Microstructural stability and mechanical behavior of a Co-20Ni-7Al-7W-4Ti at.% superalloy

The temporal evolution of the microstructure and microhardness of a polycrystalline L1(2)-strengthened Co-based superalloy (Co-19.6Ni-7.6Al-7.4W-4.1Ti, at.%) was investigated for an aging temperature of 900 degrees C. The gamma(f.c.c.)/gamma'(1:12) structure remained stable throughout aging to the longest aging time of 2000 h (83 days), with no additional phases present. While the volume fraction of gamma'(L1(2))-precipitates reached an almost constant value of 64% at 16 h, the temporal evolution of the gamma'(L1(2))-precipitates' mean-radius and their number density (temporal exponent of p = 3.49 +/- 0.18 for the equivalent radius, and q = -0.74 +/- 0.04 for the volumetric number density) deviated somewhat from the Lifshitz-Slyozov (LS) predictions (p = 3 and q = -1), suggesting that the alloy had not achieved a true stationary-state at 2000 h. At ambient temperature, a peak Vickers microhardness of 3.80 +/- 0.10 GPa was achieved after 240 h of aging. As the testing temperature was increased, an anomalous increase in flow-stress was also observed, with a peak flow-stress value of 705 MPa measured at 750 degrees C, 30 MPa higher than an archival literature value with about half the Ni concentration (10 at.%). Creep resistance at 850 degrees C was greater than an archival literature value for a ternary Co-9Al-9W alloy. After 200 h of compressive creep at 850 degrees C the microstructure evolved as follows: (i) the volume fraction of the gamma'(L1(2))-phase increased from 64% to 79%; and (ii) some grains exhibited rafts of gamma'(L1(2))-precipitates along (100)-directions. While the first change is expected to improve creep resistance, the second one is deleterious, as evidenced by a comparison with the creep resistance of a sample without rafts of gamma'(L1(2))-precipitates tested for the same conditions. (C) 2020 Elsevier B.V. All rights reserved.