Microstructure and creep performance of a multicomponent co-based L1₂–ordered intermetallic alloy

The chemistry, thermodynamics and mechanical properties of the L1₂-ordered Co₃(Al,W) γ’-phase are crucial for the understanding of γ(f.c.c.)/γ’(L1₂) cobalt-based superalloys. A single-phase γ’(L1₂) alloy with the composition Co-30Ni-11Al-5.5W-4Ti-2.5Ta-0.10B (at.%) and a γ’(L1₂)-solvus temperature of 1268 °C was recently identified using the Calphad-methodology. Scanning and transmission electron microscopy reveals that the single-phase microstructure is stable at 900 and 1000 °C for 1000 h and at 1100 °C for 168 h, without other phases being observed, resulting in similar levels of microhardness for all annealing temperatures. Atom-probe tomography confirms the presence of a single-phase γ’(L1₂)-microstructure with a composition of (Co,Ni)₃(Al,W,Ti,Ta). Grain boundaries exhibit depletion of Ni, W and Ta and enrichment of Co, Al and B. A remarkable yield stress anomaly is observed, with the yield strength increasing from ~ 300 to ~ 700 MPa from room temperature to 800 °C, which is stronger than Co₃(Al,W)(L1₂) and Ni₃Al(L1₂). The creep tests at 850 and 950 °C display power-law behavior with a stress exponent of n = ~ 3 and an activation energy of Q_n = 497 kJ•mol^-1 for (Co,Ni)₃(Al,W,Ti,Ta), similar to that of single-phase Ni₃Al(L1₂) compound (Q_n = 406–421 kJ•mol^-1) [1,2].