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, , and volume fraction, ϕ, of the γ′(L12)-phase from = 463nm and ϕ = 8% for 2at% Ti to = 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.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, SAZU, SBCE, UL, UPCLJ, UPUK
The effect of 2 at.% Ru addition on the elemental partitioning and microstructural evolution of a base Co-8.8Al-7.3W at.% superalloy, consisting of a gamma -(fcc) matrix with gamma '-(L12 structure) ...precipitates is studied using scanning electron microscopy and atom-probe tomography. Ruthenium partitions to the gamma '-precipitates in the Co-9.4Al-7.5W-2.1Ru at.% alloy with a partitioning coefficient, = 1.27, after aging at 900 degree C for 16 h, in contrast to the behavior observed in Ni-base superalloys and theoretically predicted for Co-base superalloys, for which Ru partitions preferentially to the gamma -phase. The addition of ruthenium does not significantly affect the gamma ' volume-fraction or the coarsening kinetics of the gamma ' precipitates compared to the base ternary alloy. The addition of Ru also leads, however, to a rapid discontinuous transformation of ( gamma plus gamma '), which initiates at the grain boundaries after 128 h aging at 900 degree C; ( gamma plus gamma ') is transformed into a lamellar phase mixture containing Co3W (D019), fcc solid-solution ( gamma ), and Co(Al,W) (B2). After 256 h aging at 900 degree C in the Ru-containing alloy, some grains have completely transformed, although regions of gamma plus gamma ' persist. The base ternary Co-Al-W alloy does not exhibit a discontinuous transformation and contains a ( gamma plus gamma ') microstructure up to 1024 h of aging at 900 degree C.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
The effects of micro-additions of boron and zirconium on grain-boundary (GB) structure and strength in polycrystalline γ(f.c.c.) plus γ′(L12) strengthened Co-9.5Al-7.5W-X at% alloys (X=0-Ternary, ...0.05B, 0.01B, 0.05Zr, and 0.005B-0.05Zr at%) are studied. Creep tests performed at 850°C demonstrate that GB strength and cohesion limit the creep resistance and ductility of the ternary B- and Zr-free alloy due to intergranular fracture. Alloys with 0.05B and 0.005B-0.05Zr both exhibit improved creep strength due to enhanced GB cohesion, compared to the baseline ternary Co-9.5Al-7.5W alloy, but alloys containing 0.01B or 0.05Zr additions display no benefit. Atom-probe tomography (APT) is utilized to measure GB segregation, where B and Zr are demonstrated to segregate at GBs. A Gibbsian interfacial excess of 5.57±1.04 atoms nm−2 was found for B at a GB in the 0.01B alloy and 2.88±0.81 and 2.40±0.84 atoms nm−2 for B and Zr, respectively, for the 0.005B-0.05Zr alloy. The GBs in the highest B-containing (0.05B) alloy exhibit micrometer-sized boride precipitates with adjacent precipitate denuded-zones (PDZs), whereas secondary precipitation at the GBs is absent in the other four alloys. The 0.05B alloy has the smallest room temperature yield strength, by 6%, which is attributed to the PDZs, but it exhibits the largest increase in creep strength (with an ~2.5 order of magnitude decrease in the minimum strain rate for a given stress at 850°C) over the baseline Co-9.5Al-7.5W alloy.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, SAZU, SBCE, UL, UPCLJ, UPUK
The effects of reducing the W concentration (and concomitantly mass density) of Co-10Ni-6Al-xW-6Ti at% Co-based superalloys, with a γ(f.c.c.) plus γ′(L12) microstructure, is investigated for a series ...of alloys with W concentrations of 6at% (baseline alloy), 4 and 2at% (W-reduced) and 0at% (W-free). The γ′(L12) solvus temperature decreases strongly (by 46°C per 1at% reduction in W), while the liquidus and solvus temperatures decrease mildly (3–5°C per 1at% W reduction) as measured by differential scanning calorimetry. Scanning electron microscopy was used to image phase formation in all alloys. When aging at 900 or 1000°C for 256h, the W reduction does not result in the appearance of additional phases within the grains beyond the γ(f.c.c.) and γ′(L12) phases present in the original alloy. Grain-boundary precipitates are present for all alloy compositions: W-rich or W-free precipitates after aging at 1000°C, and coarsened γ′(L12) precipitates after aging at 900°C. The composition of grain boundary precipitates were measured with energy-dispersive X-ray spectroscopy. Vickers microhardness values decrease with decreasing W content, due to a reduction in γ′(L12) precipitation and reduced solid-solution strengthening.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, SAZU, SBCE, UL, UPCLJ, UPUK
The temporal evolution of a γ(f.c.c.)/γ’ (L12) Co-8.8Al-7.3 W superalloy aged at 650 °C (10 min–4096 h) and 750 °C (10 min–256 h) is studied utilizing atom-probe tomography (APT), scanning electron ...microscopy, and Vickers microhardness testing. The evolution of the phase compositions, γ’ (L12) volume fraction, and mean precipitate radius, <R2-D(t)>, are determined. Coarsening rate constants and temporal exponents are calculated for <R-D(t)> of the γ’ (L12)-nanoprecipitates. The temporal exponents are found to be generally close to 1/p = 1/3 as required for diffusion-limited coarsening. Tungsten solid-solubility is significantly reduced in the γ(f.c.c.)-matrix at 650 °C (0.54 ± 0.04 at. %) and 750 °C (1.35 ± 0.06 at. %) when compared with aging at 900 °C (5.5 at. %). The value of <R2D(t)> of the γ’ (L12)-nanoprecipitates increases with increasing aging time corresponding to an increase in the Vickers microhardness; the peak strength was not, however, achieved for the aging times investigated. The morphology of the γ’ (L12)-nanoprecipitates begins as spheroids but transitions to cuboids at longer aging times, with final the γ’ (L12) volume fractions for aging at 650 °C and 750 °C being ϕ = 53% and 54%, respectively. The effect of quench-rate (either furnace-cooled, air-cooled, oil quenched, or water quenched) from a supersolvus temperature of 1050 °C on the microstructure of the alloy is also investigated. Slow cooling (furnace and air-cooling) is shown to result in a uniform distribution of nanometer sized γ’ (L12)-nanoprecipitates, unlike Ni-based superalloys in which the γ’ (L12)-nanoprecipitates form in a non-uniform or multimodal distribution.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
The effects of micro-additions of boron and zirconium on grain-boundary (GB) structure and strength in polycrystalline γ(f.c.c.) plus γ'(L1
) strengthened Co-9.5Al-7.5W-X at. % alloys (X = 0-Temary, ...0.05B, 0.01B, 0.05Zr, and 0.005B-0.05Zr at. %) are studied. Creep tests performed at 850 °C demonstrate that GB strength and cohesion limit the creep resistance and ductility of the ternary B- and Zr-free alloy due to intergranular fracture. Alloys with 0.05B and 0.005B-0.05Zr both exhibit improved creep strength due to enhanced GB cohesion, compared to the baseline ternary Co-9.5Al-7.5W alloy, but alloys containing 0.01B or 0.05Zr additions displayed no benefit. Atom-probe tomography is utilized to measure GB segregation, where B and Zr are demonstrated to segregate at GBs. A Gibbsian interfacial excess of 5.57 ± 1.04 atoms nm
was found for B at a GB in the 0.01B alloy and 2.88 ± 0.81 and 2.40 ± 0.84 atoms nm
for B and Zr, respectively, for the 0.005B-0.05Zr alloy. The GBs in the highest B-containing (0.05B) alloy exhibit micrometer-sized boride precipitates with adjacent precipitate denuded-zones (PDZs), whereas secondary precipitation at the GBs is not present in the other four alloys. The 0.05B alloy has the smallest room temperature yield strength, by 6 %, which is attributed to the PDZs, but it exhibits the largest increase in creep strength (with an ~2.5 order of magnitude decrease in the minimum strain rate for a given stress at 850 °C) over the baseline Co-9.5Al-7.5W alloy.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, SAZU, SBCE, UL, UPCLJ, UPUK
Two-dimensional dislocation dynamics (DD) simulations are carried out to simulate the increase in yield stress of nickel- and cobalt-based superalloys by order-strengthened γ’(L12)-precipitates, ...under conditions where precipitates are sheared and/or looped by single and super-dislocations. Parametric studies are performed for precipitate volume fractions between 5 and 70% and radii between 1 and 100 nm, resulting in yield stress increments due to a mixture of precipitate bypassing and shearing by single and super-dislocations of edge or screw character. DD yield stresses are compared to a variety of analytical models for strengthening by ordered precipitates showing that the closed-form solutions do not agree with DD predictions over all regimes of precipitate sizes and volume fractions. The DD yield stresses, converted to hardness values, are also compared to experimental hardnesses from archival literature on binary Ni-13Al (at. %) as well as Ni-10Al-8Cr(-2 W) (at.%) alloys aged at various temperatures and times, for which γ’(L12)-precipitate radii and volume fractions had been measured via atom-probe tomography and scanning electron microscopy. DD predictions are within 5–10% of experimental hardnesses in underaged alloys, but they typically overestimate hardness for peak and overaged samples. Finally, DD is used to predict the hardnesses for a γ’(L12)-strengthened Co-8.8Al-7.3 W (at. %) superalloy for 50–130 nm average precipitate radii within 10% of experimental values.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Atom-probe tomography (APT) is utilized to investigate, in a Co–9.7Al–10.8Wat.% alloy aged at 900°C for ∼1000h, the phase compositions and partitioning behavior of the two-phase γ(face-centered ...cubic)/γ′(L12) microstructure. The APT composition of the γ-matrix (Co–8.97Al–5.69Wat.%) agrees well with a previously reported measurement made by energy-dispersive X-ray spectroscopy; however, the composition of the γ′(L12) precipitates (Co–10.03Al–12.48Wat.%) is slightly richer in W and Al, and leaner in Co. Partitioning coefficients and Al interfacial excesses are also calculated.
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