Thesis--University of Florida. / Description based on print version record. Manuscript copy. Vita. Bibliography: leaves 126-127.

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

The effects of a polycrystalline surface layer on the critical resolved shear stress, yield stress and the work hardening rate of tin single crystals has been investigated. The change in these parameters was found to be independent of the thickness of the layer. The results have been interpreted in terms of the inhibition of surface sources and the higher stress level required to initiate interior sources. An exploratory investigation of the flow mechanism in tin was undertaken. Tin single crystals oriented for slip on the (110) [001] slip system were deformed in the temperature range from room temperature to -80°C, with the lower limit of temperature being sensitively dependent on the strain-rate. Twinning was observed to be the mode of deformation below this lower limit. Strain-rate change tests at temperatures from -27°C to R.T. were performed in order to assess the activation energy and activation volume. On the basis of these values, the flow mechanism was postulated to be the non-conservative motion of jogs in screw dislocations. / 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

Une théorie multi-échelles des champs de dislocations dans les solides cristallins a été étudiée dans différents cas où la distribution des dislocations est telle que leur champ de contraintes internes soit significatif devant le champ de contraintes appliquées. Cette théorie couple les dynamiques des densités de dislocations statistiques et des dislocations en excès. Ces dernières accommodent les gradients de distorsion plastique et génèrent les champs de contraintes internes. Un modèle 2D a été introduit. Sa première application a été l'intermittence de la plasticité. Ce phénomène est lié aux contraintes internes, qui, associées à l'équation de transport, organisent un mouvement collectif des dislocations, sous forme d'avalanches rapides et localisées. Le modèle a ensuite décrit le fluage en torsion de la glace monocristalline. Les dislocations vis basales en excès accommodent sa déformation. Un effet de taille adoucissant lorsque le rayon est réduit et le comportement original en torsion inverse ont été interprétés. Le modèle a étudié ensuite l'évolution des structures de dislocations lors du vieillissement de monocristaux de glace déformés par fluage en compression. La relaxation des contraintes internes lors du vieillissement augmente la vitesse de fluage alors que l'annihilation des structures reste négligeable. Le modèle a enfin montré que les contraintes internes, générées lors de la propagation d'une bande de Lüders dans les aciers doux, évitent la formation d'une nouvelle bande après vieillissement si le sens de déformation est inversé. Les contraintes internes, associées à l'équation de transport, entraînent la propagation de la bande / A field dislocation theory was studied in various plasticity problems where the dislocation distribution involves internal stresses of the order of applied stress field. This theory couples the dynamics of statistical and excess dislocation densities. The latter accommodate plastic distortion gradients and generate internal stress fields. A 2D model was introduced. As a first application, it reproduced the intermittency of plasticity. This phenomenon is linked to internal stresses which, associated with transport equation, lead to a collective movement of dislocations, in the form of fast and localized avalanches. The model then described the torsion creep of ice single crystals. Excess basal screw dislocations accommodate its deformation. A softening size effect with decreasing radius and the original behavior in reversed torsion were also reproduced. The model then studied the evolution of dislocation structures during the aging of ice single crystals deformed in compression creep. Internal stresses relaxation increase creep rate while dislocation structure annihilation remains negligible. The model finally showed that internal stresses generated during the propagation of a Lüders band in mild steels avoid the nucleation of a new band after aging if the sense of straining is reversed. Internal stresses, associated with transport equation, are responsible for band propagation

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