| Records |
Title  |
van Dalen, Marsha E. |
Year |
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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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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NU @ karnesky @ |
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9848 |
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Sebastian, Jason T. |
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Nanoscale Studies of Segregation at Ceramic/Metal Interfaces |
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2004 |
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250 |
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Three-dimensional atom-probe (3DAP) microscopy has been applied to the study of segregation at ceramic/metal interfaces. In particular, the proximity histogram (proxigram) method has been implemented to extract the relative Gibbsian interfacial excess of solute from the 3DAP reconstructions directly, without recourse to external standards. Four systems have been studied—the MgO/Cu(X) (X = Ag or Sb) systems, the CdO/Ag(Au) system, and the MnO/Ag(Sb) system. For all four systems, the relative Gibbsian interfacial excess of solute at the ceramic/metal interface is determined, and trends in this value for the different systems are discussed. The observed trends for segregation imply that the driving force for solute segregation at these ceramic/metal interfaces is not due solely to a release of elastic energy associated with the segregating solute atoms. In the case of the MnO/Ag(Sb) system, the nanoscale temporal evolution of the oxide MnO precipitates as a function of specimen heat treatment is discussed. The observations have important implications for understanding the earliest stages of nucleation and growth of metal oxide precipitates created by internal oxidation. |
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Northwestern University |
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Ph.D. thesis |
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NU @ karnesky @ |
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148 |
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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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2009 |
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Ph.D. Thesis |
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196 |
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NU @ c-booth @ |
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10592 |
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Brothers, Alan Harold |
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Processing and Properties of Advanced Metallic Foams |
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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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NU @ karnesky @ |
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1933 |
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Yoon, Kevin Eylhan |
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Temporal Evolution of the Chemistry and Nanostructure of Multicomponent Model Ni-Based Superalloys |
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2004 |
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189 |
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Ni-based superalloys are critical materials in the aerospace industry because of their excellent balance of mechanical properties including the elevated-temperature strength, which is a result of the dual-phase microstructure, consisting of the Ni-rich [gamma]-matrix (FCC) and [gamma]'-precipitates (L1[sub:2] structure). It is critical to understand the effects of each alloying element on the microstructure of Ni-based superalloys in order to further improve the mechanical properties, which are direct consequences of the microstructure.
Nanoscale chemistry, nanostructure, and temporal evolution of several Ni-based superalloys, ranging from a simple model Ni-Cr-Al ternary alloy to a complex commercial superalloy, René N6, have been investigated utilizing three-dimensional atom probe (3DAP) microscopy and conventional transmission electron microscopy (CTEM).
First, this research demonstrates the power of 3DAP microscopy, which can analyze the chemistry of the complex commercial superalloy, René N6, with nine elements. Concentration profiles and proximity histogram were obtained displaying the partitioning behavior of all alloying elements and especially Re interfacial segregation at the [gamma]/[gamma]' interface.
Next, a model Ni-based superalloy, Ni-Cr-Al alloy, was studied as a reference for the study of a more complex quaternary alloy, Ni-Cr-Al-Re alloy. The temporal evolution of chemistry and nanostructure of the alloy are determined employing 3DAP microscopy. The coarsening kinetics of the [gamma]'-precipitates is examined and compared with theory, which is in partial agreement. A new coarsening mechanism has been suggested which explains the difference between the experimental results and the theoretical predictions. In addition, experimental results are also compared with the results of kinetic Monte Carlo (KMC) simulations.
Finally, the effects of a Re addition on the temporal evolution of chemistry and microstructure of the Ni-Cr-Al alloy and coarsening kinetics of the [gamma]'-precipitates have been investigated employing 3DAP microscopy and CTEM. The Re addition stabilizes the spheroidal morphology of the [gamma]'-precipitates for extended aging times and retards the coarsening kinetics without any Re interfacial segregation. The coarsening kinetics is also compared with the theory. |
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Northwestern University |
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Ph.D. thesis |
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NU @ karnesky @ |
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15 |
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Fuller, Christian B. |
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Temporal Evolution of the Microstructures of Al(Sc,Zr) Alloys and Their Influences on Mechanical Properties |
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2003 |
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179 |
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Al-Sc |
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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. |
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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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147 |
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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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NU @ karnesky @ |
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9840 |
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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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Book Whole |
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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.
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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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