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UBC Theses and Dissertations

The tensile deformation of pure vanadium single crystals at low temperatures Snowball, Robert Forrester


An investigation of the low temperature tensile properties of vanadium single crystals was carried out, using zone-refined metal. Single crystals of predetermined axial [110] orientation were grown by a melt solidification technique, using an electron beam, floating-zone refiner. Tensile specimens were prepared from these single crystals. A plot of yield stress versus test temperature was found to be discontinuous and consisted of two curves which intersected at -125° C. The plot of log yield stress versus reciprocal temperature yielded two straight lines which also intersected at -125° C. The slip system was identified as {112}, which is different from that found for iron single crystals. X-ray, metallographic and electrical resistance data indicate that the phenomenon is primarily a yield point effect. The results of tensile tests performed on single crystals at a very low strain rate, and on polycrystalline specimens indicated that the temperature dependence of yield stress is itself orientation dependent. Three possible explanations of the unusual temperature dependence of yield stress are given: (1) A change in deformation mechanism occurs, for example, from slip to twinning. (2) A minor ordering reaction occurs. (3) A change in the mechanism by which dislocations are unlocked from their atmospheres occurs; for example, two impurity atmospheres surrounding different dislocations, each impurity showing a separate temperature dependence of yield stress.

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