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

Abstract

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 Qn = 497 kJ•mol-1 for (Co,Ni)3(Al,W,Ti,Ta), similar to that of single-phase Ni3Al(L12) compound (Qn = 406–421 kJ•mol-1) [1,2].