Microstructure and Creep Performance of a Multicomponent Co-Based L1 2-Ordered Intermetallic Alloy

Abstract

The chemistry, thermodynamics and mechanical properties of the L1₂-ordered Co₃(Al,W) γ'-phase are crucial for understanding 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'-solvus temperature of 1268°C was recently identified using a 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,B). Grain boundaries exhibit depletion of Co, Ni, W and Ta and enrichment of 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 = 3.7 and an activation energy for L26 of Q_n = 497 kJ/mol, similar to that of single-phase Ni₃Al(L1₂) compound (Q_n = 406-421 kJ/mol).