Geoffrey A. Swift

Microscale damage evolution and stress redistribution in Ti–SiC fiber composites

Abstract Local damage evolution in a composite is the primary micromechanical process determining its fracture toughness, strength, and lifetime. In this study, high energy X-ray microdiffraction was used to measure the lattice strains of both phases in a Ti–SiC fiber composite laminate. The data provided in situ load transfer information under applied tensile stress at the scale of the microstructure. To better understand damage evolution, predictions of a modified shear lag model were compared to the strain data. This comparison (1) demonstrated the importance of accounting for the matrix axial and shear stiffness, (2) optimized the stiffness ratio for load transfer, and (3) improved the interpretation of the ideal planar geometry commonly used in micromechanical composite models. In addition, the results proved the matrix within and around the damage zone sustained substantial axial load and locally yielded. It was also shown that an area detector is essential in such a diffraction study as it provides multi-axial strain data and helps eliminate the “graininess” problem.

High-temperature elastic properties of in situ-reinforced Si3N4

A high-temperature tensile stress study of a monolithic silicon nitride (Si3N4) was performed with time-of-flight neutron diffraction. A dedicated engineering diffractometer was employed at temperatures reaching 1375 °C. Rietveld refinements of diffraction spectra allowed the determination of (1) the coefficient of thermal expansion tensor during heating and (2) lattice strains during loading. The stress–strain response of individual lattice reflections was used to calculate the single-crystal elastic stiffness tensor of Si3N4 at 1375 °C via a self-consistent model.

Microscale Residual Strains in Monolayer Unidirectional Fiber Composites

Thermal residual stress is common in fiber reinforced metal matrix composites and significantly affects their mechanical properties. The calculation of these stresses typically assumes continuum mechanics holds. As the fiber diameter in most composites approaches the grain size of the matrix, the continuum assumption can become invalid. Since the mechanical properties depend on the residual strain state of the composite, it is therefore necessary to determine the residual strains using spatially resolved microscale measurements. In order to quantify these residual strains, X-ray diffraction of both the fiber and matrix was employed using a sampling volume less than the fiber diameter. Results were compared to macroscopic measurements including many fibers. The measurements were performed in transmission using high-energy synchrotron X-rays yielding strains representative of the entire thickness of the composite. Evolution of these residual strains after application of load was also investigated. Spatial variations in residual strains showed significant deviation from the macroscopically observed residual strains.

INVESTIGATION OF NOVEL ALLOY TiC-Ni-Ni3Al FOR SOLID OXIDE FUEL CELL INTERCONNECT APPLICATIONS

Solid oxide fuel cell interconnect materials must meet stringent requirements. Such interconnects must operate at temperatures approaching 800 C while resisting oxidation and reduction, which can occur from the anode and cathode materials and the operating environment. They also must retain their electrical conductivity under these conditions and possess compatible coefficients of thermal expansion as the anode and cathode. Results are presented in this report for fuel cell interconnect candidate materials currently under investigation based upon nano-size titanium carbide (TiC) powders. The TiC is liquid phase sintered with either nickel (Ni) or nickel-aluminide (Ni{sub 3}Al) in varying concentrations. The oxidation resistance of the submicron grain TiC-metal materials is presented as a function weight change versus time at 700 C and 800 C for varying content of metal/intermetallic in the system. Electrical conductivity at 800 C as a function of time is also presented for TiC-Ni to demonstrate the vitality of these materials for interconnect applications. TGA studies showed that the weight gain was 0.8 mg/cm{sup 2} for TiC(30)-Ni(30wt.%) after 100 hours in wet air at 800 C and the weight gain was calculated to be 0.5205 mg/cm{sup 2} for TiC(30)- Ni(10 wt.%) after 100 hours at 700 C and 100 hours at 800 C. At room temperature the electrical conductivity was measured to be 2444 1/[ohm.cm] for TiC-Ni compositions. The electrical conductivities at 800 C in air was recorded to be 19 1/[ohm.cm] after 125 hours. Two identical samples were supplied to PNNL (Dr. Jeff Stevenson) for ASR testing during the pre-decision period and currently they are being tested there. Fabrication, oxidation resistance and electrical conductivity studies indicate that TiC-Ni-Ni{sub 3}Al ternary appears to be a very important system for the development of interconnect composition for solid oxide fuel cells.