| Records |
| Title |
Gagliano, Michael Scott |
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Co-precipitation of copper and niobium carbide in a low carbon steel |
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2002 |
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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 × 10<super>6</super> nuclei/cm<super>3</super>s at 666°C and 1.0 nm/s at 836°C, respectively, for an austenitizing temperature of 1170°C. For BCC copper in ferrite, the maximum calculated nucleation and growth rates were determined to be 8.0 × 10<super>15</super> nuclei/cm<super> 3</super>s at 612°C and 0.038 nm/s at 682°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 “effective” 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°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> ½</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. |
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Northwestern University |
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Ph.D. thesis |
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Gerstl, Stephan S. A. |
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Three-Dimensional Nanometer Scale Analyses of Precipitate Structures and Local Compositions in TiAl Engineering Alloys |
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2006 |
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231 |
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Titanium aluminide (TiAl) alloys are among the fastest developing class of
materials for use in high temperature structural applications. Their low density
and high strength make them excellent candidates for both engine and airframe
applications. Creep properties of TiAl alloys, however, have been a limiting factor
in applying the material to a larger commercial market.
In this research, nanometer scale compositional and structural analyses of
several TiAl alloys, ranging from model Ti-Al-C ternary alloys to putative
commercial alloys with 10 components are investigated utilizing threedimensional
atom probe (3DAP) and transmission electron microscopies.
Nanometer sized borides, silicides, and carbide precipitates are involved in
strengthening TiAl alloys, however, chemical partitioning measurements reveal
oxygen concentrations up to 14 at. % within the precipitate phases, resulting in the
realization of oxycarbide formation contributing to the precipitation strengthening
of TiAl alloys.
iv
The local compositions of lamellar microstructures and a variety of
precipitates in the TiAl system, including boride, silicide, binary carbides, and
intermetallic carbides are investigated. Chemical partitioning of the microalloying
elements between the a2/g lamellar phases, and the precipitate/g–matrix phases are
determined. Both W and Hf have been shown to exhibit a near interfacial excess
of 0.26 and 0.35 atoms nm-2 respectively within ca. 7 nm of lamellar interfaces in a
complex TiAl alloy. In the case of needle-shaped perovskite Ti3AlC carbide
precipitates, periodic domain boundaries are observed 5.3±0.8 nm apart along their
growth axis parallel to the TiAl[001] crystallographic direction with concomitant
composition variations after 24 hrs. at 800°C. |
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Northwestern University |
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Ph.D. thesis |
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NU @ karnesky @ |
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803 |
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Knipling, Keith E |
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Development of a Nanoscale Precipitation-Strengthened Creep-Resistant Aluminum Alloy Containing Trialuminide Precipitates |
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2006 |
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230 |
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Al-Zr |
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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. |
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Ph.D. thesis |
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Northwestern University |
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NU @ keith.knipling @ |
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1785 |
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Brothers, Alan Harold |
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Processing and Properties of Advanced Metallic Foams |
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Book Whole |
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2006 |
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251 |
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Since the development of the first aluminum foams in the middle of the 20th century [178],
great advances have been made in the processing and fundamental understanding of metallic
foams. As a result of these advances, metallic foams are now penetrating a number of applications
where their unique suite of properties makes them superior to solid materials, such as lightweight
structures, packaging and impact protection, and filtration and catalysis [3]. The purpose of this
work is to extend the use of metallic foams in such applications by expanding their processing
to include more sophisticated base alloys and architectures.
The first four chapters discuss replacement of conventional crystalline metal foams with
ones made from high-strength, low-melting amorphous metals, a substitution that offers potential
for achieving mechanical properties superior to those of the best crystalline metal foams,
without sacrificing the simplicity of processing methods made for low-melting crystalline alloys.
Three different amorphous metal foams are developed in these chapters, and their structures
and properties characterized. It is shown for the first time that amorphous metal foams, due to
stabilization of shear bands during bending of their small strut-like features, are capable of compressive
ductility comparable to that of ductile crystalline metal foams. A two-fold improvement
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in mechanical energy absorption relative to crystalline aluminum foams is shown experimentally
to result from this stabilization.
The last two chapters discuss modifications in foam processing that are designed to introduce
controllable and continuous gradients in local foam density, which should improve mass efficiency
by mimicking the optimized structures found in natural cellular materials [64], as well as facilitate
the bonding and joining of foams with solid materials in higher-order structures. Two
new processing methods are developed, one based on replication of nonuniformly-compressed
polymer precursors, and the other based on nonuniform chemical milling of uniform foams, and
each method is demonstrated through the production of low-density aluminum foams having
simple model density gradients. |
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Northwestern University |
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Ph.D. thesis |
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no |
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NU @ karnesky @ |
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1933 |
| Permanent link to this record |
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Amouyal, Yaron |
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Thermodynamics and kinetics of grain boundaries in ultra fine grained copper produced by severe plastic deformation |
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Book Whole |
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2007 |
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148 |
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Reducing the average grain size of polycrystalline metals and alloys is a
traditional way of increasing their strength. Moreover, many other attractive properties
can be achieved by reducing average grain size: low-temperature superplasticity,
improved magnetic properties, and homogeneity of physical properties. The recently
developed technique of Equal Channel Angular Pressing (ECAP) allowed a
breakthrough in decreasing the grain size of bulk materials to the sub-micrometer level.
Its main principle is pressing a metal billet through an angular channel, a process that
involves extremely large shear deformations forming dislocation cell structure at submicron
scale. Subsequent pressings result in the formation of ultra-fine grains (UFG)
with high-angle grain boundaries (GBs).
Many unusual properties of materials produced by ECAP are attributed to nonequilibrium
grain boundaries. These GBs are expected to exhibit higher values of
energy, higher amplitude of strain fields, larger free volume, and higher diffusivity
than their relaxed counterparts. Although the concept of non-equilibrium state of GBs
is theoretically well established, its experimental foundation is still controversial. The
aim of the present study is, therefore, providing an adequate experimental proof for the
concept of non-equilibrium GBs by measurements of GB diffusivity and energy in
copper and copper alloys subjected to ECAP.
The diffusion of 63Ni radiotracer in Cu and Cu-Zr alloy was studied using the
serial-sectioning method. The diffusion annealings were performed in the temperature
range 150 °C – 350 °C for annealing times when volume diffusion is frozen and only
short-circuit diffusion occurs.
The microstructure studies by Transmission Electron Microscopy (TEM),
Atomic Force Microscopy (AFM), and Focused Ion Beam (FIB) microscopy indicated
that alloying with Zr is essential for stabilizing the ECAP-processed alloys against
grain growth and recrystallization. In all samples studied the experimentally-acquired
diffusion profiles exhibited two distinct slopes, which are associated with "slow"- and
"fast" diffusion paths. The former is very close to that of relaxed GBs in coarse-grained
Cu. Based on the analysis of the activity profiles, we proposed a hierarchical
microstructure model of the UFG Cu-Zr alloy studied. In this model, a cellular skeleton
of "fast" GBs with the characteristic cell size in the micrometer range is embedded in a
network of "slow" GBs formed by sub-micrometer grains. This model allowed a
quantitative processing of the measured activity profiles. The Arrhenius parameters of
the GB diffusivities for the "slow" and "fast" GBs were determined, indicating a 3-4
orders of magnitude difference in respective pre-exponential factors.
The measured radiotracer penetration profiles in pure ECAP-ed Cu exhibited a
bimodal shape similar to that observed in the Cu-Zr alloy. In contrast to the Cu-Zr
alloy, the pure Cu exhibited recrystallization during all thermal annealings. The
explicit expression describing the kinetics of recrystallization in ECAP-ed Cu was
obtained. A model that considers diffusion in UFG polycrystal undergoing
recrystallization was developed. Its main assumption is that diffusion flux is allowed in
the UFG phase only, while the recrystallizing grains "freeze" the concentration of
solutes existing in the UFG matrix before it was consumed by recrystallizing grain.
Application of this model enabled us deriving the slow-diffusion coefficients from the
experimentally measured penetration profiles. The Arrhenius parameters of the GB
diffusivities for the "slow" and "fast" GBs were determined, indicating about 3 orders
of magnitude difference in respective pre-exponential factors.
The relative energies of GBs in ultrafine grain copper obtained by ECAP were
determined using the thermal grooving technique. The dihedral angles at the roots of
GB grooves formed after annealings at 400 °C for 15 min and at 800 °C for 2h were
determined with the aid of AFM. The average relative GB energies in the ECAP-ed
samples annealed at 400 and 800 °C are 0.48 ± 0.11 and 0.27 ± 0.07 , respectively.
Theoretical estimates of the relaxation time of non-equilibrium GBs indicated that little
relaxation should occur after annealing at 400 °C, while full relaxation is expected
after annealing at 800 °C. It was shown that the measured difference in GB energies
can be correlated with the presence of two types of GBs in the same sample exhibiting
very different diffusivities.
We associated the fast-diffusion paths with unusually high GB diffusivities, and
the high-energy GBs observed by AFM with the non-equilibrium GBs that were
formed during ECAP. The volume fraction of such boundaries is small and they are
separated by an extensive network of normal (i.e. exhibiting usual GB diffusivities and
energies characteristic for annealed coarse grain polycrystals) GBs. These findings
provide a solid experimental foundation for the concept of non-equilibrium GBs. |
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Senate of the Technion – Israel Institute of Technology |
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Ph.D. thesis |
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no |
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NU @ karnesky @ |
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9840 |
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van Dalen, Marsha E. |
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Microstructure and Creep Properties of Al-Sc Alloys Micro-alloyed with Lanthanides (Yb or Gd) and Transition Metals (Ti or Zr) |
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Book Whole |
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2007 |
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289 |
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Al-Sc |
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This thesis examines the effects of micro-alloying additions to Al-Sc alloys on the
microstructure, coarsening resistance and creep properties. The overarching goal of this research
is to develop castable, creep-resistant aluminum alloys which can be used at temperatures in
excess of 300°C. Successful high-temperature application of aluminum based alloys offers a
lower cost and lower weight alternative to other materials commonly used at high temperatures,
including titanium- and nickel-based alloys.
To this end, this aims to improve the properties of the Al alloys by adding various
alloying elements in small quantities, on the order of several hundred atomic parts per million, to
aluminum. The thesis begins by focusing on additions of Ti to Al-Sc. Ti is a slow diffuser in Al
[1], and it will be shown that it improves the coarsening kinetics of the precipitate phase at
300°C. Since these alloys are coarsening resistant, it is found that they can be aged and crept at
temperatures of up to 425°C. The properties displayed are similar to those of Al-Sc-Zr alloys
studied previously [2, 3].
The examination of Ti additions is followed by a study of the additions of lanthanide
elements. These elements are of interest since they are known to increase the lattice parameter of
the precipitate phase [4-8], which could potentially lead to improved creep resistance [9].
Initially, binary Al-Yb alloys are studied to obtain some fundamental knowledge of the behavior
of Yb in Al. Subsequent additions of Yb to Al-Sc result in improved creep resistance. A similar
improved creep resistance is observed for additions of Gd to Al-Sc.
Finally, this dissertation concludes with the study of Al-Sc-Yb-Zr alloys. Since the goal
of this research is to obtain a creep-resistant as well as coarsening resistant alloy, both a slow
diffusing element (Zr) and an element which improves the creep resistance (Yb) are added. The
quaternary alloys are found to maintain the creep resistance and coarsening resistance of the Al-
Sc-Yb and Al-Sc-Zr alloys, respectively, which points to opportunities for future research in this
area. |
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Northwestern University |
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Ph.D. thesis |
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no |
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NU @ karnesky @ |
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9848 |
| Permanent link to this record |
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Kolli, R. Prakash |
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Kinetics of nanoscale Cu-rich precipitates in a multicomponent concentrated steel |
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Book Whole |
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2007 |
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320 |
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Kinetics; Cu; precipitate; steel |
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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. |
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Ph.D. thesis |
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Northwestern University |
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no |
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NU @ m-krug @ |
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10472 |
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Booth-Morrison, Christopher |
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Nanoscale Studies of the Early Stages of Phase Separation in Model Ni-Al-Cr Superalloys |
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Journal Article |
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2009 |
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Ph.D. Thesis |
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Ph.D. thesis |
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no |
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NU @ c-booth @ |
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10592 |
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