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

Abstract

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 256h of aging at 1000°C, the precipitates in the 6 and 8at% Ti alloys coalesce and develop an irregular, elongated morphology. After 1000h 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> = 463nm and ϕ = 8% for 2at% Ti to <R> = 722nm and ϕ = 52% for 8at% 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 1273K (1000°C) increases; and (iv) the 0.2% offset flow stress increases.