| Carbides are main precipitated phases of Ni-based superalloys during heat treatments,especially M23C6-type carbides which maintain a high proportion for a long time.Structural stabilities and mechanical elastic properties of M23C6 carbides have a significant influence on the comprehensive mechanical properties of Ni-base superalloys.Stable carbides tend to grow at the grain boundaries,thereby decreasing the strength and toughness of the alloy.M23C6 carbides are preference sites for fatigue crack initiations.There are obvious differences in the elastic mechanical properties between the carbide and the matrix phase.Under the impact of the slip zone,the alloy usually cracks at the carbide.Furthermore,the formation of carbides causes the depletion of the key elements of the alloy,such as Cr,Mo and W elements,which weakens the mechanical properties of the alloy.Using the first principle calculation method,the structural stabilities and high-temperature elastic properties of M23C6 carbides are studied,which provides theoretical basis and data support for studying the microscopic mechanism of fatigue failure of Ni-based superalloy.The main contents of this thesis include:(1)Employing the first principle method,the lattice parameters,total energies,formation energies and electronic structures are studied for Cr3C2,Fe3C,MoC and M23C6(M=Cr,Fe,Mo)in the ground state.The results indicate that there are stable bonds between metal and carbon atoms in Cr3C2,Fe3C,MoC and Cr23C6.Consequently,Cr3C2,Fe3C,MoC and Cr23C6 present high thermodynamically stabilities.(2)The high-temperature elastic properties of carbides Cr3C2,Fe3C,MoC and Cr23C6 are calculated by the stress-strain method and the Debye model.The influence of temperature on the stable and elastic properties of carbides are discussed via electronic structures.The results indicate that with the increase of temperature,the elastic properties of carbides weakens.Since the temperature has different effects on the bond energy in each carbide,the elastic properties of carbides Cr3C2,Fe3C,MoC and Cr23C6 exhibit different sensitivities when the temperature is rising.Among them,the elastic properties of MoC and Cr23C6 are relatively stable at high temperatures.(3)M23C6 carbides contain different alloying elements.For example,Cr23C6 can dissolve Fe/Mo to form ternary carbides.With first principle method,we conduct comprehensive studies for(Cr,M)23C6(M = Fe,Mo)carbides with a full composition range of Cr/M ratio(i.e.,0-1).According to the concentration of doped elements,three regions can be distinguished,the Cr-rich part(R-Cr),the M-rich part(R-M)and the transition region(R-T).To characterize the phase stability of(Cr,M)23C6(M = Fe,Mo)carbides,the reaction energy(?Hr)for a possible route relative to binary carbides(Cr3C2,Fe3C and MoC)are introduced in the present study.Then the charge density difference is adopted to further explore the influence of alloying elements on the phase stabilities.Fe atoms preferentially substitute Cr atoms in the 4(a)site,while the preferential sites for the doped Mo in Cr23C6 structure is 8(c).Fe atoms at 4(a)site or Mo atoms at 8(c)site of Cr23C6 structure strengthens the covalent bonds between the metal atoms at 8(c)site and 32(f)site,which contributes to the high stability of Cr22FeC6 and Cr21Mo2C6.(4)The high-temperature elastic properties of stable carbides(Cr,M)23C6(M= Fe,Mo)are calculated by the stress-strain method and the Debye model.The influence of temperature on the stable and elastic properties of carbides are discussed via electronic structures.The results indicate that with the increase of temperature,the elastic properties of carbides weakens.That is due to the increasing of temperature weakens the bond force between the atoms in ternary carbides.When Fe/Mo atoms are doped to Cr23C6 structure and form stable phases,the effects of temperature on the elastic properties of the carbides decrease,so that ternary carbides(Cr,M)23C6 still maintain a relatively good mechanical elastic properties at high temperature. |
PDF Full Text Request