| Records |
| Title |
Yoon, Kevin Eylhan |
Year |
Temporal Evolution of the Chemistry and Nanostructure of Multicomponent Model Ni-Based Superalloys |
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2004 |
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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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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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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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NU @ karnesky @ |
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147 |
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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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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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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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2000 |
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147 |
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Northwestern University |
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Marquis, Emmanuelle A. |
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Microstructural Evolution and Strengthening Mechanisms in Al-Sc and Al-Mg-Sc Alloys |
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2002 |
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223 |
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Al-Sc alloys are potential candidates for structural industrial applications because of their excellent mechanical properties due to the presence of small, coherent Al3Sc (L12 structure) precipitates that are formed during aging. Additional Mg alloying not only enhances the mechanical properties by solid-solution hardening but also provides corrosion resistance and better weldability. Understanding and controlling the microstructure, i.e. the temporal evolution of the Al3Sc precipitate morphologies and the effects of other alloying elements such as Mg, are critical for optimizing mechanical properties.
First, this research aims at describing the microstructural evolution of the Al3Sc precipitates during aging using transmission electron microscopies. The effects of Mg additions on precipitation are described using high-resolution transmission electron microscopy observations and three-dimensional atom-probe microscopy analyses.Results on Mg segregation, on the nanoscale level, at the coherent Al / Al3Sc interface are presented.
A second goal of this research is to understand the precipitation-strengthening parameters controlling optimal yield strength at room temperature and creep resistance at elevated temperature (0.6 Tm), the effects of precipitate size and volume fraction upon yield and creep strengths of dilute Al-Sc and Al-Mg-Sc alloys are studied. Room temperature strength is described in terms of precipitate shearing and Orowan dislocation looping. Creep threshold stresses are found to be about ten times lower than the yield stresses at 300°C, indicative of a climb-controlled bypass mechanism, which is modeled by extending existing dislocation-particle interaction models. |
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Northwestern University |
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NU @ karnesky @ |
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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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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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NU @ karnesky @ |
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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.
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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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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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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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Northwestern University |
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