First-principle Study Of The Effect Of Alloying Elements On ?' Phase In Cobalt-based Superalloys

Cobalt-based superalloys,as one of the excellent high-temperature structural materials,have good fatigue resistance,high-temperature oxidation resistance,corrosion resistance,etc.,usually through solid solution strengthening and carbide strengthening methods to improve the performance of the alloy.However,after the 1950s,because of the lack of ordered strengthening phase in cobalt-based superalloys to further improve the high-temperature performance of the alloy,while nickel-based superalloys have an ordered precipitation strengthening phase,namely y'-phase,so nickel-based superalloys has gradually occupied a leading position in the field of superalloys.In recent years,due to the continuous higher requirements of aerospace engines for thrust-to-weight ratio and thermal efficiency,materials are required to work at higher temperatures.So it is hoped that the ordered ?'-phase strengthening mechanism will be introduced into the cobalt-based superalloys with higher melting points to obtain structural materials with excellent comprehensive mechanical properties capable of higher working temperature.The ?'-Co₃(Al,W)phase with the Ll2 structure that exists stably at high temperatures has been found in the Co-Al-W ternary alloy system.This phase has a coherent relationship with the matrix Co,and the ?-?' structure in cobalt-based superalloy is similar to the nickel-based superalloy.Its mechanical properties are excellent,especially its creep performance which exceeds the existing cobalt-based alloys and is comparable to some nickel-based alloys.However,the density of Co-Al-W alloy is high.Researchers researched Mo,which belongs to group VIB in the periodic table,like the element W,and found ?'-Co₃(Al,Mo,Nb)phase with an Ll2-ordered structure which has a lower density.In order to improve the performance of the alloy,usually some alloying elements are added to the alloy.In the cobalt-based superalloy,Ti and Nb tend to enter the ?' phase,which can stabilize the ?' phase,and are beneficial to the formation of the ? ' phase.These elements can increase the volume fraction and the dissolution temperature of the ?' phase in the alloy.In view of this,this article will research effects of the alloying elements Ti and Nb on ?'phase in the cobalt-based alloys,using first-principles calculations.Their influence on the mechanical properties,structural stability,electronic structure and other properties of the ?' phase in the cobalt-based superalloy will be explored,from a microscopic view.The study of the interaction among the atoms in the alloy will provide a theoretical basis for improving the performance of the new cobalt-based superalloy with an ordered precipitation strengthening phase.The formation enthalpy of Co₃(Al,W)and Co₃(Al,W)doped with Ti and Nb are calculated,and they are all negative,indicating that these structures are stable.Ti and Nb show the strongest tendency to occupy the W6 site,followed by the Al2 site.In the DOS diagram,the DOS values at Fermi energy level are non-zero,indicating that the system,before and after doping,is conductive.The Fermi energy level is located at the left or bottom of the pseudogap,showing that Co₃(Al,W),Co₃(Al,W,Ti)and Co₃(Al,W,Nb)are stable.Ti and Nb have little influence on the DOS of Co and W,and have influence on the DOS of Al.Compared to the Al2 site,when Ti or Nb occupies the W6 site,the DOS of Al is higher and the pseudogap is narrower.Therefore,the system in which Ti or Nb occupies the W6 site is more stable than the system in which Ti occupies the Al2 site and has stronger metallic.Both the Co₃(Al,W)phase and Co₃(Al,W,X)(X=Ti,Nb)phase meet the mechanical stability conditions and have good ductility.When Ti and Nb occupy the W6 site,the ability of ?' phase to resist volume change and shape change is weakened,its hardness is reduced,its plasticity is improved,and the possibility of fracture and embrittlement is reduced.When Ti and Nb occupy the Al2 site,the results are opposite.The formation enthalpy of Co₃(Al,Mo,Nb)and Co₃(Al,Mo,Nb,Ti)are calculated,and it is found that Nb and Ti can stabilize the ?' phase.For Co₃(Al,Mo,Nb),Nb tends to occupy the Mo site,especially the Mo4 site.For Co₃(Al,Mo,Nb,Ti),both Ti and Nb firstly tend to occupy Mo site,followed by Mo site and Al site,and finally both of them occupy Al site.In the DOS diagram,the Fermi energy level is at the bottom or left of the pseudogap,indicating that the ?'phase is stable.For Co₃(Al,Mo,Nb,Ti),the hybridization between Al,Mo,Nb,and Ti atoms mainly occurs at-4.5 eV?-0.22 eV,and the hybridization between Co,Mo,Nb,and Ti atoms is mainly around-1 eV.Co₃(Al,Mo,Nb)and Co₃(Al,Mo,Nb,Ti)meet the mechanical stability conditions and both have good ductility.For Co₃(Al,Mo,Nb)with Nb occupying the Mo4 site and Co₃(Al,Mo,Nb,Ti)with Nb and Ti occupying the Mo3 site,their ratio of the bulk modulus to the shear modulus and Poisson's ratio are higher than those of Co₃(Al,W)and Co₃(Al,Mo).So Nb and Ti improve the ductility of the ?'phase.In addition,the ratio of the bulk modulus to the shear modulus and Poisson's ratio of the matrix Co are similar to those of Co₃(Al,Mo,Nb)and Co₃(Al,Mo,Nb,Ti),which is conducive to the coordination of deformation of the cobalt-based superalloy.Nb and Ti also reduce the deformation resistance and hardness of the ?' phase,especially when Nb and Ti occupy the Mo3 site,these properties are weakened the most.