Energetics of antiphase boundaries in γ′ Co₃(Al,W)-based superalloys

Although Co-based superalloys are considered as a promising next-generation higherature turbine blade material, there are issues with regard to mechanical properties that must be addressed. L1₂ planar defects such as antiphase boundaries (APBs) play a critical role in the higherature strength of γ/γ′ superalloys, particularly with the magnitude and temperature-range of the yield strength anomaly. APBs in L1₂ Co₃(Al,W) γ′ are particularly complex due to the presence of the metal mixing on the Al/W sublattice, which introduces additional degrees of freedom to APB behavior, such as Al/W composition and configuration local to the APB. We explore the behavior of APBs in L1₂ Co₃(Al,W) with density functional theory. We find a strong dependence of the APB energy on the composition, particularly from the presence of Al-Al bonds across the 010 APB and W-W bonds across the 111 APB. For both APBs, our calculations also suggest an energetically favorable segregation of Al to the APB. Therefore, future Co alloy design criteria can include alloying elements with preferential segregation to the APB over that of Al to tune the APB energy (and the behavior of the yield strength anomaly). Furthermore, the unique behavior of Al-Al and W-W bonds in the 010 and 111 APB's, respectively, indicate the possibility of preferentially modifying one APB over another with alloying.