Showing papers by "Daniel B. Miracle published in 2001"

Multi-scale characterization of spatially heterogeneous systems: implications for discontinuously reinforced metal–matrix composite microstructures

Abstract: A new multi-scale technique is presented for characterizing the spatial distribution of second-phase particles in two-dimensional distributed multi-phase systems. The implications for the characterization of reinforcement distributions in discontinuously reinforced metallic matrix composite microstructures are discussed, along with results of the analysis both for simulated and experimental discontinuously reinforced aluminum (DRA) materials. A systematic variation in the degree of spatial heterogeneity is observed with increasing length scale. This result leads to the definition of the parameter L H or homogeneous length scale. The relevance of L H measured for a real DRA microstructure is then discussed in the context of statistical variations in mechanical properties such as tensile strength, ductility, and fracture toughness.

Method of selection of alloy compositions for bulk metallic glasses

Abstract: A method for selecting alloying elements for complex, multi-component amorphous metal alloys is provided in which the solvent element is the largest atom with a concentration of 40-80 at %, the second most concentrated element has a radius of 65-83 % the radius of the solvent atom and a concentration of 10-40 at % in the alloy, with other elements selected at lower concentrations. For ternary alloys specified by this invention, the third element must have an atomic radius within 70-92 % of the solvent atom radius. In the preferred embodiment, alloys with four or more elements are specified, where the third elements must have an atomic radius within 70-80 %, the fourth element must have an atomic radius within 80-92 % of the solvent atom radius, and all other solute elements must have atomic radii within 70-92 % of the solvent atom radius. The concentrations of elements that have radii that differ by less than 1 % from one another are added together and treated as a single alloy addition for the purpose of this invention.

Application of the cruciform specimen geometry to obtain transverse interface-property data in a high-fiber-volume-fraction SiC/Titanium alloy composite

Abstract: A combined experimental and computational methodology was used to determine the relevant strength and residual-stress parameters in a manufactured, high-fiber-volume-fraction multiply metal matrix composite (MMC). The method was similar to that previously demonstrated on single-fiber composites, which had an extremely low fiber volume fraction. Variabilities in residual stresses and debond strengths in high-fiber-volume-fraction multiply composites, as well as current demands on the micromechanics-based computational prediction and validation of complex composite systems, necessitated the establishment of the test methodology described here. The model material chosen for this investigation was a plasma-processed six-ply, unidirectional Sigma-1240/Ti-6Al-2Sn-4Zr-2Mo (wt pct) MMC containing 32 vol pct continuous fibers. Room-temperature transverse tensile experiments were conducted on cruciform specimens. In addition, rectangular specimens were also evaluated in order to verify their applicability in obtaining valid interfacial property data. Debonding events, evaluated at different positions within a given specimen geometry, were captured by stress-strain curves and metallographic examination. Analytical and finite-element stress analyses were conducted to estimate the geometrical stress-concentration factors associated with the cruciform geometry. Residual stresses were estimated using etching and computational procedures. For the cruciform specimens, the experimental fiber-matrix debond strength was determined to be 22 MPa. Separation occurred within the carbon-rich interfacial layer, consistent with some previous observations on similar systems. Thus, the cruciform test methodology described here can be successfully used for transverse interfacial-property evaluation of high-fiber-volume-fraction composites. For the rectangular specimens, the strain gages at different positions along the specimen width confirmed that the interface crack had initiated from the free edge and propagated inward. Hence, rectangular specimens cannot be used for valid interface strength measurements in multiply composites.

Microstructural and mechanical characterization of carbon coatings on SiC fibers

Abstract: A series of carbon coatings was deposited on a 1040 SiC monofilament using chemical vapor deposition, and failure of the fiber-matrix interfacial region under transverse tension was studied. Deposition substrate temperatures were approximately 920, 1000, and 1080 °C, and all other deposition parameters were held constant. The microstructures of these carbon-coated fibers were examined using optical microscopy, scanning electron microscopy, and transmission electron microscopy (TEM). TEM observations were made using bright-field imaging, dark-field imaging, selected-area diffraction, and high-resolution lattice imaging. Tensile testing of single-fiber composite samples was performed transverse to the fiber axis to determine the stress required to cause debonding of the fiber from the titanium alloy matrix. Adhesion experiments were used to examine differences in bond strength of the SiC–C interfaces of the three coatings. A systematic increase in the grain size of the SiC substrate fiber within 3 μm of the SiC–C interface with increasing deposition temperature was observed. The crystallographic texturing of the basic structural units of carbon within the coatings was also found to increase with increasing deposition temperature. The SiC–C interface strength increased with increasing deposition temperature and correlates with the microstructural changes in both the SiC and carbon at the interface. The overall composite transverse strength was not affected by the change in deposition temperature, although the fracture location was affected. The carbon coating with the lowest SiC–C interface strength failed at this interface, and the coatings with more highly textured carbon failed within the coating, where the proportion of weak van der Waals bonds parallel to the tensile direction was correspondingly higher.