The stress relaxation of zone-refined iron at 77°K

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Abstract
The stress relaxation of zone-refined iron is found to be logarithmic with respect to time at 77°K. From consideration of the effect of impurity, the strain dependence above the macroscopic yield stress, and the activation volumes derived for the relaxation mechanism, it is concluded that the same rate controlling mechanism is common to the plastic yield, creep and stress relaxation of iron. The relaxation rate is strongly dependent upon the stress below the yield stress, and almost independent of stress above it. Stress relaxation is shown to be dependent upon the availability of free segments of dislocations, for pinning of dislocations at 300°K restricts completely the logarithmic form of relaxation at 77°K. The stress relaxation can also be restricted by defects produced by plastic deformation at 77°K, but in this case logarithmic relaxation is resumed after a short time. Evidence of the mobility of a species of defect at 77°K is provided by the recovery of the elastic limit, the recovery of the static Young's modulus, and the increase of the Haasen-Kelly effect.