- Since joining Pacific Northwest National Laboratory in 1976, Dr. Henager has worked in a variety of materials science... moreSince joining Pacific Northwest National Laboratory in 1976, Dr. Henager has worked in a variety of materials science areas, concentrating on radiation effects, mechanical properties and strength of materials, and computational materials science of interfaces and deformation in metals and alloys. He is currently Team Lead for the Structural Materials and Characterization Team in the Reactor Materials and Mechanical Design Group within the Nuclear Sciences Division of the Energy and Environment Directorate.(Since joining Pacific Northwest National Laboratory in 1976, Dr. Henager has worked in a variety of materials science areas, concentrating on radiation effects, mechanical properties and strength of materials, and computational materials science of interfaces and deformation in metals and alloys. He is currently Team Lead for the Structural Materials and Characterization Team in the Reactor Materials and Mechanical Design Group within the Nuclear Sciences Division of the Energy and Environment Directorate.)edit
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... 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.
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... 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.
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... 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>.
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... 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.
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... 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.
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... 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.
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... 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.
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... 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.
—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... 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 .
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.... 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.
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... 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.
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... 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.
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... 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.
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... 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.
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... 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.
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... 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.
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... 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.
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... 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.
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... 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.
Silicon carbide (SiC)-based ceramic composites have been studied for fusion applications for more than a decade. The potential for these materials have been widely discussed and is now understood to be (1) the ability to operate in... more
Silicon carbide (SiC)-based ceramic composites have been studied for fusion applications for more than a decade. The potential for these materials have been widely discussed and is now understood to be (1) the ability to operate in temperature regimes much higher than for metallic alloys, (2) an inherent low level of long-lived radioisotopes that reduces the radiological burden of the structure, and (3) perceived tolerance against neutron irradiation up to high temperatures. This paper reviews the recent progress in development, characterization, and irradiation effect studies for SiC composites for fusion energy applications. It also makes the case that SiC composites are progressing from the stage of potential viability and proof-of-principle to one where they are ready for system demonstration, i.e., for flow channel inserts in Pb–Li blankets. Finally, remaining general and specific technical issues for SiC composite development for fusion applications are identified.
SiC continuous-fiber composites are considered for nuclear applications but concern has centered on the differential materials response of the fiber, fiber/matrix interphase (fiber coating), and matrix. In our study, a continuous-fiber... more
SiC continuous-fiber composites are considered for nuclear applications but concern has centered on the differential materials response of the fiber, fiber/matrix interphase (fiber coating), and matrix. In our study, a continuous-fiber composite is simulated by four concentric cylinders to explore the magnitude of radial stresses when irradiation swelling of the various components is incorporated. The outputs of this model were input into a time-dependent crack-bridging model to predict crack growth rates in an environment where thermal and irradiation creep of SiC-based fibers is considered. Under assumed Coulomb friction the fiber–matrix sliding stress decreases with increasing dose and then increases once the pyro-carbon swelling reaches 'turn around'. This causes an initial increase in crack growth rate and higher stresses in crack bridging fibers at higher doses. An assumed irradiation creep law for fine-grained SiC fibers is shown to dominate the radiation response.
Coatings and joining materials for SiC and SiC-based composites for nuclear energy systems are being developed using preceramic polymers filled with reactive and inert powders, and using solid-state reactions. Polymer-filled joints and... more
Coatings and joining materials for SiC and SiC-based composites for nuclear energy systems are being developed using preceramic polymers filled with reactive and inert powders, and using solid-state reactions. Polymer-filled joints and coatings start with a poly(hydridomethylsiloxane) precursor, such that mixtures of Al/Al 2 O 3 /polymer form a hard oxide coating , coatings made with Al/SiC mixtures form a mixed oxide–carbide coating, while coatings made with SiC/polymer form a porous, hard carbide coating. Joints made from such mixtures have shear strengths range from 15 to 50 MPa depending on the applied pressure and joint composition. The strongest joints were obtained using tape cast ribbons of Si/TiC powders such that a solid state displacement reaction at 1473 K and 1673 K using 30 MPa applied pressure resulted in shear strengths of 50 MPa, which exceeds the shear strength of SiC/SiC composite materials. However, the polymer joints are much easier to apply and could be considered for field repair. Published by Elsevier B.V.
—Subcritical crack-growth studies in SiC f /SiC composites were conducted with composites reinforced with Hi-Nicalon fibers over a broad temperature range for comparison to earlier studies on materials reinforced with Nicalon-CG fibers.... more
—Subcritical crack-growth studies in SiC f /SiC composites were conducted with composites reinforced with Hi-Nicalon fibers over a broad temperature range for comparison to earlier studies on materials reinforced with Nicalon-CG fibers. Composites with a 0/90 plain weave architecture and carbon interphase were tested in argon from 1173 to 1473 K. Crack growth data obtained in inert environments are consistent with a proposed fiber-creep-controlled crack-growth mechanism. Measured crack-growth activation energies and time–temperature exponents in argon agree with fiber creep-activation energies and nonlinear creep equations for both fiber types. Estimates of local strains during crack growth are in reasonable agreement with estimated fiber creep strains for the given times and temperatures. The increased creep resistance of Hi-Nicalon fibers is reflected in reduced crack-growth rates for composites containing those fibers.
A continuous fiber composite was simulated by four concentric cylinders (consisting of fiber, fiber/matrix interphase coating, matrix, and surrounding composite) to explore composite stresses when irradiation swelling of the various... more
A continuous fiber composite was simulated by four concentric cylinders (consisting of fiber, fiber/matrix interphase coating, matrix, and surrounding composite) to explore composite stresses when irradiation swelling of the various components is included to study radial debonding at the fiber-coating interface as a function of neutron dose. SiC TypeS and Hi-Nicalon fibers, and three types of transversely isotropic carbons for the fiber coating were considered.
Embedded-atom method potentials and atomistic models of coherent (010) interfaces were used to study slip across interfaces in cube-on-cube oriented Cu/Ni nanolayered materials. (111) disconnections form during slip across Cu–Ni... more
Embedded-atom method potentials and atomistic models of coherent (010) interfaces were used to study slip across interfaces in cube-on-cube oriented Cu/Ni nanolayered materials. (111) disconnections form during slip across Cu–Ni interfaces and become significant barriers to continued deformation. A significant barrier exists for the flat coherent interface owing to the large coherency stresses in the Cu/Ni layers that must be overcome by applied stresses but, once these have been overcome, interface transection occurs readily. A dis-connection adds an additional barrier because of a residual dislocation with a Burgers vector magnitude equal to the difference between b Cu and b Ni. This barrier depends on the position of the disconnection relative to the glide plane of the transecting glide dislocation and on the disconnection height. Disconnections cause work hardening that prevents shear band formation during deformation and encourages homogeneous shear processes. Disconnection energies are shown to be relatively small and to depend on the disconnection type and size.
A version of the Dregia–Hirth rebound mechanism for misfit dislocation creation is observed in atomistic models of Cu–Ni nanolayered structures. Glide dislocations in Cu layers undergo a reaction at the Cu–Ni interface under applied... more
A version of the Dregia–Hirth rebound mechanism for misfit dislocation creation is observed in atomistic models of Cu–Ni nanolayered structures. Glide dislocations in Cu layers undergo a reaction at the Cu–Ni interface under applied compression to produce another glide dislocation and a misfit dislocation.
Silicon carbide composites are attractive for structural applications in fusion energy systems because of their low activation and afterheat properties, excellent high-temperature properties, corrosion resistance, and low density. These... more
Silicon carbide composites are attractive for structural applications in fusion energy systems because of their low activation and afterheat properties, excellent high-temperature properties, corrosion resistance, and low density. These composites are relatively new materials with a limited database; however, there is sucient understanding of their performance to identify key issues in their application. To date, dimensional changes of the constituents, microstruc-tural evolution, radiation-enhanced creep, and slow crack growth have been identi®ed as potential lifetime limiting mechanisms. Experimental evidence of these mechanisms, the factors that control them, and their implications on component lifetime will be discussed.
Silicon carbide has many properties that are attractive for applications in fusion energy systems. The reliability of monolithic silicon carbide, however, is insucient for its use in large components. Ceramic matrix composites oer... more
Silicon carbide has many properties that are attractive for applications in fusion energy systems. The reliability of monolithic silicon carbide, however, is insucient for its use in large components. Ceramic matrix composites oer greater ¯aw tolerance and reliability, but their failure mechanisms are less well understood. This work has focussed on studying potential failure mechanisms in silicon carbide ®ber-reinforced, silicon carbide matrix (SiC f /SiC m) composites. In the event of pre-existing cracks, subcritical crack-growth may occur due to creep of ®bers that bridge the crack faces. Irradiation-enhanced creep will enhance the subcritical crack-growth rate. The presence of oxygen leads to oxidation of the interphase material and subcritical crack-growth controlled by the rate of interphase recession. In addition , ®ber shrinkage or weakening due to exposure to radiation can promote additional failure mechanisms, including embrittlement. These mechanisms, the conditions, under which they occur, and the current state of models of the crack-growth mechanisms will be discussed.
Eshelby–Mori–Tanaka models with a continuum damage mechanics approach are developed to predict the elastic damage and fracture toughness of multiwalled-carbon-nanotube (MWCNT) reinforced ceramics as a function of MWCNT fraction. This... more
Eshelby–Mori–Tanaka models with a continuum damage mechanics approach are developed to predict the elastic damage and fracture toughness of multiwalled-carbon-nanotube (MWCNT) reinforced ceramics as a function of MWCNT fraction. This damage model is introduced in a modified boundary layer modeling approach to predict damage accumulation leading to crack propagation from a pre-existing crack tip in a process window where damage and fracture are captured under plane-strain Mode I loading. The model is validated against experimental fracture toughness data for a MWCNT 3-mol% yttria-stabilized zirconia composite and successfully predicts the observed saturation in fracture toughness at about 25% volume fraction MWCNTs.
The international fusion community has designed a miniature torsion specimen for neutron irradiation studies of joined SiC and SiC/SiC composite materials. Miniature torsion joints based on this specimen design were fabricated using... more
The international fusion community has designed a miniature torsion specimen for neutron irradiation studies of joined SiC and SiC/SiC composite materials. Miniature torsion joints based on this specimen design were fabricated using displacement reactions between Si and TiC to produce Ti 3 SiC 2 þ SiC joints with SiC and tested in torsion-shear prior to and after neutron irradiation. However, many miniature torsion specimens fail out-of-plane within the SiC specimen body, which makes it problematic to assign a shear strength value to the joints and makes it difficult to compare unirradiated and irradiated strengths to determine irradiation effects. Finite element elastic damage and elasticeplastic damage models of miniature torsion joints are developed that indicate shear fracture is more likely to occur within the body of the joined sample and cause out-of-plane failures for miniature torsion specimens when a certain modulus and strength ratio between the joint material and the joined material exists. The model results are compared and discussed with regard to unirradiated and irradiated test data for a variety of joint materials. The unirradiated data includes Ti 3 SiC 2 þ SiC/CVD-SiC joints with tailored joint moduli, and includes steel/epoxy and CVD-SiC/epoxy joints. The implications for joint data based on this sample design are discussed.
A section of a vertical gradient freeze Cd 0.9 Zn 0.1 Te boule approximately 2100 mm 3 with a planar area of 300 mm 2 was prepared and examined using transmitted infrared microscopy at various magnifications to determine the... more
A section of a vertical gradient freeze Cd 0.9 Zn 0.1 Te boule approximately 2100 mm 3 with a planar area of 300 mm 2 was prepared and examined using transmitted infrared microscopy at various magnifications to determine the three-dimensional spatial and size distributions of Te-particles over large longitudinal and radial length scales. Te-particle density distributions were determined as a function of longitudinal and radial positions in these strips and exhibited a multi-modal log-normal size density distribution that indicated a slight preference for increasing size with longitudinal growth time, while showing a pronounced cellular network structure. Higher magnification images revealed a typical Rayleigh-instability pearl string morphology with large and small satellite droplets. This study includes solidification experiments in small crucibles of 30:70 mixtures of Cd:Te performed over a wide range of cooling rates which clearly demonstrated a growth instability with Te-particle capture that is suggested to be responsible for one of the peaks in the size distribution using size discrimination visualization. The results are discussed with regard to a manifold Te-particle genesis history as Te-particle direct capture from melt–solid growth instabilities due to constitutional supercooling and as Te-particle formation from the breakup of Te-ribbons via a Rayleigh–Plateau instability.
A continuous fiber composite was simulated by four concentric cylinders (consisting of fiber, fiber/matrix interphase coating, matrix, and surrounding composite) to explore composite stresses when irradiation swelling of the various... more
A continuous fiber composite was simulated by four concentric cylinders (consisting of fiber, fiber/matrix interphase coating, matrix, and surrounding composite) to explore composite stresses when irradiation swelling of the various components is included to study radial debonding at the fiber-coating interface as a function of neutron dose. SiC TypeS and Hi-Nicalon fibers, and three types of transversely isotropic carbons for the fiber coating were considered.
Research Interests:
Research Interests: Thermodynamics, Kinetics, Scanning Electron Microscopy, Optical Spectroscopy, Optical microscopy, and 11 morePacific Northwest, Design optimization, X ray diffraction, Room Temperature, Single Crystal, Washington State, Ambient Temperature, Hall effect, Annual Report, University of Illinois at Urbana-Champaign, and Gamma radiation
Research Interests:
Research Interests:
Research Interests:
Time-dependent crack growth measurements of ceramic composites in varying PO<inf>2</inf> environments were conducted on materials consisting of chemical vapor infiltration (CVI) SiC reinforced with Nicalon fibers having C-interfaces.... more
Time-dependent crack growth measurements of ceramic composites in varying PO<inf>2</inf> environments were conducted on materials consisting of chemical vapor infiltration (CVI) SiC reinforced with Nicalon fibers having C-interfaces. Crack velocities are determined as a function of applied stress intensity and time for varying O<inf>2</inf> levels. Results are presented for crack velocity-stress intensity relationships in pure Ar and in Ar plus 2000-, 5000-, 10,000-, and 20,000-ppm O<inf>2</inf> atmospheres at 1100°C. A 2D micromechanics model is used to represent the time-dependence of observed crack bridging events and is able to rationalize the observed phenomena.
Ceramic matrix composites, CMCs, are being considered for advanced first wall and blanket structural applications because of their high-temperature properties, low neutron activation, low density and low coefficient of expansion coupled... more
Ceramic matrix composites, CMCs, are being considered for advanced first wall and blanket structural applications because of their high-temperature properties, low neutron activation, low density and low coefficient of expansion coupled with good thermal conductivity and corrosion behavior. The paper presents a review and analysis of the hermetic, thermal conductivity, corrosion, crack growth and radiation damage properties of CMCs.
Atomistic models of coherent interfaces in the CuNi system with and without (111)-steps were used to study slip transmission across interfaces in CuNi metallic bilayers. The lattice mismatch of the CuNi system results in large coherency... more
Atomistic models of coherent interfaces in the CuNi system with and without (111)-steps were used to study slip transmission across interfaces in CuNi metallic bilayers. The lattice mismatch of the CuNi system results in large coherency stresses at the interface. The (111)-steps afford a larger barrier to slip than the flat, coherent interface. The coherent flat interface dislocation barrier is largely due to the large compressive stresses in the Cu layer that must be overcome by applied tensile stresses. Additional Koehler forces are present as the dislocation in the elastically softer Cu approaches the stiffer Ni layer. The step, however, possesses a small residual edge dislocation with a Burgers vector equal to the difference of b<sub>Cu</sub> and b<sub>Ni</sub> times the height of the (111)-step in (111)-layers. We find that these steps are potent slip barriers, which suggests that homogeneous slip is preferred in such systems.
A high strength in-situ composite of MoSi<sub>2</sub>-SiC was synthesized using a solid state displacement reaction between Mo<sub>2</sub>C and silicon. Diffusion couples between Mo<sub>2</sub>C and silicon processed at 1200°C... more
A high strength in-situ composite of MoSi<sub>2</sub>-SiC was synthesized using a solid state displacement reaction between Mo<sub>2</sub>C and silicon. Diffusion couples between Mo<sub>2</sub>C and silicon processed at 1200°C revealed the formation of aligned SiC platelets in an MoSi<sub>2</sub> matrix. The reaction zone of this couple had a Vickers microhardness of 12.8 GPa (HV 1000). In-situ composites were also synthesized by blending Mo<sub>2</sub>C and silicon powders and vacuum hot pressing for 2 h at 1350°C followed by 1 h at 1700°C. The resulting microstructure consisted of 30 vol.% SiC particles 1 μm in diameter uniformly dispersed in a fine-grained MoSi<sub>2</sub> matrix. Densities of 5.53 g cm<sup>-3</sup> were obtained together with a microhardness of 14.2 GPa (HV 1000). Bend bars and chevron-notched bars cut from large-diameter, and slightly less dense, hot-pressed disks revealed a strength of 475 MPa had a fracture toughness of 6.7 MPa m<sup>1/2</sup> at room temperature. Bend strengths increased to 515 MPa at 1000°C and then decreased to 112 MPa at 1200°C. Measured fracture toughness increased to 10.5 MPa m<sup>1/2</sup> at 1050°C. Fractography revealed that the MoSi<sub>2</sub> grain size was on the order of 1-2 μm, and it was suggested that the observed SiC particle size and aspect ratio could result in ineffective dislocation pinning and relatively rapid recovery at temperatures above the ductile-to-brittle transition temperature of MoSi<sub>2</sub>. This was substantiated by comparing these results with those obtained for SiC-whisker-reinforced MoSi<sub>2</sub> composites.
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.... 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<sub>2</sub> 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 MoSi<sub>2</sub>/SiC composite are presented in which both diffusion and interfacial 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 NiAl/Ni<sub>3</sub>Al/Ni:Al<sub>2</sub>O<sub>3</sub> system is also discussed.
Time-dependent crack growth measurements of ceramic composites in varying PO<inf>2</inf> environments were conducted on materials consisting of chemical vapor infiltration (CVI) SiC reinforced with Nicalon fibers having C-interfaces.... more
Time-dependent crack growth measurements of ceramic composites in varying PO<inf>2</inf> environments were conducted on materials consisting of chemical vapor infiltration (CVI) SiC reinforced with Nicalon fibers having C-interfaces. Crack velocities are determined as a function of applied stress intensity and time for varying O<inf>2</inf> levels. Results are presented for crack velocity-stress intensity relationships in pure Ar and in Ar plus 2000-, 5000-, 10,000-, and 20,000-ppm O<inf>2</inf> atmospheres at 1100°C. A 2D micromechanics model is used to represent the time-dependence of observed crack bridging events and is able to rationalize the observed phenomena.
Solid state displacement reactions can produce in situ intermetallic matrix composites in a process where an intermetallic phases(s) and a potential reinforcing phase(s) are grown together during a solid state reaction. Interwoven and... more
Solid state displacement reactions can produce in situ intermetallic matrix composites in a process where an intermetallic phases(s) and a potential reinforcing phase(s) are grown together during a solid state reaction. Interwoven and dispersed microstructures, important for desirable composite properties, have been produced by means of displacement reaction processing techniques. Two such composites have been synthesized: MoSi<inf>2</inf> reinforced with SiC particles, and NiAl/Ni<inf>3</inf>Al reinforced with Al<inf>2</inf>O<inf>3</inf>. Strength in bending and chevron-notch fracture toughness have been determined as a function of temperature and measured properties compare favorably with composites produced by other means. The measured properties are discussed with regard to the observed microstructures. The potential for displacement reaction processing is assessed and it appears to be a cost-effective synthesis method.
Various experimental techniques were used to identify failure mechanisms that occur under occur in continuous fiber-reinforced ceramic matrix composites (SiC/SiC) containing interphases that are susceptible to oxidation. The conditions... more
Various experimental techniques were used to identify failure mechanisms that occur under occur in continuous fiber-reinforced ceramic matrix composites (SiC/SiC) containing interphases that are susceptible to oxidation. The conditions under which these mechanisms occur were dictated by environmental conditions and microstructural parameters, such as temperature, oxygen concentration, and interphase thickness. Details of the mechanical state of fibers that bridge matrix cracks, during the operation of each mechanism, were developed to allow the application of models that can predict time-dependent, subcritical crack growth. This paper presents evidence of environmentally induced crack growth and discusses a model for predicting the rate of time-dependent, subcritical crack growth.
Silicon carbide composites are attractive for structural applications in fusion energy systems because of their low activation and afterheat properties, excellent high-temperature properties, corrosion resistance, and low density. These... more
Silicon carbide composites are attractive for structural applications in fusion energy systems because of their low activation and afterheat properties, excellent high-temperature properties, corrosion resistance, and low density. These composites are relatively new materials with a limited database; however, there is sufficient understanding of their performance to identify key issues in their application. To date, dimensional changes of the constituents, microstructural evolution, radiation-enhanced creep, and slow crack growth have been identified as potential lifetime limiting mechanisms. Experimental evidence of these mechanisms, the factors that control them, and their implications on component lifetime are discussed.
Stability and properties of monolithic and SiC<sub>f</sub>/SiC composites were measured before and after irradiation in a fast neutron spectrum up to 25 dpa between 500 and 1500°C. Dimensional changes were relatively consistent with... more
Stability and properties of monolithic and SiC<sub>f</sub>/SiC composites were measured before and after irradiation in a fast neutron spectrum up to 25 dpa between 500 and 1500°C. Dimensional changes were relatively consistent with previous investigations. Strength and modulus of SiC<sub>f</sub>/SiC composites decreased after irradiation as a result of fiber/matrix decoupling. For some composites, uniform elongation was not significantly degraded by irradiation. Thermal conductivity also decreased after irradiation at low temperatures because of the introduction of lattice defects as phonon scattering sites. The retention of properties under the severe conditions of 25 dpa and 800°C suggests that a composite tailored for neutron damage resistance can be developed.
Thermal gravimetric analysis (TGA) and subcritical crack growth measurements of chemical-vapor-infiltrated SiC matrix reinforced with Nicalon fibres and with a 1μm thick C fiber-matrix interface have been conducted at 1100°C over... more
Thermal gravimetric analysis (TGA) and subcritical crack growth measurements of chemical-vapor-infiltrated SiC matrix reinforced with Nicalon fibres and with a 1μm thick C fiber-matrix interface have been conducted at 1100°C over O<sub>2</sub>-Ar mixtures ranging from 0.25% to 20.0% O<sub>2</sub>. The TGA and interface recession measurements both gave linear reaction kinetics for O<sub>2</sub> concentrations of 2.0% or less and a reaction order of unity. Subcritical crack growth measurements demonstrated that the crack velocity, in the stress-intensity-independent stage II regime, increases with increasing O<sub>2</sub>/Ar ratio. Also, the transition from stage II to the stress-intensity-dependent stage III regime is shifted to lower stress intensities with increasing O<sub>2</sub>/Ar ratio. A time-dependent crack growth model that incorporates creep of the bridging SiC fibers and the removal of the C interfacial layer by oxidation successfully explains the subcritical crack growth characteristics.
Single crystal wafers (0.25 mm thick) of molybdenum were bombarded with 5-MeV Ni<sup>++</sup> ions at 1000°C at an ion current of 9×10<sup>12</sup> ions/cm<sup>2</sup>-s. Some specimens were also bombarded simultaneously with... more
Single crystal wafers (0.25 mm thick) of molybdenum were bombarded with 5-MeV Ni<sup>++</sup> ions at 1000°C at an ion current of 9×10<sup>12</sup> ions/cm<sup>2</sup>-s. Some specimens were also bombarded simultaneously with 200-keV He<sup>+</sup>ions. To examine the damage in profile, molybdenum was sputtered deposited onto both the irradiated and back surfaces of the wafers to build up the thickness to ~2 mm. The thickened wafers were then sliced perpendicular to the original faces (parallel to the beam). These slices were subsequently electrolytically thinned for examination by TEM. Several conclusions are drawn from the damage profile; (a) the damage region is much broader than previous calculations indicate; (b) the free surface can influence the damage structure; (c) simultaneously deposited helium atoms affect void size and number density but not the net void volume fraction; and (d) the deposited nickel ions do influence void nucleation in molybdenum.
A linear stress dependence of irradiation creep was observed in pure, heavily coldworked Ni and the creep behavior was modeled using a climb-glide creep mechanism. The Ni foils were bombarded with 17 MeV deuterons at 473K with a damage... more
A linear stress dependence of irradiation creep was observed in pure, heavily coldworked Ni and the creep behavior was modeled using a climb-glide creep mechanism. The Ni foils were bombarded with 17 MeV deuterons at 473K with a damage rate of 6×10<sup>-7</sup> dpa/s. The applied tensile stress ranged from 135 MPa to 250 MPa. Transmission electron microscopy revealed that up to 50% of the material had recrystallized during the pre-irradiation thermal creep test. A high number density of interstitial loops were observed throughout the material. Small voids were observed primarily in the recrystallized regions. The creep rate magnitude and stress dependence was rationalized using models of loop hardening and rate theory predictions of point defect annihilation at sinks.
The concept of inhomogeneous slip or localized deformation is introduced to account for a weak dependence of irradiation creep on initial microstructure. Specimens of pure nickel (Ni) with three different microstructures were irradiated... more
The concept of inhomogeneous slip or localized deformation is introduced to account for a weak dependence of irradiation creep on initial microstructure. Specimens of pure nickel (Ni) with three different microstructures were irradiated at 473K with 15-17 MeV deuterons in the Pacific Northwest Laboratory (PNL) light ion irradiation creep apparatus. A dispersed barrier model for climb-glide (CG) creep was unable to account for the observed creep rates and creep strains. The weak dependence on microstructure was consistent with the stress induced preferential absorption (SIPA) creep mechanism but a high stress enhanced bias had to be assumed to account for the creep rates. Also, SIPA was unable to account for the observed creep strains. The CG and SIPA modeling utilized rate theory calculations of point defect fluxes and transmission electron microscopy for sink sizes and densities.
A climb-controlled guide (CCG) creep model, which had been used to successfully rationalize low-fluence, light-ion irradiation creep data in pure nickel, accounted for low-fluence irradiation creep rates and loop growth rates in Type 316... more
A climb-controlled guide (CCG) creep model, which had been used to successfully rationalize low-fluence, light-ion irradiation creep data in pure nickel, accounted for low-fluence irradiation creep rates and loop growth rates in Type 316 stainless steel. The model also predicted a weak stress dependence and a weak temperature dependence in agreement with typical observations. Both light-ion and neutron irradiation creep measurements and microstructural observations at low fluences were examined and compared with predictions of the CCG model. Calculated creep rates compared to measured creep rates demonstrated that dislocation glide was the dominant mode of deformation during low-fluence creep when interstitial loops were small.
Research Interests: Pacific Northwest, Cost effectiveness, Second Year, Electronic properties, Material Characterization, and 11 moreCollaborative Work, Room Temperature, Single Crystal, Schottky Barrier, Ambient Temperature, Development Strategy, Low Noise, Ionizing Radiation, Annual Report, University of Illinois at Urbana-Champaign, and Gamma radiation
Propulsion Materials FY 2004 Progress Report 41 D. Hydrogen Compatibility of Materials for Automotive Applications Russell H. Jones, James Holbery, and Thomas Gallant Pacific Northwest National Laboratory PO Box 999 Richland, Washington... more
Propulsion Materials FY 2004 Progress Report 41 D. Hydrogen Compatibility of Materials for Automotive Applications Russell H. Jones, James Holbery, and Thomas Gallant Pacific Northwest National Laboratory PO Box 999 Richland, Washington 99354 (509) 376-4276; fax:(509) ...