Chemical and Biomolecular Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2751
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Item Local Atomic Arrangments and Solution Strengthening of Ta-Mo and Ta-Nb Alloys(1975) Predmore, Roamer Edward; Arsenault, Richard J.; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)Ta-Nb alloys are shown to form random solid solutions by x-ray diffuse scattering measurements. These alloys have equal size atoms in their pure state with lattice parameters that are invariant in composition, obey Vegard's law, and exhibit an absence of solid solution hardening and an absence of fracture embrittlement at high solute concentrations. Ta-Mo atoms of about 5% difference in atomic size form short range ordered solid solutions with large atomic displacement effects. The Ta-Mo, and Ta-W, Nb-Mo and Nb-W alloys have in common a lattice parameter that varies in composition with a negative deviation from Vegard's Law. There is also a negative heat of mixing which is well correlated with short range order. In addition, all these alloys show linear solid solution hardening to high solute concentrations at room temperature and fracture embrittlement at high solute concentrations. Diffuse ex-ray scattering measurements on Ta-Mo alloys give the short range order parameters and atomic size displacements. The hardening is attributed to a combination of size effect induced substitutional solid solution hardening and short range order induced hardening.Item An Investigation of Blast Waves Generated by Constant Velocity Flames(1977) Luckritz, Robert Thomas; Marchello, Joseph M.; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)The relevant flow field parameters associated with the generation and propagation of blast waves from constant velocity flames were systematically studied through numerical integrations of the non-steady equations for mass, momentum, and energy. The flow was assumed to be that of an adiabatic inviscid fluid obeying the ideal gas law and the flame was simulated by a working fluid heat addition model. The flame velocity was varied from infinitely fast (bursting sphere) through velocities characterized by the nearly constant pressure deflagration associated with low Mach number laminar flames. The properties noted included peak pressure, positive impulse, energy distribution, and the blast wave flow field. Results were computed for the case of a methane-air mixture assuming an energy density, q = 8.0, an ambient specific heat ratio, Yo = 1.4 and a specific heat ratio behind the flame, Y4 = 1.2. In the source volume, as the flame velocity decreased to Mach 4.0 the overpressure increased. For flame velocities below Mach 4.0 the overpressure decreased, and approach the acoustic solution originally developed by Taylor. In the far field the overpressure curves for supersonic flame velocities coalesced to a common curve at approximately 70% of Baker's pentolite correlation. Far field overpressures for subsonic flame velocities decreased as the flame velocity decreased. For the flame velocities investigated the near field impulse was greater than the impulse from Baker's pentolite correlation. In the far field the flame generated impulse decreased to 60 to 75% of the pentolite impulse. In cases where the flow was expected to reduce to a self-similar solution and/or show Rayleigh line behavior it did. The calculations showed that the flow field behaved normally where expected, and for flow velocities where steady state behavior is not expected, non-steady behavior was observed.Item Thermal Displacement in Copper-Gold Alloys(1971) Gilmore, Charles M.; Bolsaitis, P.; Skolnick, L.; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)The thermal displacements and Debye temperatures are determined for single crystals of copper and Cu-Au solid solutions including Cu3Au of four degrees of long range order (0.0, 0.53; 0.8, .98). Other solid solution compositions studied were .91Cu-.09Au and .2Cu-.8Au. At the .91Cu-.09Au composition a one week anneal produced a nonequilibrium structure. After a one month anneal the thermal displacements decreased to a value nearly equal to the value for pure Cu. The thermal displacements in the quenched .75Cu-.25Au crystal were also nearly equal to the value for pure Cu, but the thermal displacements increased as the Cu3Au crystal approached the equilibrium condition of full order. In the Cu3Au crystals, which were partially or fully ordered, the thermal displacements of the individual Cu and Au atoms were determined. It was observed that the vibration amplitudes of the Cu atoms are not isotropic in this diatomic cubic crystal. The vibrations of the Au atoms are equal in the [110] and [001] directions within experimental uncertainty. Also, the thermal displacements decrease as the crystal is changed from fully ordered to fully disordered. This is consistent with calculations of the vibrational spectrum for ordered and disordered Cu3Au. The static displacements for the partially ordered S = .80 crystal were also determined from the same experiments as the thermal displacements. An Einstein model was developed to calculate thermal displacements and Einstein frequencies from interatomic potentials. The calculated thermal displacements are 10 to 20 percent less than the experimental values. This is due to the simplifying assumptions in the model. The model calculation and the experimental results do agree on the changes in the thermal displacements with alloying. The Einstein model is also used to calculate the vibrational entropy in alloys.