No abstract available

No abstract available

[en] The effect of humid air environments on behavior of B₄C-B and BORSIC reinforced Ti-6A1-4V metal matrix composites was studied. Humid environments enhanced crack propagation rates at R = 0.1. The effect was more pronounced in transverse and 45 deg specimens. A transition from fiber splitting in humid air to interfacial splitting in dry environments was observed at a load ratio of 0.1. At R = 0.5, however, no fiber splitting was observed in humid air. Fatigue crack closure load measurements revealed that closure loads were higher in humid air than in dry environments. The splitting is an environmentally induced crack closure effect, where plastically deformed matrix sets up stress fields on the fibers weakened by humidity

[en] A simple model of diffusion and evolution of the density of deuterium in metals is presented. A model of the deuterium evolution in the presence of uniform and nonuniform distributions of traps, as well as perfectly reflecting and partially permeable boundary conditions is discussed. Computers are compared with experimental results describe deuterium distribution after fatigue crack growth of 2219 and 7075 aluminum alloys in a D₂O water vapor environment and after ion implantation

[en] The effects of internal H, introduced by cathodic charging, on the room temperature tensile behavior of an aligned Ni, Al, Nb eutectic alloy were investigated. It was found that the aligned γ/γ'-delta eutectic alloy is susceptible to reversible hydrogen embrittlement. Cracking along interphase boundaries and along slip or twin bands in the nickel-rich γ phase is facilitated by hydrogen

[en] Laser pyrolysis offers a means of extending solid free-form fabrication to polymeric precursors. Laser pyrolysis of polycarbosilane (PCS) produces controlled β-SiC shapes with nanometer grain size although properties are currently limited by high porosity. By the addition of filler powders, either inert or reactive, ceramic-metal and ceramic-ceramic composite shapes are possible. The results of laser pyrolysis of PCS alone and in mixtures with β-SiC, Al, Si, Ti, and Zr are presented showing that the technique has the potential to produce a range of materials and shapes customized to particular design requirements

[en] The consolidation response of powders of the superconducting compound YBa₂Cu₃O_7-δ by itself and admixed with metal powders is reported. The processing approach relies on short duration (< Is),high current density (10⁴A/cm²), pulse resistive heating of powders under applied pressures of 200 MPa to 400 MPa. Powders and fabricated disk concepts were characterized by X-ray diffraction, optical and scanning electron microscopy

[en] A new approach to powder processing is employed in forming titanium aluminide composites. The processing consists of internal heating of a customized powder blend by a fast electrical discharge of a homopolar genertor. The high-energy high-rate lMJ in ls pulse permits rapid heating of an electrically conducting powder mixture in a cold wall die. This short time at temperature approach offers the opportunity to control phase transformations and the degree of microstructural coarsening not readily possible with standard powder processing approaches. This paper describes the consolidation results of titanium alumnide-based powder composite materials. The focus of this study was the definition of microstructure/processing relationships for each of the composite constituents, first as monoliths and then in composite forms

[en] Advances in kinetic energy storage devices have opened up a new approach to powder processing of high temperature composites. The processing consists of internal heating of a customized powder blend by a fast electrical discharge of homopolar generator. The high-energy high rate IMJ in 1s pulse permits rapid heating of a conducting powder in a cold wall die. This short time at temperature approach offers the opportunity to control phase transformations and the degree of microstructural coarsening not readily possible using standard powder processing approaches. This paper describes the consolidation results of two high temperature composite materials, (W-Ni-Fe)B₄C and (Ti₃Al + Nb)/SiC. The focus of this study was the identification of the reaction products formed at the matrix/reinforcement interface as a function of input energy and applied stress. Input energies beyond a threshold value for each system were required to produce detectable reaction products. In the (W-Ni-Fe)/B₄C system, the reaction products formed at 4000 kj/kg input energy under 420 MPa applied stress were a series of complex carbides and borides including W₂C,FeWb,Fe₃C,Fe₆W₆C and Ni₄B₃. The intermetallic Fe₇W₆ was also observed. In the (Ti₃Al + Nb)SiC System, the reaction products observed at 3400 kj/kg and 210 MPa were TiC and TiSi₂

[en] This paper focuses its attention on several research efforts performed by the authors and colleagues whose primary aim is probing interfacial and grain boundary properties. The first investigations described pertain to impurity segregation induced embrittlement which were modeled using thermodynamic considerations and were characterized by fracture testing using Auger electron spectroscopy (AES). Localized interface chemical characterization is essential in this research. The second investigation focuses on interface/impurity interaction determination. Extended X-Ray Absorption Fine Structure (EXAFS), a structure characterization tool, was used to determine the local environment of impurities in metal-impurity-metal laminate composites, where the impurities were in monolayer to submonolayer interface concentrations. The third study pertains to the interface fracture in metal-matrix (metal/metal oxide/graphite) composites where a correlation was made between electronic states and fracture path