| Records |
| Title |
Yoon, Kevin Eylhan |
Year |
Temporal Evolution of the Chemistry and Nanostructure of Multicomponent Model Ni-Based Superalloys |
Abbreviated Journal |
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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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NU @ karnesky @ |
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Sudbrack, Chantal K. |
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Decomposition Behavior in Model Ni-Al-Cr-X Superalloys: Temporal Evolution and Compositional Pathways on a Nanoscale |
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2004 |
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209 |
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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 γ-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. |
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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). |
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NU @ karnesky @ |
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16 |
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Sebastian, Jason T. |
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Nanoscale Studies of Segregation at Ceramic/Metal Interfaces |
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Book Whole |
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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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no |
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NU @ karnesky @ |
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148 |
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Shashkov, Dmitriy Alexandrovich |
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Atomic-Scale Studies of the Structure and Chemistry of Ceramic/Metal Heterophase Interfaces |
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1997 |
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275 |
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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 (Γsolute) was directly calculated. The values of Γsolute 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. |
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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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149 |
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Gorelikov, D. |
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A High-Resolution Pulsed-Laser Atom-Probe Field-Ion Microscope |
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Book Whole |
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2000 |
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147 |
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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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150 |
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Gagliano, Michael Scott |
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Co-precipitation of copper and niobium carbide in a low carbon steel |
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Book Whole |
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2002 |
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218 |
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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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no |
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NU @ karnesky @ |
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656 |
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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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no |
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NU @ karnesky @ |
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803 |
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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
4
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 |
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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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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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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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196 |
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Ph.D. thesis |
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NU @ c-booth @ |
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10592 |
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