Drawn and annealed copper wires of diameters ranging from 50 μ to 900 μ were tested in tension and the results examined for evidence of size-effects. No size-effect on yield-stress or work-hardening rate has been definitely established. The results were discussed in terms of the fraction of the number of grains in the specimen which have a free surface. The ultimate tensile strength and ductility decrease with decreasing diameter for diameters below 200 μ. An explanation has been put forward in terms of void-formation during deformation. Polycrystals and single crystals of copper were tested at room temperature with the strain-rate cycled between 10⁻¹ min⁻¹ and 10⁻³ min⁻¹. Polycrystalline copper obeys the Cottrell-Stokes law but shows a variation in the ratio of flow-stresses with varying grain diameter and with a varying value of the fraction of grains in the specimen which show a free surface. Copper specimens were also tested with the temperature cycled between 78°K and 293°K. Copper polycrystals do not obey the Cottrell-Stokes law, the deviation depending on the grain size. These results are discussed in terms of stacking-fault energy and several possible explanations are considered. Aluminum polycrystals were tested with the temperature cycled between 78°K and 293°K. Aluminum obeys the Cottrell-Stokes law for temperature variation. A work-softening effect accompanies the yield-drop found at 293°K after prior deformation at 78°K. This was discussed in terms of cross-slip and dislocation climb mechanisms. / Applied Science, Faculty of / Mining Engineering, Keevil Institute of / Graduate

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Thesis (M. Eng.)--University of Louisville, 2004. / Department of Chemical Engineering. Vita. "May 2004." Includes bibliographical references (leaves 47-48).

An investigation of the effect of temperature and interstitial composition on the yielding characteristics of high purity niobium single crystals has been carried out. Single crystals of [110] axial orientation were purified and grown using an electron beam, floating-zone, zone-refiner. Tensile specimens prepared from the single crystals were tested in tension at temperatures from +25° to -197°C. at a strain rate of 0.057 per minute. The variation of yield stress with testing temperature was found to be discontinuous and consisted of two curves which intersected at approximately -100°C. The tensile results can be correlated to both Fisher's interpretation of the Cottrell-Bilby theory of yielding and to the theory of Zener and Holloman, which involves a thermally activated process. The effect of additions of hydrogen, oxygen and nitrogen to niobium single crystals over the temperature range +25° to -197°C was to increase the yield stress. The unusual temperature dependence of yield stress has been explained in two possible ways: 1. Ordering or superlattice formation at the temperature at which the anomalous behaviour occurs. 2. The preferential release of screw dislocations at stress levels lower than that required for edge dislocations. The result is a decrease in the temperature dependence of yield stress at the temperature when edge dislocations begin to take part in yielding. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Lattice misfit and its effect on the morphology, interfacial structure and kinetics of plate shaped precipitates are investigated in this study. The 7-Ag₂Al phase in the Al-Ag system was used as the reference system and its misfit was controlled by ternary additions of Mg and Cu. The addition of 0.S1 at% Mg was found to increase the misfit from 0.8% for the binary to 1.11%. Cu additions on the other hand, reduced the misfit by 0.38% for Cu concentrations up to 0.51 at%. Electron probe microanalysis showed that the Mg atoms preferentially partition to the 7 phase whereas Cu atoms partition equally between the precipitate and matrix phases. Direct transmission election rnicroscope observations were made on the interface structure in both the equilibrated state and during precipitate dissolution. The interface structure in the ternary Mg alloy consisted of a hexagonal network of partial dislocations which essentially remained the same before and during dissolution. A single array of a/2<110> dislocations was observed in the binary and ternary Cu systems prior to dissolution. This unit array transformed to a stable hexagonal network structure having the equilibrium spacing at the onset of dissolution and remained throughout the period of dissolution. The thinning and shortening kinetics of the precipitate plates were at least five times slower than the rates for volume diffusion control in all three systems. This interfacial inhibition has further been confirmed by the consistent fall below equilibrium values of the interface concentration as determined from electron probe microanalysis. This suggests the operation of a ledge migration mechanism. A mechanism of acquiring ledge/dislocations at the interface is used as a basis to correlate the observed kinetics with misfit and ledge migration at the precipitate-matrix interphase. The mechanism involves co-ordinated motion of sets of dislocations in the network which rids the surface of the highest steps thereby accomplishing dissolution. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

LD2668 .T4 1949 Z3 / Master of Science