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Marquis, E. A.; Riesterer, J. L.; Seidman, D. N.; Larson, D. J. |
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Publication |
Mg Segregation at Coherent and Semi-Coherent Al/Al[sub:3]Sc Interfaces |
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Journal Article |
Pages |
2006 |
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Abstract |
Microscopy and Microanalysis |
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Corporate Author |
Microsc. Microanal. |
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12 |
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S2 |
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Summary Language |
914-915 |
Series Editor |
LEAP; Al-Sc |
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Abbreviated Series Title |
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. |
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Fuller, Christian B.; Seidman, David N.; Dunand, David C. |
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Creep Properties of Coarse-Grained Al(Sc) Alloys at 300°C |
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Journal Article |
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1999 |
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Scripta Materialia |
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Scripta Mater. |
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40 |
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6 |
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691-696 |
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Harada, Y.; Dunand, D.C. |
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Thermal Expansion of Al[sub:3]Sc and Al[sub:3](Sc[sub:0.75]X[sub:0.25]) |
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Journal Article |
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2003 |
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Scripta Materialia |
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Scripta Mater. |
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48 |
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3 |
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219-222 |
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intermetallic compound aluminum alloys thermal expansion scandium physical-properties al(sc) alloys |
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The thermal expansion coefficient of Al3Sc and Al3(Sc0.75X0.25), where X is Ti, Y, Zr or Hf, was measured by dilatometry between 25 and 1000 °C. The measured value, (16±1)×10-6 K-1, is constant between 25 and 900 °C and insensitive to alloying element. Good agreement is found with a literature value determined from first-principle calculations |
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Dunand, D. C. Northwestern Univ, Dept Mat Sci & Engn, 2220 Campus Dr, Evanston, IL 60208 USA Northwestern Univ, Dept Mat Sci & Engn, Evanston, IL 60208 USA Natl Inst Adv Ind Sci & Technol, Inst Mech Syst Engn, Tsukuba, Ibaraki 3058564, Japan |
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....there is an error: we should have written : alloying the matrix with magnesium, which INCREASES the lattice constant of Al ... |
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NU @ karnesky @ |
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542 |
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Marquis, Emmanuelle A.; Dunand, David C. |
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Model for Creep Threshold Stress in Precipitation-Strengthened Alloys with Coherent Particles |
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Journal Article |
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2002 |
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Scripta Materialia |
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Scripta Mater. |
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47 |
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8 |
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503-508 |
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creep threshold stress precipitation strengthening aluminum alloys dislocation mobility al(sc) alloys al3sc; Al-Sc |
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The general climb model for creep threshold stress for dislocations interacting with incoherent particles is modified for the case of coherent precipitates, by taking into account elastic interactions between matrix dislocations and particles due to particle/matrix stiffness and lattice mismatches. The model is in qualitative agreement with experimental data for the Al–Sc system. |
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Dunand, D. C. Northwestern Univ, Dept Mat Sci & Engn, MLSB 1123,2225 N Campus Dr, Evanston, IL 60208 USA Northwestern Univ, Dept Mat Sci & Engn, Evanston, IL 60208 USA |
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NU @ karnesky @ |
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569 |
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Fuller, Christian B.; Seidman, David N.; Dunand, David C. |
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Mechanical Properties of Al(Sc,Zr) Alloys at Ambient and Elevated Temperatures |
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Journal Article |
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2003 |
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Acta Materialia |
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Acta Mater. |
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51 |
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16 |
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4803-4814 |
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al-sc-zr alloys mechanical properties creep deformation mechanisms strengthened alloys threshold stress creep-properties al3sc microstructure particles additions evolution model sc |
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This study investigates the mechanical properties of ternary Al(Sc,Zr) alloys containing 0.27–0.77 vol.% of Al3(Sc,Zr) precipitates with an average radius left angle bracketrright-pointing angle bracket=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 left angle bracketrright-pointing angle bracket 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 left angle bracketrright-pointing angle bracket 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 left angle bracketrright-pointing angle bracket and precipitate volume fraction, Zr additions do not significantly improve the creep resistance of Al(Sc) alloys. |
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Dunand, D. C. Northwestern Univ, Dept Mat Sci & Engn, 2220 Campus Dr, Evanston, IL 60208 USA Northwestern Univ, Dept Mat Sci & Engn, Evanston, IL 60208 USA |
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NU @ karnesky @ |
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224 |
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Marquis, Emmanuelle A.; Seidman, David N.; Dunand, David C. |
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Effect of Mg Addition on the Creep and Yield Behavior of an Al–Sc Alloy |
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Journal Article |
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2003 |
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Acta Materialia |
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Acta Mater. |
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51 |
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16 |
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4751-4760 |
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Transmission electron microscopy; Aluminum alloys; Coarsening; Creep; Al3Sc precipitates; olid-solution alloys dislocation climb viscous glide deformation stress dependence particles fracture; Al-Sc |
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The relationships between microstructure and strength were studied at room temperature and 300 °C in an Al–2 wt% Mg–0.2 wt% Sc alloy, containing Mg in solid-solution and Al3Sc (L12 structure) as nanosize precipitates. At room temperature, the yield strength is controlled by the superposition of solid-solution and precipitation strengthening. At 300 °C and at large applied stresses, the creep strength, which is characterized by a stress exponent of ~5, is significantly improved compared to binary Al–Sc alloys, and is independent of the size of the Al3Sc precipitates. At small applied stress, a threshold stress exists, increasing from 9% to 70% of the Orowan stress with increasing Al3Sc precipitate radius from 2 to 25 nm. An existing model based on a climb-controlled bypass mechanism is in semi-quantitative agreement with the precipitate radius dependence of the threshold stress. The model is, however, only valid for coherent precipitates, and the Al3Sc precipitates lose coherency for radii larger than 11 nm. For semi-coherent precipitates with radii greater than 15 nm, the threshold stress remains high, most likely because of the presence of interfacial misfit dislocations. |
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Marquis, E. A. Sandia Natl Labs, Thin Film & Interface Dept, Livermore, CA 94550 USA Northwestern Univ, Dept Mat Sci & Engn, Evanston, IL 60208 USA |
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NU @ karnesky @ |
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212 |
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Marquis, Emmanuelle A.; Seidman, David N.; Dunand, David C. |
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Precipitation Strengthening at Ambient and Elevated temperatures of Heat-Treatable Al(Sc) Alloys [Acta Materialia 50(16), pp. 4021–4035] |
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Journal Article |
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2003 |
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Acta Materialia |
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Acta Mater. |
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51 |
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1 |
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285-287 |
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Al-Sc |
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In the final printed issue, the Publisher regrets to inform that the sequence of authors’ names given under the title on page 4021 was incorrect. The correct sequence is now shown above.
The Publisher also wishes to apologise for the poor quality reproduction of Fig. 1, Fig. 9 and Fig. 10, which are now reprinted correctly below: |
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NU @ karnesky @ |
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213 |
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Seidman, David N.; Marquis, Emmanuelle A.; Dunand, David C. |
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Precipitation Strengthening at Ambient and Elevated Temperatures of Heat-Treatable Al(Sc) Alloys |
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Journal Article |
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2002 |
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Acta Materialia |
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Acta Mater. |
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50 |
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16 |
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4021-4035 |
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Aluminum alloys; Scandium; Mechanical properties; Precipitation strengthening; Creep; Threshold stress; Al-Sc |
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Yield strength at ambient temperature and creep resistance between 225 and 300C were investigated in dilute Al(Sc) alloys containing coherent Al3Sc precipitates, which were grown by heat-treatments to radii in the range 1.49.6 nm. The dependence of the ambient-temperature yield stress on precipitate size is explained using classical precipitation strengthening theory, which predicts a transition from precipitate shearing to Orowan dislocation looping mechanisms at a precipitate radius of 2.1 nm, in good agreement with experimental data. At 300C creep threshold stresses are observed and found to be much lower than the yield stresses, indicative of a climb-controlled bypass mechanism. The threshold stress increases with increasing precipitate radius, in qualitative agreement with a climb model taking into account stiffness and lattice mismatches between matrix and precipitates [1]. |
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NU @ karnesky @ |
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618 |
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Marquis, E. A.; Seidman, D. N. |
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Nanoscale Structural Evolution of Al[sub:3]Sc Precipitates in Al(Sc) Alloys |
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Journal Article |
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2001 |
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Acta Materialia |
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Acta Mater. |
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49 |
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11 |
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1909-1919 |
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aluminum scandium transmission electron microscopy (tem) phase transformations microstructure al-sc alloys creep-properties nucleation scandium aluminum intermetallics kinetics growth energy; Al-Sc |
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Precipitation of the Al3Sc (L12) phase in aluminum alloys, containing 0.1, 0.2 or 0.3 wt% Sc, is studied with conventional transmission and high-resolution (HREM) electron microscopies. The exact morphologies of the Al3Sc precipitates were determined for the first time by HREM, in Al–0.1 wt% Sc and Al–0.3 wt% Sc alloys. The experimentally determined equilibrium shape of the Al3Sc precipitates, at 300°C and 0.3 wt% Sc, has 26 facets, which are the 6 {100} (cube), 12 {110} (rhombic dodecahedron), and 8 {111} (octahedron) planes, a Great Rhombicuboctahedron. This equilibrium morphology had been predicted by first principles calculations of the pertinent interfacial energies. The coarsening kinetics obey the (time)1/3 kinetic law of Lifshitz–Slyozov–Wagner theory and they yield an activation energy for diffusion, 164±9 kJ/mol, that is in agreement with the values obtained from tracer diffusion measurements of Sc in Al and first principles calculations, which implies diffusion-controlled coarsening. |
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Seidman, D. N. Northwestern Univ, Dept Mat Sci & Engn, 2225 N Campus Dr, Evanston, IL 60208 USA Northwestern Univ, Dept Mat Sci & Engn, Evanston, IL 60208 USA |
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NU @ karnesky @ |
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568 |
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Harada, Y.; Dunand, D.C. |
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Creep Properties of Al[sub:3]Sc and Al[sub:3](Sc, X) Intermetallics |
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Journal Article |
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2000 |
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Acta Materialia |
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Acta Mater. |
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48 |
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13 |
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3477-3487 |
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Journal Article; Aluminum compounds, Mechanical properties; Scandium compounds, Mechanical properties; Intermetallics, Mechanical properties; Titanium, Alloying additive; Yttrium, Alloying additive; Zirconium, Alloying additive; Hafnium, Alloying additive; Creep (materials), Alloying effects; Dislocation mobility, Deformation effects; Al-Sc; mechanical properties creep intermetallic compounds bulk diffusion scandium microstructure high-temperature creep l12 trialuminides self-diffusion behavior fracture films al3ti compression chromium aluminum |
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A systematic creep study was undertaken for the binary intermetallic Al sub 3 Sc and the ternary single-phase intermetallic Al sub 3 (Sc sub 0.74 X sub 0.26 ), where X is one of the transition-metals Ti, Y, Zr or Hf. Creep tests were conducted in the temperature range from 673-1200K under a constant compressive stress ranging from 30-300 MPa. The binary Al sub 3 Sc exhibits a stress exponent of 4.4-4.9 indicative of creep controlled by climb of dislocations. The activation energy for creep of Al sub 3 Sc was 128plus/minus6 kJ/mol, close to that for self-diffusion for pure aluminum, in agreement with the Cu sub 3 Au rule, indicating that diffusion on the Al-sublattice is controlling. Ternary Al sub 3 (Sc sub 0.74 X sub 0.26 ) exhibits a decrease in creep rate by about one order of magnitude for Zr and Hf and by about two orders of magnitude for Ti and Y. For all ternary alloys, a stress exponent of 3.9-5.5 was observed, indicative of dislocation creep. Activation energies for creep of 202plus/minus8 kJ/mol were found, showing that ternary substitution for scandium with transition metals affects diffusion on the Al sublattice. |
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Northwestern University (Evanston); Ministry of International Trade and Industry (Japan) |
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English |
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1359-6454 |
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37 ref., Photomicrographs, Graphs, Numerical Data |
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NU @ karnesky @ |
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544 |
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