Studies on the Creep Recovery and Annealing of Zinc Single Crystals

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Abstract
Part I is a description of some of the factors involved in the recovery by pure zinc single crystals at 35°C of the ability to creep following plastic deformation. The following observations were made: (1) Complete recovery of the ability to show an initial instantaneous plastic extension occurs much less readily than recovery of the ability to show transient creep. (2) Once the quasi-viscous creep stage is reached no recovery of transient creep occurs when the load is partially or completely removed for two minutes, but removal of only 6 percent of the load for 100 minutes permits partial recovery of transient creep at the full load. A 300 minute ``rest'' at 91.5 percent of the full load resulted in complete recovery of the transient creep at full load, even though creep occurred at the reduced stress. (3) The recovery of quasi-viscous creep at a reduced stress after exposure to a higher stress was also investigated. The recovery was slower, the larger the initial load. (4) The rate of the time-dependent contraction of zinc crystals on removal of the load probably follows a power law in the time. There is also an instantaneous nonelastic contraction which is about ten times larger than the elastic contraction. These results show that the role of thermal fluctuations in activating the units of flow is very different from the role of the stress. The applications of the Eyring rate equation to quasi-viscous creep which have been made in the past are, therefore, not valid. A variable activation energy must be postulated in order to account for the kinetics of creep and recovery. These results are interpreted in terms of the dislocation theory of plastic flow. In Part II a phenomenon is described in which pure zinc single crystals are consistently found to become temporarily hardened by annealing in vacuum above 200–260°C. The hardness thus introduced can be removed by straining the crystal and then allowing it to stand at temperatures below 200°C. The crystal is in this way returned to its normal plastic state. The hardening is not removed by electropolishing the crystal, so is not a surface phenomenon. Though similar to Orowan's thermal hardening effect, it differs by requiring a much higher temperature and by not requiring the addition of impurities.

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