.The elastic constant of W2B2C?R3m?,W3B2C2?R3m?and W₃B₂C₂?R3m?in the direction of C₃₃ are as high as 928,916 and 933GPa respectively which are quite comparable with that in the direction of C11?1024GPa?in diamond whichrevealed that the compressible property of W2B2C?R3m?,W3B2C2?R3m?and W₃B₂C₂?R3m?in the direction of C₃₃ is almost the same with diamond.Even though the Vickers hardness of W₃B₆C?P6m2?is a little lower,its C₃₃is 890GPa which is almost the same with other harder structures.The crystal structure,electronic structure,elastic constants ideal tensile and shear strength of YB₂C were studied with the first principles calculations.From the electronic structure it can be known that YB2C owns metallic property.There is strong hybridization between B-p,C-p and Y-d orbits.The obvious and strong electron localization function?ELF?among the B-C layer in YB₂C means that there exits strong covalent bond between boron and carbon atoms.The Vickers hardness of YB₂C is calculated to be 23.46 GPa which is comparable with the incompressible material oP6-OsB2.The shear and tensile strength in different slip system show that YB₂C owns quite high strength.The bulk modulus,shear modulus,Pugh's ratio,Young's modulus,Vickers hardness,Passion's ratio and the thermal expansion coefficient of hard rare earth oxygen sulfidesin rare earth steels like Ce₂O₃,Ce₂SO₂,CeS,La₂O₃,La₂SO₂ and LaS in the rare earth steels were studied.In order to compare the effects of rare earth on the steels,the original inclusions in steels were also studied.The results showed that when compared with the original inclusions,the new rare earth oxygen sulfides could reduce the thermal stress in the steels which could reduce or even avoid the appearance of cracks in the steels.So the property of rare earth steels could be improved.