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Type Fuller, Christian B.
  Publication Temporal Evolution of the Microstructures of Al(Sc,Zr) Alloys and Their Influences on Mechanical Properties Volume Book Whole
Pages 2003
  Abstract  
  Corporate Author  
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  Summary Language 179 Series Editor Al-Sc  
Abbreviated Series Title Al(Sc) alloys represent a new class of potential alloys for aerospace and automotive applications. These alloys have superior mechanical properties due to the presence of fine, coherent, unshearable Al3Sc precipitates, which form upon the decomposition of an supersaturated Al(Sc) solid-solution. Additions of Zr to Al(Sc) are found to improve alloy strength and coarsening resistance, but the operating mechanisms are not well understood. In this thesis, the relationships between the mechanical and microstructural properties of Al(Sc,Zr) alloy are presented. Three-dimensional atom probe microscopy (3DAP) and conventional and high-resolution transmission electron microscopies (CTEM and HREM) are utilized to study the temporal evolution of Al3Sc1-xZrx (L12 structure) precipitates in dilute Al(Sc,Zr) alloys (precipitate volume fractions < 1%) aged between 300 and 375°C. Concentration profiles, obtained with 3DAP, show Sc and Zr to partition to Al3Sc1-xZrx precipitates, and Zr to segregate near the Al/Al3Sc1-xZrx interface. CTEM and 3DAP are utilized to determine the temporal evolution of Al(Sc,Zr) alloys, which is discussed employing diffusion-limited coarsening theories. Zirconium additions are found to retard the precipitate coarsening kinetics and stabilize precipitate morphologies. Mechanical properties of Al(Sc,Zr) alloys are investigated utilizing Vicker’s microhardness and creep. Deformation at ambient-temperature is explained by classic precipitation-strengthening mechanisms, where a transition between precipitate shearing and Orowan looping is calculated to occur at an average precipitate radius, <r>, of 2-3 nm. Al(Sc,Zr) alloys deformed by creep at 300°C are found to exhibit a climb controlled threshold stress, which is shown to increase with <r>, in agreement with previous results in Al(Sc) alloys and a previous general climb model considering the interaction between dislocations and coherent misfitting precipitates. Finally, the effect of various heat-treatments upon the microstructure and mechanical properties of a rolled 5754 aluminum alloy modified with 0.23 wt.% Sc and 0.22 wt. % Zr are investigated. The presence of the Al3Sc1-xZrx precipitates is found to improve the alloy strength, by pinning subgrain and grain boundaries, as shown by hardness, tensile, and fatigue measurements.
  Series Issue ISSN Northwestern University  
Medium Ph.D. thesis
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no NU @ karnesky @ 147
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Marquis, Emmanuelle A.; Seidman, David N. Nanostructural Evolution of Al[sub:3]Sc Precipitates in an Al-Sc-Mg Alloy by Three-Dimensional Atom Probe Microscopy Journal Article 2004 Surface and Interface Analysis Surf. Interface Anal. 36 5-6 559 - 563 atom probe microscopy Mg segregation coherent heterophase interface Gibbsian excess; Al-Sc The effects of Mg alloying on the precipitation of Al3Sc precipitates were investigated, focusing on nanostructural evolution during isothermal aging at 300C. Three-dimensional atom probe microscopy was performed on samples both in the as-quenched state and after aging for various times. Magnesium tends to segregate at the coherent Al/Al3Sc interface, with a measured value of 1.9 0.5 atom nm-2 for the relative Gibbsian excess of Mg with respect to Al and Sc. This value is constant for all heat treatments, thereby demonstrating that the system is in global thermodynamic equilibrium. This study provides direct experimental evidence for first-principles calculations, which explain morphological changes of Al3Sc precipitates in the presence of Mg observed by high-resolution electron microscopy. Evidence for the presence of Mg atoms in the center of the precipitates is also found, and is discussed in terms of heterogeneous nucleation of the precipitates on MgSc atomic clusters. Marquis, E. A. Sandia Natl Labs, Livermore, CA 94551 USA Northwestern Univ, Dept Mat Sci & Engn, Evanston, IL 60208 USA no NU @ karnesky @ 659
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Fuller, Christian B.; Seidman, David N. Temporal Evolution of the Nanostructure of Al(Sc,Zr) Alloys: Part II—Coarsening of Al[sub:3]Sc[sub:1-x]Zr[sub:x] Precipitates Journal Article 2005 Acta Materialia Acta Mater. 53 20 5415-5428 Al-Sc-Zr; Phase Transformations; nanostructure; Transmission electron microscopy (TEM), The coarsening behavior of four Al(Sc,Zr) alloys containing small volume fractions (< 0.01) of Al3(Sc1-xZrx) (L12) precipitates was investigated employing conventional transmission electron microscopy (CTEM) and high-resolution electron microscopy (HREM). The activation energies for diffusion-limited coarsening were obtained employing the Umantsev-Olson-Kuehmann-Voorhees (UOKV) model for multi-component alloys. The addition of Zr is shown to retard significantly the coarsening rate and stabilize precipitate morphologies. HREM of Al(Sc,Zr) alloys aged at 300C reveals Al3(Sc1-xZrx) precipitates with sharp facets parallel to {100} and {110} planes. Coarsening of Al-0.07 Sc-0.019 Zr at.%, Al-0.06 Sc-0.005 Zr at.%. and Al-0.09 Sc-0.047 Zr at.% alloys is shown to be controlled by volume diffusion of Zr atoms, while coarsening of Al-0.14 Sc-0.012 Zr et al. is controlled by volume diffusion of Sc atoms. no NU @ karnesky @ 235
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Marquis, Emmanelle A.; Seidman, David N Coarsening Kinetics of Nanoscale Al[sub:3]Sc Precipitates in an Al-Mg-Sc Alloy Journal Article 2005 Acta Materialia Acta Mater. 53 15 4259-4268 Al3Sc precipitate; Morphology; Coarsening; Transmission electron microscopy; Atom-probe tomography; Al-Sc The effects of Mg alloying on the temporal evolution of Al3Sc (L12 structure) nanoscale precipitates are investigated, focusing on the morphology and coarsening kinetics of Al3Sc precipitates in an Al2.2 Mg0.12 Sc at.% alloy aged between 300 and 400 C. Approximately spheroidal precipitates are obtained after aging at 300 C and irregular morphologies are observed at 400 C. The coarsening behavior is studied using conventional and high-resolution transmission electron microscopies to obtain the temporal evolution of the precipitate radius, and atom-probe tomography is employed to measure the Sc concentration in the &#945;-matrix. The coarsening kinetics are analyzed using a coarsening model developed by Kuehmann and Voorhees for ternary systems [Kuehmann CJ, Voorhees PW. Metall Mater Trans A 1996;27:937]. Values of the interfacial free energy and diffusion coefficient for Sc diffusion in this AlMgSc alloy at 300 C are independently calculated, and are in good agreement with the calculated value of interfacial free energy [Asta M, Ozolins V, Woodward C. JOM 2001;53:16] and the experimental diffusivity obtained for the AlSc system [previous termMarquisnext term EA, previous termSeidmannext term DN. Acta Mater 2001;49:1909; previous termMarquisnext term EA. Ph.D. Thesis. Materials Science and Engineering Department, Northwestern University, 2002; Fujikawa SI. Defect Diffusion Forum 1997;143147:115]. no NU @ karnesky @ 202
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van Dalen, M. E.; Dunand, D. C.; Seidman, D. N. Effects of Ti Additions on the Nanostructure and Creep Properties of Precipitation-Strengthened Al-Sc Alloys Journal Article 2005 Acta Materialia Acta Mater. 53 15 4225-4235 Aluminum alloys; Precipitation; Coarsening; Three-dimensional atom-probe tomography; Creep; Al-Sc Three-dimensional atom-probe tomography, transmission electron microscopy and microhardness were used to investigate the temporal evolution of nanosize, coherent Al3(Sc1&#8722;xTix) precipitates (L12 structure) in a coarse-grained Al0.06at.%Sc0.06at.%Ti alloy aged between 300 and 450 C. At 300 C, most scandium atoms partition within one day to the precipitates, whereas titanium atoms, even after 64 days of aging, remain predominantly in solid solution in the matrix, resulting in precipitates with average composition Al3(Sc0.94Ti0.06). While titanium is very effective at retarding the coarsening kinetics of the precipitates, the low levels of titanium substitution result in only modest hardness increases over the binary Al0.06at.%Sc alloy. When crept at 300 C, the peak-aged alloy exhibits a threshold stress, which when normalized by the Orowan stress, increases with increasing radius, as predicted by a recent model considering elastic interactions between dislocations and coherent precipitates, and as previously observed in AlScZr alloys. no NU @ vandalen @ 201
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Fuller, Christian B.; Murray, Joanne L.; Seidman, David N. Temporal Evolution of the Nanostructure of Al(Sc,Zr) Alloys: Part I—Chemical Compositions of Al[sub:3](Sc[sub:1-x]Zr[sub:x]) Precipitates Journal Article 2005 Acta Materialia Acta Mater. 53 20 5401-5413 Al-Sc-Zr alloys; Precipitation; Microstructure; High-resolution electron microscopy; Three-dimensional atom-probe microscopy Atom-probe tomography (APT) and high-resolution transmission electron microscopy are used to study the chemical composition and nanostructural temporal evolution of Al3(Sc1-xZrx) precipitates in an Al-0.09 Sc-0.047 Zr at.% alloy aged at 300C. Concentration profiles, via APT, reveal that Sc and Zr partition to Al3(Sc1-xZrx) precipitates and Zr segregates concomitantly to the &#945;-Al/Al3(Sc1-xZrx) interface. The Zr concentration in the precipitates increases with increasing aging time, reaching a maximum value of 1.5 at.% at 576 h. The relative Gibbsian interfacial excess () of Zr, with respect to Al and Sc, reaches a maximum value of 1.240.62 atoms nm-2 after at 2412 h. The temporal evolution of Al3(Sc1-xZrx) precipitates is determined by measuring the time dependence of the depletion of the matrix supersaturation of Sc and Zr. The time dependency of the supersaturation of Zr does not follow the asymptotic t-1/3 law while that of Sc does, indicating that a quasi-stationary-state is not achieved for both Sc and Zr. no NU @ karnesky @ 172
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Knipling, Keith E.; Dunand, David C.; Seidman, David N. Criteria for Developing Castable, Creep-Resistant Aluminum-Based Alloys–A 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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Karnesky, Richard A.; Seidman, David N.; Dunand, David C. Creep of Al-Sc Microalloys with Rare-Earth Element Additions Journal Article 2006 Materials Science Forum Mater. Sci. Forum 519-521 1035-1040 Scandium, Rare-Earth Elements, Creep, Precipitation Strengthening; Al-Sc Cast and aged Al-Sc microalloys are creep-resistant to 300C, due to the blocking of dislocations by nanosize, coherent Al[sub:3]Sc (L1[sub:2]) precipitates. Rare-earth elements substitute for Sc in these precipitates, leading to a higher number density of smaller precipitates, which have a greater lattice-parameter mismatch with Al than in the Al-Sc binary microalloy. This leads to an improvement in both ambient temperature microhardness and high temperature creep. Creep threshold stresses of Al-Sc-RE (RE = Y, Dy, or Er) at 300C; are higher than for Al-Sc and Al-Sc-M (M = Mg, Ti, or Zr) microalloys. This is in agreement with a dislocation climb model that includes the elastic stress fields of the precipitates. ICAA10 no NU @ karnesky @ 660
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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 &#945;/&#949;, where &#949; = 0.62% is the lattice parameter misfit between the &#945;-Al matrix containing 2.2 at.% Mg and the Al3Sc phase [3,4], and &#945; = 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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van Dalen, Marsha E.; Dunand, David C.; Seidman, David N. Nanoscale Precipitation and Mechanical Properties of Al-0.06 at.% Sc Alloys Micro-Alloyed with Yb or Gd Journal Article 2006 Journal of Materials Science 41 23 7814-7823 Aluminum alloys; Precipitation; Nucleation; Three-dimensional atom-probe tomography; Rare earth elements; Al-Sc Dilute Al-0.06 at.% Sc alloys with micro-alloying additions of 50 at. ppm of Yb or Gd are studied with 3-D local-electrode atom-probe (LEAP) tomography for different aging times at 300C. Peak-aged alloys exhibit Al3(Sc1-xYbx) or Al3(Sc1-xGdx) precipitates (L12 structure) with a much higher number density (and therefore higher peak hardness) than a binary Al-0.06 at.% Sc alloy. The Al-Sc-Gd alloy exhibits a higher number density of precipitates with a smaller average radius than the Al-Sc-Yb alloy, leading to a higher hardness. In the Al-Sc-Gd alloy, only a small amount of the Sc is replaced by Gd in the Al3(Sc1-xGdx) precipitates, where x = 8.0%. By contrast, the hardness incubation time is significantly shorter in the Al-Sc-Yb alloy, due to the formation of Yb-rich Al3(Yb1-xScx) precipitates to which Sc subsequently diffuses, eventually forming Sc-rich Al3(Sc1-xYbx) precipitates. For both alloys, precipitate radii are found to be relatively stable up to 24 h of aging, although the concentration and distribution of the RE elements in the precipitates continues to evolve temporally. Similar to microhardness at ambient temperature, the creep resistance at 300C is significantly improved by RE microalloying of the binary Al-Sc alloy. no NU @ karnesky @ 767
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