Nonelastic strain recovery in zinc single crystals

Abstract

The rapid and time dependent strain recovery which occurs in deformed zinc single crystals has been studied as a function of prestrain, specimen purity and surface condition. The total strain recovery of zinc crystals was found to increase with prestrain for a constant unloading rate or to increase when the surface is copper coated over that for an uncoated crystal at equal prestrains greater than 10 −4. Doping zinc with cadium is found to decrease the amount of total strain recovery as compared to undoped crystals at equal prestrains greater than 10 −5. The recovered strain under various conditions was found to be logarithmic in time at constant applied stress. The time dependent recovery is explainable as a creep recovery process, the inverse of the transient creep behavior discussed by Seeger. The increase of the total strain recovery with prestrain can be accounted for by an increase of dislocation density with prestrain. Only cadmium doped crystals were observed to exhibit a jerky unloading behavior at room temperature and after a certain prestrain. This jerky unloading behavior can be explained in a similar manner to that presented by Cottrell to explain the Portevin-Le Chatelier effect in iron and aluminum alloys. Interpretation of the results is not dependent upon a precise knowledge of the proportion of the dislocations partaking in strain recovery which are held up just beneath the surface of the crystal to those in the interior. The surface effect results suggest, however, that a uniform increase in dislocation density may proceed from the center to the outside of the crystal after prestrain. The internal stress associated with this type of dislocation configuration may be expected to relax almost linearly with the recovered strain.