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

The chemistry, thermodynamics and mechanical properties of the L1(2)-ordered Co-3(Al,W) gamma'-phase are crucial for the understanding of gamma(f.c.c.)/gamma'(L1(2)) cobalt-based superalloys. A single-phase gamma'(L1(2)) alloy with the composition Co-30Ni-11Al-5.5W-4Ti-2.5Ta-0.10B (at.%) and a gamma'(L1(2))-solvus temperature of 1268 degrees 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 degrees C for 1000 h and at 1100 degrees 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 gamma'(L1(2))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 similar to 300 to similar to 700 MPa from room temperature to 800 degrees C, which is stronger than Co-3(Al,W)(L1(2)) and Ni3Al(L1(2)). The creep tests at 850 and 950 degrees C display power-law behavior with a stress exponent of n = similar to 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(L1(2)) compound (Q(n) = 406-421 kJ.mol(-1)) [1,2]. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.