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Title (up) 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)
Abbreviated Journal Book Whole
Issue 2007 Keywords
Place of Publication Language 289
Original Title Al-Sc
Series Title 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. Series Volume
Edition Northwestern University
ISBN Ph.D. thesis Area
Serial Orig Record
no NU @ karnesky @ 9848
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Sebastian, Jason T. Nanoscale Studies of Segregation at Ceramic/Metal Interfaces Book Whole 2004 250 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. Northwestern University Ph.D. thesis no NU @ karnesky @ 148
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Booth-Morrison, Christopher Nanoscale Studies of the Early Stages of Phase Separation in Model Ni-Al-Cr Superalloys Journal Article 2009 Ph.D. Thesis 196 Ph.D. thesis no NU @ c-booth @ 10592
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Brothers, Alan Harold Processing and Properties of Advanced Metallic Foams Book Whole 2006 251 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. Northwestern University Ph.D. thesis no NU @ karnesky @ 1933
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Yoon, Kevin Eylhan Temporal Evolution of the Chemistry and Nanostructure of Multicomponent Model Ni-Based Superalloys Book Whole 2004 189 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. Northwestern University Ph.D. thesis no NU @ karnesky @ 15
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Fuller, Christian B. Temporal Evolution of the Microstructures of Al(Sc,Zr) Alloys and Their Influences on Mechanical Properties Book Whole 2003 179 Al-Sc 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. Northwestern University Ph.D. thesis no NU @ karnesky @ 147
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Amouyal, Yaron Thermodynamics and kinetics of grain boundaries in ultra fine grained copper produced by severe plastic deformation Book Whole 2007 148 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. Senate of the Technion – Israel Institute of Technology Ph.D. thesis no NU @ karnesky @ 9840
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Gerstl, Stephan S. A. Three-Dimensional Nanometer Scale Analyses of Precipitate Structures and Local Compositions in TiAl Engineering Alloys Book Whole 2006 231 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. Northwestern University Ph.D. thesis no NU @ karnesky @ 803
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