Jump-like creep of zirconium at helium temperatures

Jump-like creep of polycrystalline zirconium at helium temperatures has been studied over a wide range of applied stresses. It has been shown that unlike the case in other metals studied, the dependence of the strain and the creep rate of zirconium on the applied stress is nonmonotonic. At stresses exceeding those at which jump-like strain begins (more than 25 kg/mm2) the values of ε and ε decrease as σ grows, and creep is absent when σ ≥ 40 kg/mm2. At stresses above 25 kg/mm2 the strain is observed to undergo jumps which can occur with equal probability at any time during the creep process in the specimen. Characteristic features of the structural state of zirconium have been studied at stresses before and after the onset of jump-like strain. On the basis of the data obtained we conclude that the jump-like strain and the interruption of creep in zirconium are due to mass twinning initiated by large dislocation pile-ups against obstacles. The experimental results are analyzed in terms of present-day ideas as to the nature of the instability of plastic flow in materials at low temperatures.

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