Effects of Ti and Cr Additions in a Co–Ni–Al–Mo–Nb-Based Superalloy

Abstract

In the present work, we show the feasibility of microstructural control by additions of Ti and Cr to a γ/γ′ Co-30Ni–10Al–5Mo–2Nb superalloy. Solutioning at 1300 °C followed by aging at 900 °C leads to homogenous distribution of L12 ordered cuboidal γ′ precipitates in face-centered-cubic (fcc) γ matrix. Compositional measurements show Al, Mo, and Nb partition to γ′ that indicates the γ′ can be described as (Co, Ni)3(Al, Mo, Nb). An addition of 2 at.% Ti leads to increases in the γ′ volume fraction from 56 to 70% and solvus temperature from 990 to 1030 °C. Ti strongly partitions to γ′ with respect to γ matrix. Similarly, an addition of 10 at.% Cr to the base alloy leads to a morphological transition of γ′ precipitates from cuboidal to near spherical shape, indicating a direct influence on the γ/γ′ lattice misfit. Unlike Ti, Cr partitions to γ matrix, and additionally, Cr influences Mo to partition into γ matrix. A combined addition of Ti and Cr leads to high γ′ volume fraction ~76% and an increase in γ′ solvus temperature to 1045 °C, while maintaining the spherical γ′ morphology. These superalloys show 0.2% proof strength comparable to those of Co–Al–W-based superalloys. At 870 °C, 10 at.% Cr and 2 at.% Ti added alloys show higher specific 0.2% proof stress than Co–Al–W-based superalloys. The obtained results show the microstructural sensitivity of these Co-based superalloys toward their designing for better performance.