Operation of near-surface dislocation sources
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I.
THE arrangement
INTRODUCTION
and density of dislocations in the near-surface region affect important properties of materials, e.g., fatigue, stress corrosion cracking, friction, wear, etc. Kramer ~,2reported an increase in the density of dislocations in the near-surface region (i.e., to a depth of - 1 0 0 / x ) in samples that were deformed in tension or in tensioncompression fatigue. This increase in dislocation density in the near-surface as compared with the interior of the sample is defined as the "hard layer" phenomenon. Support for the hard layer phenomenon can be found in investigations in which transmission electron microscopy, 3'4'5 etch pits, 6-~~ and a birefringent technique ~z'~2were used. In a simple tensile test, the dislocation density decreases to a constant value as the depth beneath the surface increases. However, in the case of samples subjected to tension-compression fatigue, the dislocation density decreases, attains a minimum, then increases to a constant value as the depth beneath the surface increases. This increase in dislocation density was determined by an X-ray technique, t3 A double crystal diffraction X-ray technique was also used to determine the density of dislocations of one sign in excess of the density of dislocations of the opposite sign. For example, when the density of positive dislocations is 5 x 108 cm -2 and the density of negative dislocations is 3 x 108cm -2, the density determined by this technique is 2 x 108cm -2. Kitajima and co-workers 6 have conducted extensive dislocation etch pit experiments. They have shown that a higher dislocation density exists in the near-surface region, and they have produced plots of dislocation density vs distance into samples that were deformed by various amounts. They also have determined the activation volume and energy for plastic deformation from the same samples. From the activation volume data it is possible to calculate the spacing between the short range barriers, i.e., the barriers jumped by thermal activation. If the density of forest dislocations is related to the measured dislocation density, and if the forest dislocations are the short range barriers, then the density of the short range barriers (SRB) would be comparable to the measured dislocation density, making it possible to check values reported for the activation volume. It has been argued that this increase in dislocation density is due to a strong adherent oxide layer on the metal. ~4How-
ever, it has been shown that an increase in the dislocation density in the near-surface region occurs in Au single crystals also. 15 One possible explanation for the existence of a hard layer in metals that have a strong adherent oxide layer can be found in a dislocation pile-up model at the oxidemetal interface. The stress field of this dislocation pile-up would activate secondary sources near the surface, which would then result in an increase in dislocation density, i.e., a hard layer. Fourie ~6has obtained data which strongly suggest that the surface region of a deformed sam
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