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Title (down) Karnesky, Richard A.
Year Mechanical Properties and Microstructure of Al–Sc with Rare-Earth Element or Al[sub:2]O[sub:3] Additions
Abbreviated Journal Book Whole
Issue 2007 Keywords
Place of Publication Language 258
Original Title Al-Sc
Series Title Aluminum alloys strengthened with coherent (L1[sub:2]), nanosize Al[sub:3]Sc precipitates are structural materials that have outstanding strength at ambient and elevated temperatures. They are creep resistant at 300 °C and exhibit a threshold stress, below which creep is not measurable. Introducing ternary alloying additions, such as rare-earth elements (RE=Y, Dy, Er), that segregate within Al[sub:3]Sc precipitates improves this creep resistance by increasing the lattice parameter misfit of precipitates with Al. In this thesis, Al–600 Sc–200 RE and Al–900 Sc–300 Er (at. ppm) are studied. These elements are an order of magnitude less expensive than Sc, so reduce alloy costs. As an alternative or supplement to ternary additions, submicron (incoherent) Al[sub:2]O[sub:3] dispersoids impart additional strengthening. The dispersion-strengthened cast alloys, DSC–Al–1100 Sc and DSC–Al–800 Sc–300 Zr, studied in this thesis contain 30 vol.% Al[sub:2]O[sub:3]. In this thesis, the temporal evolution of Al–Sc–RE and DSC–Al–Sc(–Zr) alloys are measured using Local-Electrode Atom-Probe (LEAP) tomography, conventional transmission electron microscopy, and electrical conductivity. These techniques measure the changes in precipitate number density, size, volume fraction, chemical composition, and interprecipitate distance and are compared to models. They are also employed to measure the diffusivity and solid solubility of Er in Al in Al–300 Er, Al–450 Er, and Al–600 Er. The mechanical behavior (microhardness, yield, and creep) of the alloys is studied at 25, 300, and 350 °C. The effect of Al[sub:3](Sc[sub:1-x]Er[sub:x]) precipitate size and interprecipitate distance is studied by varying isochronal and isothermal aging treatments. Various models and simulations are compared to experimental data. At ambient temperatures, very small Al[sub:3](Sc[sub:1-x]M[sub:x]) precipitates contribute to order strengthening and larger (unshearable) precipitates are bypassed by dislocations through Orowan bowing. Dislocation dynamics simulations allow both processes to operate in a glide plane, where precipitate distributions may be gathered directly or be informed by LEAP tomography data. At elevated temperatures, the lattice parameter and modulus mismatches of Al[sub:3](Sc[sub:1-x]M[sub:x]) oppose both dislocation climb over Al[sub:3](Sc[sub:1-x]M[sub:x]) and dislocation detachment from Al[sub:2]O[sub:3]. Series Volume
Edition Northwestern University
ISBN Ph.D. thesis Area
Conference Evanston, IL
Serial English Orig Record
no NU @ karnesky @ 10000
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Kolli, R. Prakash Kinetics of nanoscale Cu-rich precipitates in a multicomponent concentrated steel Book Whole 2007 320 Kinetics; Cu; precipitate; steel The kinetics of nanoscale Cu-rich precipitates of multicomponent concentrated steels has been investigated utilizing primarily APT and supplemented with a synchrotron radiation experiment, first-principles calculations, Thermo-Calc study, and CTEM (at the longest aging time). Results on mechanical properties and microstructure at a greater length scale are also presented. The studied steels, NUCu-170 and NUCu-140-x, are HSLC steels, and are primarily strengthened by nanoscale Cu-rich precipitates. NUCu-170 contains 1.82 at. % Cu, whereas NUCu-140-x contains nominally ca. 1.15 at. % Cu. This study focused on a 900 °C solutionizing treatment followed by isothermal aging at 500 °C between 0.25 and 1024 h for NUCu-170 and NUCu-140-1, and aging at 550 °C between 0.25 and 4 h for NUCu-140-3. In addition, a double aging treatment of 550 °C aging followed by 200 °C for 2 h was investigated for NUCu-140-3. Ph.D. thesis Northwestern University no NU @ m-krug @ 10472
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Knipling, Keith E Development of a Nanoscale Precipitation-Strengthened Creep-Resistant Aluminum Alloy Containing Trialuminide Precipitates Book Whole 2006 230 Al-Zr This research is toward developing a castable and heat-treatable precipitation-strengthened aluminum alloy exhibiting coarsening- and creep resistance at temperatures exceeding 400°C. Criteria for selecting alloying elements capable of producing such an alloy are established. Those systems forming Al3M trialuminide compounds with a cubic L12 crystal structure are favored, and based on a review of the existing literature, these are assessed in terms of solid-solubility and diffusivity in α-Al (satisfying the need for slow coarsening kinetics), and castability (which is discussed based on the binary phase diagrams). The first Group 3 element, Sc, and the second Group 4 element, Zr, are shown to be most promising. These expectations are confirmed by an initial study on the Al-Ti system, which demonstrates that conventionally-solidified alloys are not capable of precipitation strengthening. The Al-Zr system, by contrast, exhibits precipitation of nanometer-scale Al3Zr (L12) producing pronounced precipitation hardening when aged at 375, 400, or 425°C. The Al3Zr precipitates are coarsening resistant and have the metastable L12 structure up to 500°C, a result of very sluggish diffusion of Zr in α-Al. Ternary additions of Ti are also investigated, forming Al3(Zr,Ti) (L12) precipitates with a reduced lattice parameter mismatch with α-Al, potentially improving the coarsening resistance. The composition of Al3(Zr,Ti) precipitates formed at 375 or 425°C are measured directly using 3-D atom-probe tomography. At these temperatures, the Zr:Ti atomic ratio in the precipitates is about 10 and 5, respectively, indicating that most of the available Ti fails to partition to the Al3(Zr,Ti) phase. This is consistent with prior studies on Al-Sc alloys, where the slower-diffusing ternary solute species make up a small fraction of the Al3Sc-based precipitates. Despite the confirmed presence of Ti, Al3(Zr,Ti) precipitates exhibit no improvement in terms of coarsening resistance compared to binary Al3Zr. Mechanical properties of the Al-Zr and Al-Zr-Ti alloys are investigated utilizing Vickers microhardness and creep. The alloys deformed by creep at 300−400°C exhibit a dislocation climb-controlled threshold stress, ca. 6−12 MPa. The binary Al-Zr and ternary Al-Zr-Ti alloys behave similarly under ambient- and high temperature loading, consistent with the similar microstructures of the two alloys. Ph.D. thesis Northwestern University no NU @ keith.knipling @ 1785
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Sudbrack, Chantal K. Decomposition Behavior in Model Ni-Al-Cr-X Superalloys: Temporal Evolution and Compositional Pathways on a Nanoscale Book Whole 2004 209 In model Ni-Al-Cr-X superalloys, the compositional pathways and temporal evolution of coherent [gamma]' (L1[sub:2])-precipitation from an isothermally aged solid-solution, [gamma] (FCC), is investigated at: (i) 600°C, to study nucleation, growth, and coarsening; and (ii) 800°C, to study the influence of W on coarsening of a Ni-Al-Cr alloy. In the quenched Ni-5.2 Al-14.2 Cr at.% alloy, radial distribution functions establish Ni3(Al,Cr)-type short-range ordering that extends 0.6 nm and is Cr depleted. Phase separation at 600°C occurs by nucleation and growth, and the [gamma]'-precipitates’ morphology is a mixture of isolated spheroids and spheroids in various stages of coalescence. Sub-nanometer scale compositional profiles across the [gamma]/[gamma]' interfaces reveal: (i) transient chemical gradients of Al depletion and Cr enrichment adjacent to the precipitates; (ii) trapped Cr atoms in the growing precipitates; (iii) the interfacial width is component dependent; and (iv) increased Al solubility in the [gamma]'-precipitates resulting from capillarity. For a quasi-steady state, the governing power-law time dependencies during coarsening are compared to extant models and discussed in light of recent KMC simulations performed at Northwestern. Independent of the solute diffusivities, the [gamma]/[gamma]' interfacial free-energy is determined from coarsening data to be 22 to 23 mJ m[super:-2]. In Ni-9.8 Al-8.3 Cr at.% and Ni-9.7 Al-8.5 Cr at.%, spheroidal precipitates (5-15 nm diameter) form during quenching. Initially, chemical gradients exist in the [gamma]'-precipitates, however, they disappear after 1 h. After 16 h aging at 800°C, the precipitates have a cuboidal morphology and align along the elastically soft <100>-type directions. Particle size distributions and spatial pair correlation functions evolve temporally, and are discussed in context of the morphological development of the [gamma]'-precipitates. The coarsening kinetics of the mean radius and interfacial area per unit volume obey t1/3 and t–1/3 law, where the addition of W decreases the coarsening rate by a third. The slower kinetics are attributed to W’s influence on elemental partitioning, which leads to stronger partitioning of Al to the [gamma]'-phase and Cr to the &#947;-phase, and to its smaller diffusivity. Finally, an inflection-point method for determining reproducible phase compositions from three-dimensional atom-probe data is described, which is important for determining partitioning ratios. Northwestern University Ph.D. thesis I want to bring to your attention a mistake in my thesis with respect to the units on the compositional rate constant, kappa, which was a caught in the review process of recent submission of mine to Acta Mater. Kappa is obtained by fitting experimental data of the supersaturation del C of a particular phase, to: delC = C(t)- C(eqm)= kappa * t^(-1/3) so the units of kappa are at.% s^(1/3) NOT at.% s^(-1/3). Please make note of this. In my thesis, it applies to Figures: 4.1 (page 89), 4.2 (page 90), 4.3 (page 94) and Table 4.1 (page 95). no NU @ karnesky @ 16
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Gagliano, Michael Scott Co-precipitation of copper and niobium carbide in a low carbon steel Book Whole 2002 218 Co-precipitation of niobium carbide and body-centered cubic (BCC) copper in ferrite was investigated as a high strength, low carbon, chromium-free alternative to conventional high performance structural steels that rely on a tempered martensitic microstructure. Theoretical nucleation and growth rate models for BCC copper and niobium carbide were constructed using calculated thermodynamic driving forces in conjunction with classical theories for the homogeneous nucleation and subsequent growth of coherent, spherical precipitates. The maximum calculated nucleation and growth rates for niobium carbide were found to be 1.0 &times; 10<super>6</super> nuclei/cm<super>3</super>s at 666&deg;C and 1.0 nm/s at 836&deg;C, respectively, for an austenitizing temperature of 1170&deg;C. For BCC copper in ferrite, the maximum calculated nucleation and growth rates were determined to be 8.0 &times; 10<super>15</super> nuclei/cm<super> 3</super>s at 612&deg;C and 0.038 nm/s at 682&deg;C, respectively, for all austenitizing temperatures. Three-dimensional atom probe (3DAP) microscopy revealed the presence of nano-scale BCC copper clusters in approximately the same number density predicted by the theoretical nucleation model. Using an experimentally determined &ldquo;effective&rdquo; activation energy for copper in iron, the normalized theoretical nucleation rate curve compared very well with the normalized hardness response after 5 minutes of aging and effectively described the experimental short-time aging behavior of a low carbon, niobium bearing steel. The size and morphological evolution as well as the growth and coarsening behavior of copper precipitates were investigated through conventional TEM during isothermal direct aging at 550&deg;C for a niobium and niobium-free steel. Although niobium carbide precipitation was not characterized, niobium additions provided increased hardness upon direct aging and showed a much higher resistance to overaging, than a niobium-free steel, for long isothermal aging times. In both steels for aging times up to five hours, both 9R type and BCC copper precipitates were present within the ferrite matrix and the average precipitate size scaled with a time dependence of <italic>t</italic><super> &half;</super>, indicative of diffusion controlled growth. For aging times between 5 and 20 hours, only 9R precipitates were observed with a kinetic exponent of t<super>0.28</super>, representative of a coarsening process. Northwestern University Ph.D. thesis no NU @ karnesky @ 656
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Mulholland, Michael Co-precipitation Kinetic Pathways in a Blast Resistant Steel for Naval Applications Book Whole 2011 150 The nanoscale co-precipitation of Cu and M2C carbides in a high strength, low carbon quenched and tempered steel is characterized in detail along with the mechanical properties and precipitated austenite. Correlations between the nanostructure and the mechanical properties are drawn. The co-precipitation of Cu, M2C (M is any combination of Cr, Mo, or Ti), and austenite (f.c.c.) is characterized for 5 h isochronal aging times by synchrotron x-ray diffraction and 3-D atom-probe tomography for a HSLC steel, BA (BlastAlloy) 160. High number densities, ca. 1023 m-3, of co-located Cu and M2C precipitates were observed. Only small austenite volume percentages (<1.5%) were measured after aging at temperatures up to 625 °C for 5 h. Nanoscale co-precipitation is studied in detail after isothermal aging. Atom-probe tomography is utilized to quantify the co-precipitation of co-located Cu precipitates and M2C (M is any combination of Cr, Mo, Fe, or Ti) carbide strengthening precipitates. Coarsening of Cu precipitates is offset by the nucleation and growth of M2C carbide precipitate, resulting in the maintenance of a yield strength of 1047 ± 7 MPa (152 ±1 ksi) for as long as 320 h of aging time at 450 °C. Impact energies of 153 J (113 ± 6 ft-lbs) and 144 J (106 ± 2 ft-lbs) are measured at - 30 °C and – 60 °C, respectively. The co-location of Cu and M2C precipitates results in non- stationary state coarsening of the Cu precipitates. Synchrotron-source x-ray diffraction studies reveal that the measured 33% increase in impact toughness after aging for 80 h at 450 °C is due to dissolution of cementite, Fe3C, which is the source of carbon for the nucleation and growth of M2C carbide precipitates. Less than 1 volume percent austenite is observed for aging treatments at temperatures less than 600 °C, suggesting that TRIP does not play a significant role in the toughness of specimens aged at temperatures less than 600 °C. Aging treatments at temperatures greater than 600 °C produce more austenite, in the range 2-7%, but at the expense of yield strength. The differences in artifacts associated with voltage-pulsed and laser-pulsed (wavelength = 532 or 355 nm) atom-probe tomographic (APT) analyses of nanoscale precipitation in a high- strength low-carbon steel are assessed using a local-electrode atom-probe (LEAP) tomograph. It is found that the interfacial width of nanoscale Cu precipitates increases with increasing specimen apex temperatures induced by higher laser pulse-energies (0.6-2 nJ pulse-1 at a wavelength of 532 nm). This effect is probably due to surface diffusion of Cu atoms. Increasing the specimen apex temperature by using pulse energies up to 2 nJ pulse-1 at a wavelength of 532 nm is also found to increase the severity of the local magnification effect for nanoscale M2C metal carbide precipitates, which is indicated by a decrease of the local atomic density inside the carbides from 68±6 nm-3 (voltage-pulsing) to as small as 3.5±0.8 nm-3. Methods are proposed to solve these problems based on comparisons with the results obtained from voltage-pulsed APT experiments. Essentially, application of the Cu precipitate compositions and local atomic-density of M2C metal carbide precipitates measured by voltage-pulsed APT to 532 or 355 nm wavelength laser-pulsed data permits correct quantification of precipitation. Based on detailed three-dimensional (3-D) local-electrode atom-probe (LEAP) tomographic measurements of the properties of Cu and M2C precipitates, the yield strength of a high-toughness secondary-hardening steel, BA160, as a function of aging time is predicted using a newly developed 3-D yield strength model. Contributions from each strengthening constituent are evaluated with the model and superposition laws are applied to add each contribution. Prediction of the yield strength entirely based on 3-D microstructural information is thus achieved. The accuracy of the prediction depends on the superposition laws and the LEAP tomographic measurements, especially the mean radius and volume fraction of M2C precipitates. Northwestern University Ph.D. thesis Evanston, IL English no NU @ karnesky @ 11305
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Shashkov, Dmitriy Alexandrovich Atomic-Scale Studies of the Structure and Chemistry of Ceramic/Metal Heterophase Interfaces Book Whole 1997 275 Materials containing metallic and ceramic phases play an important role in modern technology. The structure and chemistry, however, of ceramic/metal interfaces are not well understood. This study, therefore, was conducted to characterize fully the atomic-scale structure and chemistry of metal oxide/metal interfaces produced by internal oxidation of dilute binary and ternary metallic alloys. Ternary alloys were oxidized under conditions whereby only one of the two solute species is oxidized. The remaining solute species segregates to the ceramic precipitate/matrix interfaces. Scanning transmission electron microscopy was used to determine the atomic structure and chemistry of the ceramic/metal interfaces. Electron energy loss and energy-dispersive X-ray spectroscopies were used to determine the chemistry and nature of bonding at the interfaces. Atom-probe microscopy was used to measure quantitatively the chemical composition of the interfaces. Four alloys were internally oxidized: Pd-2.3 at.% Mg, Cu-2.5 at.% Mg, Cu-2.5 at.% Mg-0.8 at.% Ag, and Ag-1.5 at.% Cd-1 at.% Au. MgO precipitates are formed in Pd, Cu and Cu(Ag) matrices and CdO precipitates are formed in a Ag(Au) matrix. In all cases, the precipitates are octahedral-shaped, bound by {222} planes, and maintain a cube-on-cube orientation relationship with the matrix. Atom-probe microscopy revealed a significant level of segregation in both ternary systems. The Gibbsian excess of solute at the interface (&#915;solute) was directly calculated. The values of &#915;solute are (4.0±1.9)×1014 atoms cm-2 for the {222}MgO/Cu(Ag) and (3.0±1.0)×1014 atoms cm-2 for the {222}CdO/Ag(Au) interfaces. Scanning transmission electron microscopy of {222}MgO/Cu(Ag) interfaces showed that the interfaces are semicoherent and contain misfit dislocations spaced at 1.45±0.19 nm. Double and quadruple-height steps were observed. Stand-off misfit dislocations were found at one {111} interplanar distance (0.208 nm) in the metal matrix. It was proven by measurements of the interface separation that the interface is terminated by oxygen ions. Silver enrichment at this interface was detected by Z-contrast imaging and by electron energy loss spectroscopy (EELS). A study of {222}MgO/Cu interfaces by EELS revealed that this interface is terminated by oxygen ions and showed that copper remains metallic at the interface, introducing electronic states inside the band gap of MgO. A study of {222}MgO/Pd interfaces by energy-dispersive X-ray spectroscopy (EDX), however, could not reveal the interfacial chemistry due to experimental limitations. The conditions necessary for the successful interface analysis by EDX are examined. Northwestern University Ph.D. thesis no NU @ karnesky @ 149
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Gorelikov, D. A High-Resolution Pulsed-Laser Atom-Probe Field-Ion Microscope Book Whole 2000 147 Northwestern University Ph.D. thesis no NU @ karnesky @ 150
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