Journal Papers by Charles H Henager
The recession rates for 10<sup>--6</sup>m thick C interfaces in chemical vapor infiltrated SiC re... more The recession rates for 10<sup>--6</sup>m thick C interfaces in chemical vapor infiltrated SiC reinforced with Nicalon fibers were calculated from thermogravimetric data, assuming all of the mass losses were due to C oxidation, and found to be consistent with the measured recession distances of the C interface, which were surprisingly uniform across the composite. Agreement between the two approaches for a microstructurally complex material indicates thermogravimetric analysis could be an important tool for understanding environmental effects in ceramic composites with reactive interfaces. Mass losses were linear within the first 1.08×10<sup>4</sup> s to 2.16×10<sup>4</sup> s between 1073 and 1373 K and between 3.1×10<sup>2</sup> and 2.5×10<sup>3</sup> Pa O<sub>2</sub>. Calculated reaction orders with respect to O<sub>2</sub> were between 0.5 and 1.0 at 1373 K, and activation energies were about 50 kJ.mol<sup>-1</sup>. Analysis of the kinetic data and estimates of gas boundary layer thickness suggest the mechanism for the C-interface oxidation involved reaction control, but the possibility of diffusion control for some conditions cannot be ruled out.
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A novel technique for measuring interphase recession in ceramic-matrix composites (CMCs) due to o... more A novel technique for measuring interphase recession in ceramic-matrix composites (CMCs) due to oxidation is described. The technique involves fiber push-in testing and analysis of the load-displacement curves. Fiber push-in tests were conducted on carbon-coated Hi-Nicalon SiC fibers in a CVI SiC matrix, where the carbon interphase had recessed due to oxidation. Estimates of interphase recession distances from analysis of fiber push-in tests are in reasonable agreement with measurements made by optical microscopy. Besides measuring the recession distance, the fiber push-in test can be used to investigate environmental effects on fiber bridging.
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High-temperature exposures of SiC/SiC composites to oxidizing environments can lead to substantia... more High-temperature exposures of SiC/SiC composites to oxidizing environments can lead to substantial changes in mechanical behavior. Results from flexure and crack growth experiments are used to demonstrate such effects. Flexure tests of graphite-coated Nicalon-reinforced SiC previously oxidized in air at 950°C revealed that degradation of fracture resistance began after very short exposure times (less than 1 h) and could be described in terms of distinct oxidation effects on strength and fiber pullout. Crack velocities were determined as a function of applied stress intensity and time for varying O<sub>2</sub> levels. It was observed that crack velocities increased at 1,100°C in the presence of oxygen, which also shifted the onset of stage III (power law) growth to lower values of applied stress intensity. The crack growth observations were described using a two-dimensional micro-mechanical model developed to simulate cracks bridged by continuous fibers. Fiber creep relaxation predicted the correct crack velocity and time-dependence in argon, but other mechanisms, such as interface removal, are required to explain the data in Ar+O<sub>2</sub>.
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Although matrix creep accommodation occurs for a variety of diffusion controlled phase transforma... more Although matrix creep accommodation occurs for a variety of diffusion controlled phase transformations, the focus is usually on the diffusion kinetics. However, interface control of kinetics, associated with the development of elastic stresses at a growing interface, is also possible. The displacement reaction Fe+Cu<sub>2</sub>O→FeO+Cu is used as a model system to determine the role of such stresses. The reaction is shown to occur in a regime where the metal matrix phase undergoes either Nabarro-Herring or power law creep. The models developed herein to describe the interface and matrix stresses are tested on several diffusional phase transformations. The general results are that matrix stresses very near growing precipitates are often appreciable. The results have implications for the control of microstructure in in situ composites produced by such reactions.
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The ability to use HVEM techniques to examine and evaluate the microdeformation characteristics o... more The ability to use HVEM techniques to examine and evaluate the microdeformation characteristics of Alloy 600 was documented. Macroscopic deformation was accommodated by localized deformation within planar dislocation arrays. Grain boundary carbides were identified as the primary dislocation source, activated at lower macroscopic stresses than other sources (e.g. grain boundary triple points, matrix precipitates, etc.). Most dislocation movement during macroscopic deformation was confined to these planar arrays. At a sufficient stress level, cracks were initiated and propagated along these arrays. Microdeformation characteristics may have significant implications on mechanism(s) controlling IGSCC of the Alloy 600 steam generator tubing. It is suggested that grain boundary microdeformation characteristics and microchemistry are the essential components specifying the relative susceptibility of Alloy 600 to IGSCC in primary-water and certain caustic environments.
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Synthesis of SiC-reinforced ternary phase Ti<inf>3</inf>SiC<inf>2</inf>-matrix composites is show... more Synthesis of SiC-reinforced ternary phase Ti<inf>3</inf>SiC<inf>2</inf>-matrix composites is shown to be readily accomplished in solid state displacement reactions between Si and TiC. The Ti<inf>3</inf>SiC<inf>2</inf>-matrix composite reinforced with SiC had a fracture toughness of 9.1 MPa√m and a hardness of 9 GPa, both of which are higher than values for unreinforced Ti<inf>3</inf>SiC<inf>2</inf>. Although evidence for true plastic deformation in the Ti<inf>3</inf>SiC<inf>2</inf>-matrix was not observed, the layered structure apparently promoted the formation of many crack-bridging ligaments during crack propagation. Reinforcing TiSi<inf>2</inf> with SiC particles increased the fracture toughness relative to that of unreinforced TiSi<inf>2</inf> more than two-fold to 4.2 MPa√m and the hardness from 8.5 to 12 GPa.
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The use of contact tension specimen for measuring slow crack growth was studied by comparing the ... more The use of contact tension specimen for measuring slow crack growth was studied by comparing the data obtained in single edge notched bar tests. The mechanism of subcritical crack growth driven by relaxation of crack-bridging tractions due to fiber creep controls the crack growth rate in the specimen. Displacement rates were determined as a function of temperature in argon and used to calculate effective crack velocities and activation energies for crack growth in pure argon.
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Nicalon-CG and Hi-Nicalon fibers were characterized by measuring their density and tensile streng... more Nicalon-CG and Hi-Nicalon fibers were characterized by measuring their density and tensile strength in the unirradiated, thermal annealed, and irradiated conditions. The results indicate the fibers that perform best after irradiation to 43 dpa SiC at 1000°C are those that approach stoichiometric and crystalline SiC. Hi-Nicalon fiber exhibited less than 1% densification, accompanied by a slight increase in tensile strength after irradiation. Nicalon-CG, in contrast, was significantly weakened in the annealed and irradiated conditions. In addition, Nicalon-CG exhibited substantial irradiation-induced shrinkage. Loss of fiber tensile strength after irradiation is shown to reduce the flexural strength of irradiated composites while fiber shrinkage, and resultant debonding from the matrix, are linked to a reduced composite elastic modulus.
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—A dynamic crack-growth model has been developed to predict slow crack growth in ceramic composit... more —A dynamic crack-growth model has been developed to predict slow crack growth in ceramic composites containing nonlinear, creeping fibers in an elastic matrix. Mechanics for frictional bridging and nonlinear fiber-creep equations are used to compute crack extension dynamically. Discrete, two-dimensional fiber bridges are employed, which allows separate bridge " clocks " , to compute slow crack-growth rates for composites containing Nicalon-CG and Hi-Nicalon fibers. Predictions for activation energies, time-temperature exponents, crack lengths, and crack-velocity data for composites in bending at 1173 K to 1473 K in inert environments are in good agreement with experimental data. In addition, calculated creep strains in the bridges agree with experimental damage-zone strains. The implications of multiple-matrix cracking are discussed .
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Displacement reactions can produce in situ intermetallic and ceramic matrix composites in a proce... more Displacement reactions can produce in situ intermetallic and ceramic matrix composites in a process where an intermetallic or ceramic phase(s) and a potential reinforcing phase(s) are grown together during a reactive phase transformation. Various forms of interpenetrating-phase and dispersed-phase microstructures are produced by means of these reactions. It is also apparent that both composition and morphology can be manipulated to some degree in order to tailor composite structures. The composition and morphology of MoSi 2 reinforced with SiC particles was explored over a wide range by controlling starting reactant compositions and hot-pressing conditions. Preliminary results of a model for the formation of the MoSi2/SiC composite are presented in which both diffusion and interracial reactions are included. Strength in bending and chevron-notch fracture toughness were determined as a function of temperature and composition and the measured properties are discussed with regard to the observed microstructures. A novel, graded composite structure in the NiA1/Ni3A1/Ni:AI:O 3 system is also discussed.
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Density functional theory (DFT) is used to calculate the thermodynamic and kinetic properties of ... more Density functional theory (DFT) is used to calculate the thermodynamic and kinetic properties of trans-mutant Mg in 3C–SiC due to high-energy neutron irradiation associated with the fusion nuclear environment. The formation and binding energies of intrinsic defects, Mg-related defects, and clusters in 3C–SiC are systematically calculated. The minimum energy paths and activation energies during point defect migration and small cluster evolution are studied using a generalized solid-state nudged elastic band (G-SSNEB) method with DFT energy calculations. Stable defect structures and possible defect migration mechanisms are identified. The evolution of binding energies during Mg 2 Si formation demonstrates that the formation of Mg 2 Si needs to overcome a critical nucleus size and nucleation barrier. It is found that C vacancies promote the formation of the Mg 2 Si nucleus, and formation of which results in a compressive stress field around the nucleus. These data are important inputs in meso-and macro-scale modeling and experiments to understand and predict the impact of Mg on phase stability, microstructure evolution, and performance of SiC and SiC-based materials during long-term neutron exposures. Published by Elsevier B.V.
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Molecular dynamics (MD) simulations were employed with empirical potentials to study the effects ... more Molecular dynamics (MD) simulations were employed with empirical potentials to study the effects of multilayer interfaces and interface spacing in Al–Ti nanolayers. Several model interfaces derived from stacking of close-packed layers or face-centered cubic {1 0 0} layers were investigated. The simulations reveal significant and important asymmetries in defect production with $60% of vacancies created in Al layers compared to Ti layers within the Al–Ti multilayer system. The asymmetry in the creation of interstitials is even more pronounced. The asymmetries cause an imbalance in the ratio of vacancies and interstitials in films of dissimilar materials leading to >90% of the surviving interstitials located in the Al layers. While in the close-packed nanolayers the interstitials migrate to the atomic layers adjacent to the interface of the Al layers, in the {1 0 0} nanolayers the interstitials migrate to the center of the Al layers and away from the interfaces. The degree of asymmetry and defect ratio imbalance increases as the layer spacing decreases in the multilayer films. Underlying physical processes are discussed including the interfacial strain fields and the individual elemental layer stopping power in nanolayered systems. In addition, experimental work was performed on low-dose (10 16 atoms/cm 2) helium (He) irradiation on Al/Ti nanolayers (5 nm per film), resulting in He bubble formation $1 nm in diameter in the Ti film near the interface. The correlation between the preferential flux of displaced atoms from Ti films to Al films during the defect production that is revealed in the simulations and the morphology and location of He bubbles from the experiments is discussed. Published by Elsevier B.V.
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Advanced characterization tools, such as electron backscatter diffraction and transmitted IR micr... more Advanced characterization tools, such as electron backscatter diffraction and transmitted IR microscopy, are being applied to study critical microstructural features and orientation relations in as-grown CZT crystals to aid in understanding the relation between structure and properties in radiation detectors. Even carefully prepared single crystals of CZT contain regions of slight misorientation, Te-particles, and dislocation networks that must be understood for more accurate models of detector response. This paper describes initial research at PNNL into the hierarchy of microstructures observed in CZT grown via the vertical gradient freeze or vertical Bridgman method at PNNL and WSU.
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SiC/SiC composites used in fusion reactor applications are subjected to high heat fluxes and requ... more SiC/SiC composites used in fusion reactor applications are subjected to high heat fluxes and require knowledge and tailoring of their in-service thermal conductivity. Accurately predicting the thermal conductivity of SiC/SiC composites as a function of temperature will guide the design of these materials for their intended use, which will eventually include the effects of 14-MeV neutron irradiations. This paper applies an Eshelby–Mori–Tanaka approach (EMTA) to compute the thermal conductivity of unirradiated SiC/SiC composites. The homogenization procedure includes three steps. In the first step EMTA computes the homogenized thermal conductivity of the unidirectional (UD) SiC fiber embraced by its coating layer. The second step computes the thermal conductivity of the UD composite formed by the equivalent SiC fibers embedded in a SiC matrix, and finally the thermal conductivity of the as-formed SiC/SiC composite is obtained by averaging the solution for the UD composite over all possible fiber orientations using the second-order fiber orientation tensor. The EMTA predictions for the transverse thermal conductivity of several types of SiC/SiC composites with different fiber types and interfaces are compared to the predicted and experimental results by Youngblood et al.
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This article appeared in a journal published by Elsevier. The attached copy is furnished to the a... more This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/authorsrights Abstract There is increased interest in improved methods for in situ non-destructive interrogation of materials for nuclear reactors in order to ensure reactor safety and quantify material degradation (particularly embrittlement) prior to failure. Therefore, a prototypical ferritic/ martensitic alloy, HT-9, of interest to the nuclear materials community was investigated to assess microstructure effects on micromag-netics measurements (Barkhausen noise emission, magnetic hysteresis measurements, and first order reversal curve analysis) for samples undergoing three different heat treatments. Microstructural and physical measurements consisted of high precision density, resonant ultrasound elastic constant, Vickers microhardness, grain size, and texture determination. These were varied in the HT-9 alloy samples and related to various magnetic signatures. In parallel, a mesoscale microstructure model was created for a-iron and the effects of poly-crystallinity and the demagnetization factor were explored. It was observed that Barkhausen noise emission decreased with increasing hardness and decreasing grain size (lath spacing), while coercivity increased. The results are discussed in terms of the use of magnetic signatures for the non-destructive interrogation of radiation damage and other microstructural changes in ferritic/martensitic alloys.
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Atomistic simulations of CdTe using a Stillinger-Weber SW interatomic potential were undertaken t... more Atomistic simulations of CdTe using a Stillinger-Weber SW interatomic potential were undertaken to model the solid-liquid phase equilibria of this important compound semiconductor. Although this potential has been used by others to study liquid CdTe and vapor-liquid interface, it is based on fitting parameters optimized only for the zincblende solid. It has not been fully explored as a potential for solid-liquid phase equilibria until this work. This research reports an accurate determination of the melting temperature, T M = 1305 K near P = 0, the heat of fusion at melting, and on the relative phase densities with a particular emphasis on the melting line. The SW potential for CdTe predicts a liquid with a density slightly less than that of the solid and, hence, the pressure-temperature melting line has a positive slope. The pair-correlation structure of the liquid is determined and favorably compared to neutron-scattering data and to ab initio simulations. The liquid-solid interface is discussed using density profiles and a short-range order parameter for models having principal orientations along 100, 110, and 111 crystallographic directions.
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The growth-tip region of a high-purity 4.2-cm-diameter Ge boule grown using low-pressure Bridgman... more The growth-tip region of a high-purity 4.2-cm-diameter Ge boule grown using low-pressure Bridgman methods in a vertical gradient freeze furnace was sectioned and polished in preparation for scanning electron microscopy and was characterized using electron backscatter diffraction (EBSD). The boule had a characteristic conical tip region with cone angle of 401 of a right circular cylinder from which a section was taken along the boule longitudinal centerline with an approximate surface area of 4 cm 2. The majority of this surface area was characterized using EBSD and an image collage was assembled for the tip region. The grain structure, grain boundary orientation, twin structure, and overall crystal growth direction were determined. A crystal growth direction of approximately /11 2S was observed, which was also identified as the growth direction of several prominent twins observed in the tip region. The grain structure of the tip region appeared to be controlled by the sidewall nucleation of a stray grain that competed for dominance during growth. Grain boundaries and triple grain junctions were identified as low-energy coincident-site-lattice (CSL) boundaries and junctions of the S3 and S9 types.
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This article appeared in a journal published by Elsevier. The attached copy is furnished to the a... more This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright a b s t r a c t SiC is a candidate for nuclear applications at elevated temperatures but has not been fully studied under typical light-water reactor operating conditions, such as moderate temperatures and high pressures. Coupons of high-purity chemical vapor deposited SiC were exposed to deoxygenated, pressurized water at 573 K and 10 MPa for up to 5400 h. Ceramographic examination of the exposed SiC surfaces revealed both embryonic and large, d > 300 lm, pits on the surface after initial exposure for 4000 h. The pits were characterized using scanning electron microscopy for structure and chemistry analysis. Pit densities were also determined by standard counting methods. The chemical analysis revealed that the pits are associated with the formation of silica and subsequent loss of Si, which is expected due to several suggested reactions between SiC and water. Subsequent exposure under nominally identical water chemistry conditions for an additional 1400 h removed the pits and the samples exhibited general corrosion with measurable loss of Si from the surface.
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A dynamic crack-growth model using discrete, two-dimensional fiber bridges developed for ceramic ... more A dynamic crack-growth model using discrete, two-dimensional fiber bridges developed for ceramic composites containing nonlinear, creeping fibers in an elastic matrix is used to develop a crack growth mechanism map. In addition to nonlinear creep, fiber oxidation and fiber/matrix interphase oxidation are treated and discussed. The model aids in the development of a crack-growth mechanism map based on available experimental crack growth data as a function of temperature and oxygen concentration and in terms of proposed crack-growth mechanisms; fiber relaxation (FR), interface removal (IR), viscous sliding (VS), oxidation embrittlement (OE), and fiber stress rupture (SR). Transitions between the various mechanisms are identified and discussed.
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Journal Papers by Charles H Henager