Microstructural evolution and high-temperature strength of a γ(f.c.c.)/γ’(L12) Co–Al–W–Ti–B superalloy

Abstract We characterized the microstructural features and mechanical performance of a model Co-5.6Al-5.8W-6.6Ti-0.12B (at.%) alloy consisting of γ(L12)-precipitates in a γ(f.c.c.)-matrix. Scanning electron microscopy (SEM) was used to follow the isothermal aging of the microstructure at 900 and 1000 °C for 256 h, and 950 °C for 1000 h. The compositions of the γ'(L12)-precipitates and γ(f.c.c.)-matrix were evaluated by atom-probe tomography (APT) for solution-treated and air-cooled conditions, as well as in specimens aged at 950 °C for 16 and 100 h. Boron was shown to partition preferentially to the γ'(L12)-precipitates, and profiles taken across the γ(f.c.c.)-matrix channels in both aged specimens revealed confined segregation of Al at one of the two γ(f.c.c.)/γ′(L12) heterophase interfaces. After aging at 950 °C for 16 h, Co-5.6Al-5.8W-6.6Ti-0.12B (at.%) exhibited anomalous flow-strength behavior in the range 625–900 °C with a peak yield stress of 822 MPa between 800 and 825 °C. Compressive creep tests performed at 850 °C demonstrated a creep strength comparable to archival literature results for Co–9Al–9W-0.12B (at.%), despite a smaller γ′(L12)-volume fraction and lack of strengthening borides along the grain boundaries (GBs). The activation energy for creep in the temperature range 800–900 °C was 606 kJ mol−1. The post-creep microstructure consists of rafted γ′(L12)-precipitates perpendicular to the compression axis, consistent with the positive γ(f.c.c.)/γ'(L12) lattice parameter misfit character of this class of alloys. Creep failure could occur due to GB embrittlement caused by deleterious Ti-rich (L21 or B2) and D019 phases formed at the GBs during creep.