Charles H. Henager

Synthesis of Ti3SiC2SiC and TiSi2SiC composites using displacement reactions in the TiSiC system

This paper reports on efforts to synthesize composites in the Ti-Si-C system. Selection of the composite system(s) was based on the Ti-Si-C ternary equilibrium-phase diagram. However, interest in the Ti-Si-C system was motivated by recent reports of not-so-brittle behavior of the complex ternary phase Ti{sub 3}SiC{sub 2}. Apparently, Ti{sub 3}SiC{sub 2} is a soft ceramic compound, like BN or graphite in its behavior. It has a layered crystal structure (D{sub oh}{sup 4}-P6{sub 3}/mmc, hexagonal), a melting temperature greater than 3000C, and a density of 4.53 g/cm{sup 3}. Reportedly, its microhardness as a function of load indicates ductile, or not-so-brittle, behavior, and is anisotropic. The Knoop hardness is about 3.5 GPa when measured along (hk0)(uv0) (parallel to soft, TiC-like orientation) and about 13 GPa when measured in other orientations (perpendicular to TiC-like orientation). Bulk, polycrystalline Ti{sub 3}SiC{sub 2} has a Vickers hardness of 6 to 7 GPa.

Synthesis of a MoSi2SiC composite in situ using a solid state displacement reaction

Abstract A high strength in-situ composite of MoSi2SiC was synthesized using a solid state displacement reaction between Mo2C and silicon. Diffusion couples between Mo2C and silicon processed at 1200°C revealed the formation of aligned SiC platelets in an MoSi2 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 Mo2C 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 MoSi2 matrix. Densities of 5.53 g cm−3 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 1 2 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 1 2 at 1050 °C. Fractography revealed that the MoSi2 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 MoSi2. This was substantiated by comparing these results with those obtained for SiC-whisker-reinforced MoSi2 composites.

Microstructure and Microdeformation Effects on IGSCC of Alloy 600 Steam Generator Tubing

Abstract Microdeformation characteristics in alloy 600 tubing have been examined after various tensile deformations. Microstructure developed during processing was found to control subsequent microdeformation behavior. Grain boundary carbides were the most effective source of dislocations, activating at lower macrostrains and continuing to operate at higher macrostrains than other sources. Ledges within grain boundaries, twin boundaries, and matrix carbides also acted as dislocation sources. Most dislocation activity at low strains was confined to planar arrays. A conceptual model is presented to explain the effects of interfacial and matrix microstructure on microdeformation and primary side stress corrosion cracking (SCC) of alloy 600 tubing. Microstructure is linked to intergranular stress corrosion cracking (IGSCC) resistance through its influence on microdeformation behavior and the resultant crack tip stress state. Dislocation source activity at grain interfaces is proposed to be critical in control...

Grain Boundary Junctions in Microstructure Generated by Multiple Twinning

The microstructure of a Cu-Ni alloy after static recrystallization was investigated using electron backscatter diffraction in a scanning electron microscope and the existence of orientationally related clusters of crystallites formed by multiple twinning has been established. Grain boundary and triple junction character within the clusters are analyzed. While the outer boundaries of the cluster are crystallographically random, all the inner boundaries have Σ 3n misorientations. A newly developed crystallographic theory of triple junctions and multicrystallite ensembles consisting of CSL boundaries is used to describe the structure of the cluster. The presence of an α ≠ 1 triple junction is confirmed. Apparently, the microstructure of recrystallized materials susceptible to annealing twinning consists of multiple-twinned clusters. The cluster size cannot be reduced to the “grain size excluding twins.”

Interactions of dislocations with disconnections in fcc metallic nanolayered materials

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 disconnection 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.

High-temperature plasticity effects in bridged cracks and subcortical crack growth in ceramic composites

The time-dependent plasticity of the crack-wake bridging zone at high temperatures is found to control the rate of subcritical crack growth in continuous-fiber-reinforced ceramic composites. Subcritical crack growth measurements of ceramic matrix composites were conducted on materials consisting of chemical-vapor infiltrated SiC matrix reinforced with Nicalon fibers (SiC/SiCf) having C and BN fiber-matrix interfaces. Crack velocities were determined as a function of stress intensity in pure Ar and in Ar + 2000 ppm O2 at 1100 °C. A stage II regime, where the crack velocity is weakly dependent on the applied stress intensity, characterized the V − K data over a range of stress intensities corresponding to the R curve of the material. This stage II behavior was followed by a stage III, or power-law, regime at higher stress intensities. Oxygen increased the crack velocity in the stage II regime and shifted the stage II to stage III transition to lower stress intensities. A two-dimensional micromechanics approach modeled the time-dependence of crack bridging by allowing fiber creep and was able to rationalize the observed velocity measurements and the stage II to stage III transition.

High temperature corrosion and crack growth of SiCSiC at variable oxygen partial pressures

Abstract Thermal gravimetric analysis (TGA) and subcritical crack growth measurements of chemical-vapor-infiltrated SiC matrix reinforced with Nicalon fibres and with a 1 μm thick C fiber-matrix interface have been conducted at 1100 °C over O 2 Ar mixtures ranging from 0.25% to 20.0% O 2 . The TGA and interface recession measurements both gave linear reaction kinetics for O 2 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 2 /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 2 /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.

Investigation of the polymorphs and hydrolysis of uranium trioxide

This work focuses on the polymorphic nature of the UO3 and UO3–H2O system, which are important materials associated with the nuclear fuel cycle. The UO3–water system is complex and has not been fully characterized, even though these species are key fuel cycle materials. Powder X-ray diffraction, and Raman and fluorescence spectroscopies were used to characterize both the several polymorphic forms of UO3 and the certain UO3-hydrolysis products for the purpose of developing predictive capabilities and estimating process history; for example, polymorphic phases of unknown origin. Specifically, we have investigated three industrially relevant production pathways of UO3 and discovered a previously unknown low temperature route to the production of β-UO3. Several phases of UO3, its hydrolysis products, and key starting materials were synthesized and characterized as pure materials to establish optical spectroscopic signatures for these compounds for forensic analysis.

Subcritical crack growth in CVI SiCf/SiC composites at elevated temperatures: effect of fiber creep rate

Abstract 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.

High-temperature properties of SiC/SiC for fusion applications

Ceramic matrix composites (CMCs) such as SiC/SiC exhibit novel mechanical properties relative to their monolithic counterparts. The crack velocity (dadt) versus stress intensity (K) relationship for monolithic ceramics can be described by a simple power law relationship whereas SiC/SiC was found to exhibit a multistage dadt versus K relationship similar to that for stress corrosion of metals. A K-independent stage II was followed by a strongly K-dependent stage III similar to monolithic materials. Evidence also exists that the fracture resistance of these materials is greater if cracks are produced by subcritical growth processes relative to machined notches. Oxygen was found to increase dadt and decrease the K for the stage II to stage III transition while cyclic loads produced little damage at low K values but there was some evidence for increasing damage with increasing number of cycles and K.