The Creep of Zinc Single Crystals

Abstract

The transient creep of single crystals prepared from 99.999 percent zinc was studied at 35°C and at strain rates of the order of 10−5/min. The results are given by the relation: creep rate=at−n, where a and n are constants and t is the time after applying the load. This is of the form expected from Andrade's equation, but the exponent n had only approximately the value ⅔ found by Andrade and varied from experiment to experiment. An additional instantaneous plastic strain occurs on applying the stress at the start of a creep test. It was found to be proportional to the subsequent transient creep rate (as measured by a in the above equation) in a series of tests made on a given crystal. In addition to the process of recovery from strain-hardening whereby a zinc crystal completely resoftens in a few days at 35°C, there appears to be a rapid, but very limited, stage of recovery, the effects of which quickly disappear upon further deformation. Some crystals exhibited a decrease in plasticity when allowed to rest for about two days at 35°C after a creep test. The development of this ``rest-hardening'' is dependent to some extent on the occurrence of creep prior to the rest period. Zinc crystals were observed to contract with time after removing the load in a creep test. The extent of this contraction varied among specimens and among different tests made with the same specimen and was of the order of 10−5 cm/cm or less. This appears to be the first report of such an after effect in single crystals of a pure metal. The results are discussed in relation to the findings of other workers and are interpreted in terms of recent models of the metallic state.