Stacking Fault Energy in Co and Dilute Co Alloys

In the present work, generalized stacking fault energies (GSFE) of Co-base alloys at different magnetic states are investigated using ab initio calculations. The energy difference between the hcp and fcc structure for pure Co is reasonably well predicted and is in good agreement with the available theoretical data. Theoretical calculations show that the SFEs of dilute Co alloys are sensitive to their chemical composition and magnetic state. The stacking fault energies (SFE) of binary dilute Co_1−xM_x (M represents W, Al, and Ni, respectively) systems and ternary CoAl_18.3−yW_y (0 ≤ y ≤ 18) alloys are calculated to study their dependence from chemical composition. The calculated results agree well with available experimental and theoretical data in the literature. We further study the temperature dependence of the SFE for pure Co and ternary Co9.3Al9W alloy calculated by using the corresponding experimental lattice parameter at each temperature. It reveals that the SFE increases as temperature increases in the ferromagnetic (FM) state while it shows an opposite trend in the paramagnetic (PM) state for pure Co. For the ternary Co9.3Al9W alloy, the FM SFE shows similar trends as the FM SFE of pure Co. The PM SFE decreases with increasing temperature in the range from 700 K to 1173 K, but the PM SFE increases with increasing temperature at higher temperatures due to the magnetic entropy contribution. The negative SFE of the dilute Co alloys hints to the possibility of twin formation according to empirical relationships between the value of SFE and the deformation mode. The predicted SFE of CoCrNi base alloys indicates the formation of twinning at room temperature, which is confirmed by experimental observations [Acta Materialia, 252 (2023) 118928, Acta Materialia, 128 (2017) 292-303]. The present study identifies potential plastic deformation mechanisms of metastable dilute Co alloys at elevated temperatures using the same empirical relationship between SFE and twin formation validated by experimental and theoretical investigations [Materials Science and Engineering, 26 (1976) 123-132, PNAS Nexus, 2022, 2,1-11]. This is of special interest as it gives insights into the favorable plastic deformation mechanisms of typical matrix compositions in superalloys at typical service temperatures and could support the design of advanced alloys.