Thesis (Ph.D.)--Brown University, 2005. / Vita. Thesis advisor: Clyde L. Briant. Includes bibliographical references (leaves 95-99). Also available online.

A nonuniform annealing rate of color centers in LiF is attributed to a nonuniform density of dislocations which, in turn, is a direct consequence of the cleaving process. The tensile deformation properties of annealed LiF crystals were determined over the temperature range – 196 ⁰C. to +60 ⁰C. Several tests were also carried out on ɣ - irradiated specimens at ambient temperatures. A transition occurs in the work hardening rate during stage II deformation. Experiments involving both strain-rate and temperature cycling were performed over the limited temperature range of - 60 to +60 ⁰C. The results were analyzed in terms of rate theory expressions and indicated that the rate controlling mechanism for dislocation motion in stage IIA is probably the nonconservative motion of jogs in screw dislocations. Stage IIB hardening is more likely controlled by dislocation intersections. Evidence is presented which indicated that stress relaxation experiments may provide an extremely easy technique for the determination of the internal flow stress. Values obtained from such experiments on LiF agree remarkably well with those obtained from rate theory experiments. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Single crystals of cobalt have been deformed in tension over the temperature range 20°C to 600°C. On heating a transformation from the hexagonal close packed (hep) structure to the face centred cubic (fee) structure occurs at 430°C. The deformation behaviour in the hep phase has been compared to that for other hep metals, and it has been shown that only basal slip occurs even in unfavourable orientations. Twinning may occur, but has not been found to be associated with an increase in the work hardening rate. Similarly the properties in the fee phase have been compared to those of fee metals. The effect of deformation on the transformation has been studied on specimens which have been thermally cycled through the transformation during tensile tests. It has been found that the (111)/ (0001) transformation habit plane may be controlled by deformation. Recrystallisation may occur if two slip systems operate, but this is a function of crystal orientation. Slip may be induced on an unfavourably orientated glide plane in the fee phase, and this indicates that deformation dislocations on the (0001)[subscript: h], /(111)[subscript: c] plane are not affected by the transformation. However, glide dislocations on any {111} plane, which does not form the basal plane on cooling do not affect the work hardening behaviour in the hep phase. In previously deformed specimens, the flow stress is a function only of the deformation history, i.e. the yield point in the hep phase may be raised by a factor of 10 or the yield point in the fee phase may be lowered by a factor of 3. However, the work hardening rate depends only on the crystal structure and is always higher (by 20 to 100 times) in the fee phase than in the hep phase. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

The recovery of mechanical properties following deformation of single crystal cadmium has been studied. Such recovery has been observed above 0.26 TM [subscript omitted] (-120°C). Crystals covering a range of orientation were deformed in tension at -196°C and recovered at elevated temperatures. Transmission electron microscopy to relate tensile and recovery behaviour to dislocation structures was found to be impossible. It was observed that work hardening during the initial portion of the easy glide region is completely recoverable. At higher strains in easy glide, a portion of the work hardening was not recoverable. It is believed that in this latter section, dislocations are generated on the second order pyramidal system {1122} <1123>. These dislocations will combine with basal dislocations to form stable obstacles in the lattice which will be responsible for the non-recoverable work hardening. The end of easy glide was found to occur at x = 20°, independent of recovery or initial orientation. This phenomenon is associated with flow on the second order pyramidal system which will produce a much higher density of obstacles at this point, resulting in a higher work hardening rate. Recovery in stage II was observed to increase the amount of strain attainable. It was also observed that while recovery up to intermediate strains in stage II affected only basal dislocations, both basal and pyramidal dislocations appear to be recovered at high strains. Pyramidal dislocations may recover by the processes observed by Price. The rate controlling mechanism for yield and flow of cadmium single crystals is thought to be one of the non-conservative motion of jogs. An attempt was made to calculate an activation energy for the recovery process, but the data did not yield any meaningful numbers. This may be a result of the definition of recovery adopted for this work. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

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The tables describing the geometry of the yield surface are derived. They concern only fcc metals which undergo isotropic hardening, and which glide on the 24 {111} slip systems. The tables list all five dimensional vertices, and edges of the fourth, third and second order limiting the yield surface as well as the slip systems with which they are associated. A direct method for calculating the yield vectors for any orientation, in five dimensional stress space is established, which enables the user to obtain the yield surface without need of knowing the six deviator stress components. / The relaxed constraint theory, which, by contrast to the classical theory, prescribes only part of the strain increment tensor, is applied to the case of the fixed end torsion test. By taking account of the deformed shape of the grains, three strain components are prescribed; axisymmetry of the sample, which is assumed for every grain, leads to the prescription of another strain component. A further equilibrium condition on the stress associated with the absence of surface tractions leads to an additional constraint. The Yield Subsurface Analysis, which consists of cutting the five dimensional yield surface by the planes of the prescribed stresses, and selecting the p dimensional vertices (p being the number of prescribed strains) that provide maximum work, is carried out. The results are compared with the ones of the classical theory where all the strain components are known. / The simulations are carried out to shear strains of 10, and the orientation changes are calculated incrementally assuming the shear plane and shear direction to be fixed in space. / The texture results are similar up to shear strains of about 3, but differ significantly at larger strains. The relaxed constraint theory predicts the strengthening of the {100} texture component and the development of an equally strong {hk1} fiber component at the expense of the {111} fiber component, whereas the classical theory predicts a very strong {111} fiber component. At large strains, the experimental results agree better with the relaxed constraint predictions. / The average Taylor factors do not exhibit the same trends, since the Bishop and Hill theory predicts a continuous decrease and the RC theory a slight decrease followed by a continuous increase. . . . (Author's abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UMI

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