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Type Knipling, Keith E.; Dunand, David C.; Seidman, David N.
  Publication Nucleation and Precipitation Strengthening in Dilute Al-Ti and Al-Zr Alloys Volume Journal Article
Pages 2007
  Abstract Metallurgical and Materials Transactions A  
  Corporate Author  
Publisher 38  
Editor 10
  Summary Language 2552-2563 Series Editor Al-Zr; Al-Ti  
Abbreviated Series Title Two conventionally solidified Al-0.2Ti alloys (with 0.18 and 0.22 at. pct Ti) exhibit no hardening after aging up to 3200 hours at 375 C or 425 C. This is due to the absence of Al3Ti precipitation, as confirmed by electron microscopy and electrical conductivity measurements. By contrast, an Al-0.2Zr alloy (with 0.19 at. pct Zr) displays strong age hardening at both temperatures due to precipitation of Al3Zr (L12) within Zr-enriched dendritic regions. This discrepancy between the two alloys is explained within the context of the equilibrium phase diagrams: (1) the disparity in solid and liquid solubilities of Ti in alpha-Al is much greater than that of Zr in alpha-Al; and (2) the relatively small liquid solubility of Ti in alpha-Al limits the amount of solute retained in solid solution during solidification, while the comparatively high solid solubility reduces the supersaturation effecting precipitation during post-solidification aging. The lattice parameter mismatch of Al3Ti (L12) with alpha-Al is also larger than that of Al3Zr (L12), further hindering nucleation of Al3Ti. Classical nucleation theory indicates that the minimum solute supersaturation required to overcome the elastic strain energy of Al3Ti nuclei cannot be obtained during conventional solidification of Al-Ti alloys (unlike for Al-Zr alloys), thus explaining the absence of Al3Ti precipitation and the presence of Al3Zr precipitation.
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no NU @ karnesky @ 1908
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Knipling, Keith E.; Dunand, David C.; Seidman, David N. Criteria for Developing Castable, Creep-Resistant Aluminum-Based AlloysA Review Journal Article 2006 Zeitschrift für Metallkunde Z. Metallkd 97 3 246-265 Aluminum alloys; Trialuminides; Precipitation strengthening; Creep; Al-Sc 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) showlow 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 Al3Mtrialuminide compounds with a cubic L12 crystal structure, which are structurally analogous to Ni3Al ( 0) 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 rare-earth elements (Er, Tm, Yb, Lu). Based on a review of the existing literature, these elements are discussed in terms of criteria (ii) and (iii), which satisfy the need for a dispersion in Al with slow coarsening kinetics, and criteria (iv), which is discussed based on the binary phase diagrams. no NU @ karnesky @ 236
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Knipling, Keith E.; Karnesky, Richard A.; Lee, Constance P.; Dunand, David C.; Seidman, David N. Precipitation Evolution in Al-0.1Sc, Al-0.1Zr, and Al-0.1Sc-0.1Zr (at.%) Alloys during Isochronal Aging Journal Article 2010 Acta Materialia 58 15 5184-5195 Aluminum alloys, Precipitation, Scandium, Zirconium, Atom-probe tomography; Al-Sc-Zr Precipitation strengthening is investigated in binary Al-0.1Sc, Al-0.1Zr, and ternary Al-0.1Sc-0.1Zr (at.%) alloys aged isochronally between 200 and 600 °C. A pronounced synergistic effect is observed when both Sc and Zr are present. Above 325 °C, where peak microhardness (670 MPa) occurs in the binary Al-Sc alloy due to Al[sub:3]Sc (L1[sub:2]) nanometer-scale precipitates, Zr additions result in a secondary increase in strength due to additional precipitation of Zr-enriched outer shells onto these precipitates. The ternary alloy reaches a peak microhardness of 780 MPa at 400 °C, delaying overaging by >100 °C compared with the binary Al-Sc alloy and increasing strength compared with the binary Al-Zr alloy (peak microhardness of 420 MPa at 425450 °C). Compositions, radii, volume fractions, and number densities of the Al[sub:3](Sc[sub:1-x]Zr[sub:x]) precipitates are measured directly using atom-probe tomography. This information is used to quantify the observed strengthening increments, attributed to dislocation shearing of the Al[sub:3](Sc[sub:1-x]Zr[sub:x]) precipitates. no NU @ karnesky @ 10705
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Krug, M.E.; Werber, A.; Dunand, D.C.; Seidman, D.N. Core-shell nanoscale precipitates in Al-0.06 at.% Sc microalloyed with Tb, Ho, Tm or Lu Journal Article 2010 Acta Materialia Acta Mater. 58 1 134-145 Rare earth; Precipitation; Microhardness; Atom-probe field-ion microscopy (AP-FIM); Aluminum alloys; Al-Sc The age-hardening response at 300 C of Al-0.06Sc-0.02RE (at.%, with RE = Tb, Ho, Tm or Lu) is found to be similar to that of binary Al-0.08Sc (at.%), except that a shorter incubation period for hardening is observed, which is associated with nanoscale RE-rich Al-3(RE1-xScx) precipitates. In addition, Al-0.06Sc-0.02Tb (at.%) has a much lower peak microhardness than that of Al-0.08Sc (at.%) due to the small solubility of Tb in alpha-Al(Sc). Peak-age hardening occurs after 24 h. and is associated with a high number density of nanoscale Sc-rich Al-3(Sc1-xREx) precipitates. Analysis by three-dimensional local-electrode atom-probe tomography shows that x increases with increasing atomic number, and that the REs partition to the core of the precipitates. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. [Krug, Matthew E.; Werber, Alexandra; Dunand, David C.; Seidman, David N.] Northwestern Univ, Evanston, IL 60208 USA, Email: m-krug@northwestern.edu Pergamon-Elsevier Science Ltd English 1359-6454 ISI:000272405600015 no NU @ m-krug @ 10807
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Krug, Matthew E. Microstructural Evolution and Mechanical Properties in Al-Sc Alloys With Li and Rare Earth Additions Book Whole 2011 372 Al-Sc, Al-Li, Al-Li-Sc, rare earth, LEAP, atom probe Aluminum-scandium alloys have excellent mechanical properties at ambient and elevated temperatures due to the presence of coherent, nano-scale, L12-ordered Al3Sc precipitates. In this thesis, a variety of Al-Sc alloys with additions of Li and RE elements, primarily Yb, are studied. An addition of ytterbium reduces the cost of Al-Sc alloys by replacing some of the more-expensive Sc. Lithium is a unique alloying addition to Al-Sc alloys, because it has significant solubility in both the matrix and precipitate phases. Lithium also provides solid solution strengthening, and a large strengthening increment on aging through the formation of Al3Li precipitates. The effects of these alloying additions on Al-Sc alloys are investigated in detail, and discussed in the context of physical models linking the microstructure to measured mechanical properties. The alloys undergo a variety of aging treatments between 170 450 °C, producing a range of precipitate distributions. Their aging response is assessed using Vickers microhardness to monitor ambient-temperature strength, and electrical conductivity to monitor the progress of the precipitation reaction. The alloys are creep-tested in compression at 300 °C, and exhibit threshold stresses, below which no measurable creep occurs. Detailed microstructural investigations rely primarily on local electrode atom probe tomography, as well as transmission electron microscopy. The volume fractions, number densities, and chemical compositions of precipitates are measured at the nano-scale, and their size and spatial distributions are quantitatively determined. Compared to binary Al-Sc alloys, Al-Li-Sc and Al-Li-Sc-Yb alloys contain a finer distribution of Al3(Sc1-x-yLixYby) precipitates at a greater number density and volume fraction, as well as solid-solution strengthening in the Al(Li) matrix, all of which lead to a greater peak strength at ambient-temperature. Because partitioning of Li to the precipitates results in a smaller lattice parameter mismatch with the matrix, a Li addition is detrimental to the elevated temperature strength of Al-Sc alloys, but this effect is mitigated if additions of both Li and Yb are made. A model for threshold stresses at elevated temperature semi-quantitatively captures experimentally-observed trends in threshold stress data in Al-Sc-X alloys. Dislocation dynamics simulations on directly-measured precipitate arrangements lead to a rule for superposition of strength contributions from dissolved solutes, α′Al3(Li,Sc,Yb) precipitates, and δ′Al3Li precipitates. Northwestern University Ph.D. thesis English no NU @ m-krug @ 11430
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Krug, Matthew E.; Dunand, David C.; Seidman, David N. Effects of Li additions on precipitation-strengthened Al-Sc and Al-Sc-Yb alloys Journal Article 2011 Acta Materialia 59 1700-1715 aluminium alloys, atom-probe field-ion microscopy (AP-FIM), rare earth, nanostructure, precipitation; Al-Sc Two Al-Sc based alloys (Al-0.12 Sc and Al-0.042 Sc-0.009 Yb, at. %) and their counterparts with Li-additions (Al-2.9 Li-0.11 Sc and Al-5.53 Li-0.048 Sc-0.009 Yb, at. %) are aged at 325 °C. For both base alloys, the addition of Li results in greater peak hardness from incorporation of Li in the L12-structured alpha'Al3(Sc,Li) and alpha'Al3(Sc,Li,Yb) precipitates, and a concomitant increase in number density and volume fraction of the precipitates and a reduction in their mean radius. These changes result from a combination of (i) an increase in the driving force for precipitate nucleation due to Li, (ii) a decrease in the elastic energy of the coherent misfitting precipitates from a decrease in their lattice parameter mismatch due to their Li content, and (iii) a decrease in the interfacial free energy, as determined from measurements of relative Gibbsian interfacial excess of Li. In Al-2.9 Li- 0.11 Sc (at. %), the Li content of the precipitates drops from 9.1 at. % in the peak-aged state (8 h) to 5.7 at. % in the overaged state (1536 h). As a result, the precipitate volume fraction decreases from 0.56% at peak-age to 0.45% at 1536 h. In Al-5.53 Li-0.048 Sc-0.009 Yb (at. %), the relatively limited Li concentration produces only a small increase in Vickers microhardness from precipitation of metastable delta'Al3Li upon a second aging at 170 °C following the primary aging at 325 °C. no NU @ karnesky @ 10984
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Krug, ME; Dunand, DC; Seidman, DN Composition Profiles within Al3Li and Al3Sc/Al3Li Nanoscale Precipitates in Aluminum Journal Article 2008 Applied Physics Letters App. Phys. Let. 92 124107-1 - 124107-3 Al; Sc; Li; Aluminum; Scandium; Lithium; Core-shell Precipitates; Al-Sc An Al11.3Li0.11Sc (at. %) alloy was double-aged to induce first Alpha'-Al3Sc and then delta'-Al3Li precipitates. Atom-probe tomography revealed both single-phase delta'-precipitates and core-shell alpha'/delta'-precipitates (with respective average radii of 16 and 27 nm, and respective volume fractions of 12 and 9%) conferring a high strength to the alloy. Although the delta'-shells contain little Sc (~0.027 at. %), the alpha'-cores have a high Li content, with an average composition of Al0.72(Sc0.17Li0.11). The Li concentrations within the delta'-phase and the Li interfacial excess at the delta'/alpha'-interface both exhibit wide precipitate-to-precipitate variations. no NU @ m-krug @ 10263
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Mao, Z.; Chen, W.; Seidman, D.N.; Wolverton, C. First-principles study of the nucleation and stability of ordered precipitates in ternary Al-Sc-Li alloys Journal Article 2011 Acta Materialia 59 3012-3023 Al-Sc-Li; First principles; Interfacial energy; Core/shell structures; Site substitution First-principles density functional calculations are used to study the nucleation and stability of L12-ordered precipitates in Al-Sc-Li alloys. For dilute Al alloys, there are three possible ordered L12 precipitates: Al3Sc, Al3Li and an Al3Sc/Al3Li core/shell structure. To calculate the nucleation behavior, information about bulk thermodynamics (both static total energies and vibrational free energies), interfacial energetics and coherency strain is required. The study finds the following: (1) the coherency strain energies for forming coherent interfaces between Al/Al3Sc, Al/Al3Li and Al3Sc/Al3Li are relatively small, owing to the small atomic size mismatches in these systems; (2) the sublattice site preferences of Sc and Li are calculated, and it is demonstrated that Sc and Li share the same sublattice sites in both Al3Sc(L12) and Al3Li(L12), in agreement with recent experimental results; (3) the calculated solubilities of Sc and Li in [alpha]-Al alloys are in good agreement with experimental values and, for Sc, agree well with prior first-principles results; (4) the interfacial energies for Al/Al3Sc, Al/Al3Li and Al3Sc/Al3Li for (1 0 0), (1 1 0) and (1 1 1) interfaces are calculated: the values of the Al/Al3Sc interfacial energies are significantly larger than those of the Al/Al3Li and Al3Sc/Al3Li interfaces; (5) combining the bulk and interfacial energies yields the nucleation barriers and critical radii for Al3Sc and Al3Li precipitates; and (6) the energetic stability of the Al3Sc/Al3Li core/shell structure is compared with individual Al3Sc and Al3Li nuclei, and the range of precipitate sizes for which the core/shell structure is energetically favored is determined quantitatively. 1359-6454 no NU @ karnesky @ 11034
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Marquis, E. A.; Riesterer, J. L.; Seidman, D. N.; Larson, D. J. Mg Segregation at Coherent and Semi-Coherent Al/Al[sub:3]Sc Interfaces Journal Article 2006 Microscopy and Microanalysis Microsc. Microanal. 12 S2 914-915 LEAP; Al-Sc Aluminum alloys containing Sc are promising materials for high-temperature structural applications due to the high strengthening effect of the Al3Sc (L12 structure) precipitates [1]. Further improvements in strength and nanostructural stability of Al-Sc based alloys are achieved by adding alloying elements, such as Mg, as solid-solution strengtheners. It is important to understand the effects of Mg in order to control not only the specific contribution of Mg to the properties of Al-Sc alloys (strengthening effect and creep resistance) but also the changes in the nanostructure. From previous work, it is known that Mg tends to segregate to the coherent Al/Al3Sc interface due to positive interactions between Mg and Sc atoms [2]. This paper reports measurements of Mg segregation at the Al/Al3Sc interface and compares the segregation level between coherent and semicoherent Al/Al3Sc interfaces. A cast Al-2 wt.% Mg-0.2 wt.% Sc alloy was annealed at 618C in air for 24 hours, quenched into cold water, and then aged in air at 300C for 24 hours. One sample was subsequently aged at 400oC for 240 hours. Three-dimensional atom probe (3DAP) microscopy tips were obtained by a double electro-polishing technique. Field evaporation was performed at 30 K with a pulse fraction of 20 % at a frequency of 200 kHz using a LEAP microscope. Transmission electron microscopy (TEM) imaging was performed on a JEOL 1200 microscope. During aging at 300oC, Al3Sc precipitates are formed with a high number density (~2 1022 precipitates/m3), which is advantageous for random 3DAP microscope observations. The average radius of the precipitates is 2 nm and the interface is coherent (Fig. 1). After aging at 400oC, however, the average radius of the precipitates is ~19 nm and dislocations loops are observed at the matrix/precipitate interface. The number density of precipitates has also decreased dramatically (~1019 precipitates/m3) and is no longer sufficient for random atom probe observations. Atom probe tips were therefore observed by TEM to confirm the semi-coherent nature of the Al/Al3Sc interface and to determine the position of the precipitates with respect to the tip apex. Micro-polishing was used to position precipitates to within ~100 nm of the apex. A TEM image of a tip is shown in FIG.2. Al3Sc precipitates are visible with dislocations at the matrix/precipitate interfaces. Coherency loss may occur when the precipitate diameter is larger than the spacing between the misfit dislocations. This spacing is of the order of α/ε, where ε = 0.62% is the lattice parameter misfit between the α-Al matrix containing 2.2 at.% Mg and the Al3Sc phase [3,4], and α = 0.20 nm is the spacing between {200} planes. The calculated equilibrium dislocation spacing is therefore 32 nm, in good agreement with the presence of interfacial dislocations for precipitates with diameter of ~38 nm. Comparison is made between the segregation levels measured for coherent and semicoherent interfaces. The role of the interfacial dislocations will be discussed. References [1] Toporova L.S., Eskin D.G., Kharakterova M.L., Dobatkina T.B. Advanced aluminum alloys containing scandium. Amsterdam: Gordon & Breach; 1998. [2] Marquis E.A., Seidman D.N., Asta M., Woodward C., Ozolins V., Phys. Rev. Letters 91 (2003) 036101 1-3 [3] Hatch J.E. Aluminum: properties and physical metallurgy. Metals Park (OH): ASM; 1984. no NU @ karnesky @ 753
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Marquis, E. A.; Seidman, D. N. A Subnanoscale Study of Segregation at Al/Al[sub:3]Sc Interfaces Journal Article 2002 Microscopy and Microanalysis 8 S2 1100-1101 Al-Sc no NU @ karnesky @ 1905
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