We examine the precipitation and creep behavior of Al-0.5Mn-0.02Si (at.%) alloys, with and without the L12-forming elements Zr and Er (0.09 and 0.05 at.%, respectively), utilizing isochronal aging ...experiments as well as compressive and tensile creep tests performed between 275 and 400 °C. The Al-0.5Mn-0.09Zr-0.05Er-0.05Si alloy exhibits an unusually high creep resistance in the peak-aged state, which is significantly better than that observed generally in its Mn-free L12-strengthened counterparts; for example, the creep threshold stresses at 300 °C are 34-37 MPa, about three times higher than those in a Mn-free Al-0.11Zr-0.005Er-0.02Si alloy. Scanning transmission electron microscopy illustrates that nanoscale Al3(Zr,Er) L12-precipitates are formed in the dendritic cores and micron-sized Al(Mn,Fe)Si α-precipitates in the interdendritic channels. Moreover, the Al(f.c.c.)-matrix remains supersaturated with randomly distributed Mn solute atoms, as determined by atom-probe tomography and electrical conductivity measurements, for months at creep temperatures. Creep experiments on the Zr- and Er-free Al-0.5Mn-0.02Si solid-solution alloy reveal a small primary creep strain, a high apparent stress exponent, na ∼9-7, and a threshold-stress-type behavior. After ruling out other possible mechanisms, we provide evidence that the threshold stress in this precipitate-free alloy originates from dislocation/solute elastic interactions leading to a strong drag force exerted on edge dislocations, hindering their ability to climb. The relatively high creep resistance of Al-0.5Mn-0.09Zr-0.05Er-0.05Si is interpreted in terms of the synergy between this solute-induced threshold stress (SITS, from Mn in solid-solution) and the known precipitate-bypass threshold stress (from the L12-nanoprecipitates).
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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
Precipitation-strengthening at ambient and high temperatures is examined in Al-0.5Mn-0.3Si (at.%) alloys with and without 0.02 at.% Sn micro-additions. Isochronal aging experiments reveal that Sn ...inoculation results in a pronounced age-hardening response: a hardening increment of 125 MPa is achieved at peak-aging (475 °C), which is five times greater than that of a Sn-free alloy. Scanning electron microscopy and synchrotron x-ray diffraction analyses demonstrate that, while the structure of the α-Al(Mn,Fe)Si precipitates formed in the peak-aged alloys is identical, their mean radius is smaller (R ∼ 25 vs. 100–500 nm) and their number density is greater (∼1021 vs. ∼1019–20m− 3) in the Sn-modified alloy. Atom-probe tomography analyses reveal that the enhanced dispersion of the α-precipitates is related primarily to the formation of Sn-rich nanoprecipitates at intermediate temperatures, which act as nucleation sites for Mn-Si-rich nanoprecipitates. High-resolution transmission electron microscopy analyses demonstrate that these Mn-Si-rich nanoprecipitates exhibit icosahedral quasicrystal ordering (I-phase), which transform into the cubic-approximant α-phase upon peak aging. Significant Sn segregation at the semi-coherent interfaces of the α-precipitates in the peak-aged Sn-modified alloy is observed via APT, which promotes homogeneous nucleation of the I/α-precipitates at aging temperatures > 400 °C. At 300 °C, creep threshold stresses are observed in both alloys in the peak-aged state, which increases from ∼30 MPa in the Sn-free alloy to ∼52 MPa in the Sn-modified alloy. This boost in creep resistance is consistent with the enhanced aging response (higher Orowan stress).
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The ferritic alloy Fe–10Cr–10Ni–5.5Al–3.4Mo–0.25Zr–0.005B (wt.%), strengthened by coherent B2-structured (Ni,Fe)Al precipitates with a volume fraction of 13vol.% and average precipitate radius of ...62nm, was subjected to creep in the stress range 30–300MPa and the temperature range 600–700°C. The stress dependence of the steady-state strain rate can be represented by a power law with high apparent stress exponents of 6–13 and high apparent activation energies of 510–680kJmol−1. Threshold stresses at all studied temperatures were observed, ranging from 69 to 156MPa, from which a true stress exponent of ∼4 and a true activation energy of 243±37kJmol−1 were determined, which are equal to those for dislocation creep and lattice diffusion in the ferritic matrix, respectively. Based on these mechanical results and detailed electron microscopy observations, the creep mechanism was rationalized to be general dislocation climb with repulsive elastic interaction between coherent precipitates and the matrix dislocations.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
This study investigates the mechanical properties of ternary Al(Sc,Zr) alloys containing 0.27–0.77 vol.% of Al
3(Sc,Zr) precipitates with an average radius
〈r〉=2−24
nm
. Microhardness values at ...ambient temperature follow predictions of the Orowan dislocation bypass mechanism, with a transition to the precipitate shearing mechanism predicted for
〈r〉 larger than 2 nm. Addition of Zr to binary Al(Sc) alloys delays the onset and kinetics of over-aging at 350 and 375 °C, but has little influence on the magnitude of the peak microhardness. Creep deformation at 300 °C is characterized by a threshold stress, which increases with
〈r〉 in the range 2–9 nm, in agreement with prior results for binary Al(Sc) alloys and a recently developed general climb model considering elastic interactions between dislocations and coherent, misfitting precipitates. At constant
〈r〉 and precipitate volume fraction, Zr additions do not significantly improve the creep resistance of Al(Sc) alloys.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
We describe four criteria for the selection of alloying elements capable of producing castable, precipitation-strengthened Al alloys with high-temperature stability and strength: these alloying ...elements must (i) be capable of forming a suitable strengthening phase, (ii) show low solid solubility in Al, (iii) low diffusivity in Al, and (iv) retain the ability for the alloy to be conventionally solidified. With regard to criterion (i), we consider those systems forming Al
M trialuminide compounds with a cubic L1
crystal structure, which are chemically and structurally analogous to Ni
Al in the Ni-based superalloys. Eight elements, clustered in the same region of the periodic table, fulfill criterion (i): the first Group 3 transition metal (Sc), the three Group 4 transition metals (Ti, Zr, Hf) and the four latest lanthanide elements (Er, Tm, Yb, Lu). Based on a review of the existing literature, these elements are assessed in terms of criteria (ii) and (iii), which satisfy the need for a dispersion in Al with slow coarsening kinetics, and criterion (iv), which is discussed based on the binary phase diagrams.
The precipitate nanostructure and the strength of an Al-0.055Sc-0.005Er-0.02Zr at% alloy with Si additions, in the range 0–0.18at%, were investigated utilizing micro-hardness, electrical ...conductivity, scanning electron microscopy and atom-probe tomography techniques. Si-containing alloys are cost-effective due to the existence of Si in commercial purity Al. In all studied alloys, homogenization for at least 0.5h at 640°C is needed to eliminate Al3Er primary precipitates. Alloys containing the higher Si concentrations achieve higher microhardness by increasing the heterogeneous nucleation current of (Al, Si)3 (Sc, Zr, Er) precipitates. The alloy containing 0.18at% Si achieves an 60% improvement in peak-microhardness compared to the Si-free alloy, during isothermal aging at 400°C. Silicon additions reduce the peak-aging time in the temperature range 300–400°C, indicating that the Er and Sc diffusion kinetics are accelerated. Silicon also enhance the Zr diffusion kinetics, accelerating precipitate growth during aging at 300°C and precipitate coarsening at 400°C. Addition of Si modifies the concentration profiles within the nanoprecipitates, enhancing the chemical homogeneity of Sc and Er in their cores, rather than forming Er-enriched-cores/Sc-enriched-shells that we have observed in prior research. Finally, the microhardness of the alloys, containing 0.12 and 0.18at% Si, only diminishes slightly from the peak values after isothermal aging at 375°C for about 2000h, suggesting that the studied alloys can be practically utilized at this operating temperature.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
A model for creep threshold stresses in alloys strengthened by coherent, misfitting precipitates is developed for the case where the precipitate is not sheared, and where there are elastic ...interactions between a dislocation and the precipitate over which it climbs. Calculations of the particle stress field due to a positive stiffness and lattice parameter mismatch between precipitate and matrix predict that the mismatch forces help the dislocation climb/glide process over the precipitates but that they trap it at the departure side of the particle. This results in a true threshold stress, rather than a slowing of the kinetics of dislocation climb as in previous models, which is given by the applied stress necessary to free the dislocation by a glide mechanism. Model predictions and experiment are compared for precipitation-strengthened aluminum alloys containing nanosize Al
3Sc, Al
3(Sc, Li) and Al
3(Sc, Yb) precipitates with various sizes and mismatches. In agreement with experimental creep results, the model predicts that the threshold stress increases nearly linearly with precipitate radius, and also with the magnitude of the precipitate/matrix lattice mismatch.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Solid-oxide iron-air batteries have potential for applications in large-scale energy storage systems, but their storage materials, iron and iron oxides, have limited cycle life due to powder ...sintering and choking of gas flow. To address this issue, Fe foams are synthesized with either equiaxed or directional dendritic pore structures by camphene-based freeze casting of Fe2O3 powders, followed by H2 reduction to Fe and sintering. For each pore architecture, Fe foams are created with three different initial porosities, ranging from 47 to 63 vol %, and are then cycled at 800 °C under alternating oxidation (via H2O) and reduction (via H2) conditions. The redox-cycled foams are examined by optical microscopy, scanning electron microscopy, and synchrotron X-ray tomography to assess the evolution of their porosity driven by the redox volume changes, sintering, and micropore formation via the Kirkendall effect. After 5 redox cycles, the Fe foams have lost the majority (39 ± 2 vol %) of their initial porosity.
•Solid-oxide iron-air batteries have potential for large-scale energy storage systems.•Fe foams are synthesized by camphene-based freeze casting.•3D microstructural analysis is used to determine degradation mechanism.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, 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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