Graduate Thesis Or Dissertation
 

Low temperature deformation characteristics of high purity zirconium single crystals

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  • High purity single crystals of zirconium were produced and mechanically strained in the temperature range from 4.2°K to 293°K. The crystals were produced by thermal treatment with a specially constructed electron beam zone refiner which provided the abrupt temperature gradient necessary for crystal growth. Specimens were electrolytically machined to have a reduced center section and chemically polished. Both examination under polarized light and Laue back reflection patterns were necessary to select specimens without alpha variants. Single crystals were tested at liquid helium temperature in a cryostat which was constructed for this study. Intermediate test temperatures were obtained with the specimen immersed in the proper refrigerant. The flow stress was determined for strains of 0.02%, 0.1%, and 0.2%. Strain rate was cycled from 0.002 inches per minute to 0.02 inches per minute to obtain data for calculation of activation volumes. The flow stress versus temperature curve was found to level off at a temperature of approximately 120°K which is 330°K lower than the lowest temperature observed previously for the phenomina. A pronounced peak in the curve of activation volume versus temperature was found at 230°K. Two possible explanations are proposed for the peak in activation volume. Inverse strain rate sensitivity was observed in three specimens two of which were tested at room temperature and one in liquid nitrogen. No evidence of basal slip or twinning was found at any temperature. A complete description of the design and operation of the zone refiner and cryostat is given in separate appendixes.
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