M.C. Nichols

X-ray tomographic study of chemical vapor infiltration processing of ceramic composites

The fabrication of improved ceramic-matrix composites will require a better understanding of processing variables and how they control the development of the composite microstructure. Noninvasive, high-resolution methods of x-ray tomography have been used to measure the growth of silicon carbide in a woven Nicalon-fiber composite during chemical vapor infiltration. The high spatial resolution allows one to measure the densification within individual fiber tows and to follow the closure of macroscopic pores in situ. The experiments provide a direct test of a recently proposed model that describes how the surface area available for matrix deposition changes during infiltration. The measurements indicate that this surface area is independent of the fiber architecture and location within the preform and is dominated by large-scale macroporosity during the final stages of composite consolidation. The measured surface areas are in good agreement with the theoretical model.

High resolution tomography with chemical specificity

A very fast method of computerized critical absorption tomography featuring approx.10 ..mu..m spatial resolution and high chemical sensitivity is described. Synchrotron radiation is used and the method is especially suited to investigating small samples. From a preliminary experiment it is found that layers of neighboring elements only 0.2 ..mu..m thick can be distinguished at medium atomic numbers.

Energy‐modulated x‐ray microtomography

We describe an instrument for computerized x‐ray tomography which can provide fully three‐dimensional examination of small samples (a few millimeters to a few centimeters) with resolution variable from 5 to 100 μm. Elemental and chemical‐state distributions are obtained with high resolution by modulating the energy of the incident x‐ray beam. An essential component is the monochromator we designed and optimized for chemical microtomography. It has a narrow energy bandpass and a large degree of harmonic suppression, provides a uniform x‐ray beam with small angle of divergence, and is cooled in a way that eliminates thermal and vibrational instabilities.

Nondestructive investigation of damage in composites using x-ray tomographic microscopy (XTM)

X-ray tomographic microscopy (XTM), utilizing intense, highly collimated synchrotron radiation, has been used to nondestructively image materials structures in three dimensions. The spatial resolution of the technique approaches that of conventional optical microscopy, but XTM does not require polished surfaces or serial sections. We present the results of an XTM investigation of a composite material composed of silicon-carbide fibers in an aluminum matrix. The results reveal the aluminum/silicon-carbide interfaces and show microcracks running along many of the interfaces as well as in the matrix. Excellent contrast is observed between the silicon-carbide sheath of the fiber surrounding the graphite core and the coating at the fiber-matrix interface. The sensitivity to small changes in the linear absorption coefficient allows nondestructive imaging of variations in chemistry between graphite and silicon carbide and between silicon carbide and aluminum. The experimentally determined values of the absorption coefficients of these phases are identical to values published in the literature. For the first time, XTM will allow observation of damage accumulation and crack growth {ital during} deformation testing. The availability of such data will greatly improve our understanding of failure in advanced materials.

X-ray tomographic microscopy of fibre-reinforced materials

Aluminium composites containing Al2O3 fibres and precipitates of various intermetallic phases are investigated by high-resolution computerized microtomography. Individual fibres 15 μm in diameter and intermetallic phases forming a network with about 15 μm mesh size have been imaged. The capabilities of the method and its further development down 1 μm and less spatial resolution are discussed.

Elemental and chemical-state imaging using synchrotron radiation.

The near-edge structure in the x-ray absorption coefficient of an element is affected by chemistry and local environment. Experiments demonstrate that this property can be exploited in x-ray imaging both to identify and enhance the detectability of different chemical states of the same element. Chemical contrast images are obtained by digital subtraction of absorption images taken at carefully selected energy values.

Optimization of CCD‐based energy‐modulated x‐ray microtomography

Employing asymmetric Bragg reflection at the monochromator we obtain a wide and practically parallel synchrotron‐radiation beam which, when combined with the use of a CCD detector, enables us to perform chemically specific, high‐speed and high‐resolution tomography. We present recent results obtained with this new method. The actual resolution achieved was determined from the measured line‐spread function of the complete detection system which includes the CCD, the fluorescent screen, and the lens used for optical magnification. The modulation‐transfer function (MTF) calculated from the line‐spread function shows that with only twofold magnification and at 3% contrast detectability about 28 line pairs per mm are resolved. Extrapolating from this result we find that with an optical magnification of 7:1 about 100 line pairs per mm should be resolved. Ways to optimize the method further are discussed.

Heterogeneous fibre microstructures and their influence on failure

A detailed microscopic, experimental and micromechanical evaluation of fibre damage initiation in a unidirectional aluminium matrix‐silicon carbide fibre (SCS‐8TM) composite has been performed for a monotonic load sequence. The salient fibre features include a 33‐μm‐diameter monofilament turbostratic carbon (C) core, a ∼ 1·5‐μm pyrolytic C layer, an interior sheath of β‐phase silicon carbide (SiC) crystallites imbedded in an amorphous C matrix, and an exterior sheath of radially orientated β‐SiC. Quantitative microscopy shows that the interior sheaths surplus C varies smoothly from ∼ 35% by volume near the core to zero at the mid‐radius. The multi‐phase structure of the fibre produces an internal mechanical stiffness that increases with distance from the core, and thus peak stresses result in the exterior sheath. X‐ray tomographic microscopy (XTM) reveals that cores fracture randomly, without failure of the surrounding SiC, at stress levels above half the ultimate strength of the composite. Three‐dimensional XTM reconstructions show planar, non‐planar and spiral cracks in the failed fibre, suggesting multiple and competing initiation mechanisms. Qualitative fracture assessments suggest that flaws near the C core grow outward in a curved manner through the SiC–C and planar beyond the mid‐radius, whereas cracks originating near the fibre–matrix interface favour planar trajectories inward across the whole fibre.

Nondestructive Imaging of Materials Microstructures Using X-Ray Tomographic Microscopy

A technique for nondestructively imaging microstructures of materials in situ, especially a technique capable of delineating the time evolution of chemical changes or damage, will greatly benefit studies of materials processing and failure. X-ray tomographic microscopy (XTM) is a high resolution, three-dimensional inspection method which is capable of imaging composite materials microstructures with a resolution of a few micrometers. Because XTM is nondestructive, it will be possible to examine materials under load or during processing, and obtain three-dimensional images of fiber positions, microcracks, and pores. This will allow direct imaging of microstructural evolution, and will provide time-dependent data for comparison to fracture mechanics and processing models. 23 refs., 8 figs.

An investigation of the tellurium-rich uranium tellurides using X-ray powder diffraction

Abstract A comprehensive X-ray powder diffraction study of synthesized uranium telluride phases produced new and improved diffraction pattern data compared to published values. Experimental work at the tellurium-rich end of the U-Te phase diagram, using a high intensity rotating anode diffraction system, has yielded data for the phases UTe 2 , U 2 Te 5 , UTe 3 , UTe 3.4 and UTe 5 . Indexing of this experimental diffraction data has produced more complete patterns for the previously published phases UTe 2 , UTe 3 and UTe 5 . The first conclusive experimental diffractometer data are presented proving the existence of the phase U 2 Te 5 . The phase UTe 3.4 is structurally very similar to a reported β-UTe 3 , but is considered to be a new phase based on its composition and computer indexing.