Effects of titanium substitutions for aluminum and tungsten in Co-10Ni-9Al-9W (at%) superalloys

Polycrystalline Co-10Ni-(9 – x)Al-(9 – x)W-2xTi at% (x = 0, 1, 2, 3, 4) alloys with γ(f.c.c.) plus γ′(L1₂) microstructures are investigated, where the γ′(L1₂)-formers Al and W are replaced with Ti. Upon aging, the initially cuboidal γ′(L1₂)-precipitates grow and develop a rounded morphology. After 256 h of aging at 1000 °C, the precipitates in the 6 and 8 at% Ti alloys coalesce and develop an irregular, elongated morphology. After 1000 h of aging, replacement of W and Al with Ti increases both the mean radius, <R>, and volume fraction, ϕ, of the γ′(L1₂)-phase from <R> = 463 nm and ϕ = 8% for 2 at% Ti to <R> = 722 nm and ϕ = 52% for 8 at% Ti. Composition measurements of the γ(f.c.c.)-matrix and γ′(L1₂)-precipitates demonstrate that Ti substitutes for W and Al in the γ′(L1₂)-precipitates, increases the partitioning of W to γ′(L1₂), and changes the partitioning behavior of Al from a mild γ′(L1₂)-former to a mild γ(f.c.c.)-former. The grain boundaries in the aged alloys exhibit W-rich precipitates, most likely μ(Co₇W₆)-type, which do not destabilize the γ(f.c.c.) plus γ′(L1₂) microstructure within the grains. Four important benefits accrue from replacing W and Al with Ti: (i) the alloys’ mass density decrease; (ii) the γ′(L1₂)-solvus temperature increases; (iii) the γ′(L1₂) volume fraction formed during aging at 1273 K (1000 °C) increases; and (iv) the 0.2% offset flow stress increases.