An extensive creep study of a superplastic material has not previously been made. The present study was carried out to determine if there are any basic differences between the creep of coarse grained materials and fine grained super-plastic materials. The results give information about the mechanical properties of superplastic alloys and are relevent to an understanding of the mechanics of superplasticiy. At high strain rates the superplastic lead-tin eutectic deforms by recovery creep and a 3-stage creep curve is observed, similar to that found for coarse grained materials. As the strain rate is decreased, the initial transient (primary creep) disappears and the creep curve is linear until necking occurs and tertiary creep ends in failure. In the principal superplastic range, at medium strain rates, creep curves are linear to at least 50 % strain. The recovery rate is immediately equal to the strain hardening rate and there is no primary creep. At low strain rates the creep curve is slightly convex as the creep rate decreases with time. This may be due to the self extinguishing nature of diffusional creep or possibly strain induced grain growth. These results are consistent with the grain boundary sliding theories of superplasticity although details of the accommodation processes are not known. At the lowest strain rates, diffusional creep may operate. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

A Zn - 1 wt % Al alloy has been studied in an attempt to relate tensile data and metallographic observations to assumed modes of superplastic deformation. Tensile testing, was carried out at a constant crosshead speed to determine the stress versus strain rate relationships as a function of grain size and temperature. Further analysis included activation energy determinations, low temperature (-100°C) deformation behavior, deformation grain growth characteristics, and surface and internal metallography. Stage II deformation was consistent with a process combining boundary sliding and migration and boundary diffusion. Deformation in stage I was characterized by a low strain rate sensitivity and a variable activation energy. Normal slip processes were operative during stage III deformation. Deformation grain growth and anisotropic grain shape changes were pronounced and variable in stages I and II. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

A qualitative analysis of current creep theory has been made by studying the creep of polycrystalline lead. The behaviour of the low temperature stress-strain curve with prior creep history, and the strain response to decreases in creep stress were examined. The effect of temperature and creep strain on the low temperature stress-strain curve was investigated over the temperature range 0.5Tm to 0.8Tm. Specimens were quenched to 77°K. after creep and strained to determine the stress-strain curve. The 77°K. yield stress was found to increase during primary creep and remain constant in steady state. Increasing the creep temperature drastically lowered the low temperature yield stress. The reversible flow stress ratio was found to decrease with increasing temperature. These observations were in qualitative agreement with both a reaction rate theory and a rearrangement model. Stress decrease tests were carried out by reducing the creep stress after deforming the specimen varying amounts into primary and steady state in the temperature range 0.5Tm to 0.85Tm . The strain response to a stress decrease in steady state was in best agreement with the simple recovery theory. The variation In yield stress due to non-regular obstacle spacing was found to be extremely small at all temperatures and did not behave in accordance with the qualitative predictions of the rearrangement theory. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

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Thesis (Ph. D.)--University of Wisconsin, 1981. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 119-127).

Rectangular beams of high density single-crystal UO₂ were prepared from large grains of fused UO₂.They were deformed under constant load in four-point bending in hydrogen at temperatures from 1340 to 1420°C. The results appear to fit a creep equation of the form (formula omitted) where e = steady-state creep rate (formula omitted) Q = 118 ± 23 kcal/mole C = constant ; and 3 < n <4 in the regions of stress and temperatures examined. Faint slip lines were observed. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

An investigation of creep deformation in pure zirconia, using a creep-in-bending method with programmed temperature increase, showed the occurrence of superplasticity at temperatures near the monoclinic to tetragonal phase transformation. An interesting phenomenon was observed in a temporary halt to the creep process which occurred with continuing temperature increase beyond the phase transformation. Temperature dependence tests at a maximum fibre stress of 3140 psi gave overall activation energies for the creep process in pure zirconia in the monoclinic and tetragonal phases of 44 and 103 kcal/mole respectively. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

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