Most of the mass of the interstellar medium is in neutral phases that are heated by far-ultraviolet radiation from star forming regions. The gas, in turn, emits line radiation in the infrared and submillimeter that potentially has great diagnostic value. In this paper, we will review the physical processes (including heating, cooling, gas phase and grain surface chemistry) that give rise to this line emission and present diagnostic diagrams. We will also discuss several examples where models of neutral gas emission have been used to interpret line emission, including observations of H2O and O2 in molecular clouds and an example which points out the caution that must be used when applying the diagnostics to kiloparsec-sized regions in other galaxies.

We have expanded our model of photodissociation regions (PDRs) to include the freezing of O- and C-bearing species on dust grains, simple grain surface chemistry, and desorption processes, including photodesorption, that may be important in the surface layers of diffuse, translucent, and dense molecular clouds. The main result of including these processes is that a number of important gas-phase species, including H2O and O2, peak in abundance at AV ~ few into the cloud. Most of the gas-phase column, and most of the emission, from these species arises in the peak. Closer to the surface, H2O and O2 are photodissociated, while deeper into the cloud, they freeze out onto grain surfaces. The result is H2O and O2 column densities that are nearly constant for a wide range of gas densities, n, and for FUV fields G0$\lesssim$ 500. The roughly constant column densities of these species provides an explanation for the low line-of-sight average abundances of H2O observed toward GMCs. The model results also suggest that regions of high FUV field are the best places to search for O2.

The evolution of nearly dense stoichiometric silicide compacts via powder processing is presented in this paper. For specific single-phase Mo(Si,Al)2 compacts, room-temperature environmental degradation, phenomenologically similar to the pesting behavior of binary MoSi2 was observed. This degradation occurs over a series of a few months and results in grain boundary decohesion, which leads to crumbling of polycrystalline compacts in air at room temperature. Auger electron spectroscopy, transmission electron microscopy, and energy dispersive spectroscopy of the boundaries revealed the presence of an array of lens-shaped particles each comprised of silicon carbide and aluminum. The reaction of these phases with atmospheric species was accelerated by the presence of humidity. The trace presence of carbon was unavoidable due to the use graphite pressing dies. Alloying additions were made to tie up carbon by forming more stable carbides while maintaining the desired matrix phase stoichiometry. The pest phenomenon and alloying remedy were proven applicable to other silicide systems through experimentation and ThermoCalc modeling.

MoSi2-based composites have been synthesized through the mechanical alloying (MA) of elemental molybdenum and silicon powders with and without carbon additions. The interplay between the phase formation sequence in the powders and the microstructural evolution in the consolidated samples is described. It is shown that the glassy SiO2 phase characteristic of conventional powder processed MoSi2 can be effectively eliminated by combining mechanical alloying, carbon additions, and an in situ carbothermal reduction reaction. Using this approach, composites consisting of uniformly distributed micron-size SiC in an MoSi2 matrix can be formed. The effect of important processing variables such as the extent of carbon additions, extraneous iron pickup during MA, partial pressures of oxygen, consolidation temperatures, and consolidation atmospheres is discussed based on the evidence obtained from DTA, TGA, TEM, and XRD.

Compositionally tailored MoSi2/SiC composites with silicon carbide content ranging from 0 to 60 volume percent were synthesized through a novel processing scheme involving the mechanical alloying of elemental molybdenum, silicon, and carbon. The effects of important processing parameters such as the nominal powder composition and the processing temperature on the microstructural evolution during mechanical alloying and subsequent heating are described based on the results obtained from DTA and XRD.

An investigation of the phase equilibria and phase transformation behavior in the Al-Ti-Ta system was conducted with an emphasis on developing alloys with stable microstructures and properties desirable for advanced aerospace applications. A series of ternary alloys containing 5 to 50 atomic percent Ta and varying amounts of Ti and Al were prepared by arc melting and characterized in the as-cast and heat treated (1450°C) conditions. Three stable two-phase regions (β-σ, β-α2. and Al2 Ta-η) were identified and characterized at 1450°C. A stable ternary B2 phase was identified near the composition 25Al-5OTi-25Ta1 and several binary phases were found to exhibit extensive solid solubilities. The results were used to construct a partial isothermal section at 1450°C for this system and to assess the complex phase transformations that occur upon cooling.

Mechanical alloying (MA) has been used to synthesize MoSi2 from elemental Mo and Si powders. SiC and A12O3 dispersoids were also incorporated as a means of increasing high temperature strength. After synthesis, the powders were consolidated using vacuum hot pressing. It is shown that the size and volume fraction of porosity increases anomalously with increasing hot pressing temperature over the range 1200 to 1700°C. The strengths of the A12O3 composite are, in most instances, considerably higher than the monolithic material produced under similar conditions.

Submicron powders, amorphous films and melt spun ribbons of various Al-Ge alloys have been analyzed to determine the relative roles of undercooling and cooling rate in the production of non-equilibrium structures. All analyses were performed in transmission electron microscopes equipped with energy dispersive x-ray spectrometers. The submicron powders, produced by electro-hydrodynamic atomization, were analyzed in their as-received condition and then annealed and/or melted using the electron beam as a local heating source. Once molten, the liquid droplets were undercooled at different cooling rates by varying the rate of beam obstruction. In this manner, a number of different microstructures were produced. These included metastable crystalline phases and mixed amorphous/crystalline structures. By combining this technique with a microscope heating stage, it was possible to carry out controlled dynamic undercooling experiments and determine phase selection as a function of undercooling and composition. The amorphous films were rapidly heated with the electron beam in the microscope and metastable as well as stable phases were produced. The results of these complementary analyses will be compared and discussed with reference to current models and theories of rapid solidification.