Effect of Cr addition on γ-γ′ cobalt-based Co-Mo-Al-Ta class of superalloys: a combined experimental and computational study

The present article deals with effect of Cr addition (10 at.%) on the partitioning behavior and the consequent effect on mechanical properties for tungsten-free gamma-gamma' cobalt-based superalloys with base alloy compositions of Co-30Ni-10Al5Mo- 2Ta (2Ta) and Co-30Ni-10Al-5Mo-2Ta-2Ti (2Ta2Ti). Cr addition leads to a change in the morphology of the strengthening cuboidal-shaped gamma' precipitates to a spherical shape. The site preference of Cr atoms in two alloy systems (with and without Ti) has been experimentally investigated using atom probe tomography with the supportive prediction from first principles DFT-based computations. Cr partitions more to the c matrix relative to gamma'. However, Cr also has a strong effect on the Ta and Mo partitioning coefficient across gamma/gamma' interfaces. The value of partition coefficient for Mo (K-Mo) becomes <1 with Cr addition to the alloys. Results from ab initio calculations show that the Cr atoms prefer to replace Mo atoms in the sublattice sites of the Ll(2) unit cell. The solvus temperature of about 1038 and 1078 degrees C was measured for 10Cr2Ta and 10Cr2Ta 2Ti alloy, respectively, and these Cr-containing alloys have very low densities in the range of similar to 8.4-8.5 gm/cm(-3). The 0.2% compressive proof strength of 10Cr2Ta2Ti alloy yields a value of 720 MPa at 870 degrees C, substantially better than most Co-Al-W-based alloys and many of the nickel-based superalloys (e.g., MAR-M-247).