Etch-Pit Observations of Dislocation Arrangements under Reverse Stress in Copper 9at. % Aluminum Alloy Single Crystals

With a view of solving the cause and essence of the Bauschinger effect, dislocation behaviour under reverse stress has been investigated in Cu-9at% Al alloy single crystals, using an etch-pitting technique. From the direct measurements of the beginnings of backward movements of dislocations, the frictional resistance force to moving dislocations due to solid solution hardening is estimated to be approximately 0.8kg/㎟, which is ≃4/5 of easy glide stress. It is found that pile-up dislocations against a barrier move well under the reverse stress range from 0.6 to 0.7 to the pre-stress level, but then hardly move more than the reverse stress ratio of 0.8. Evidence of almost complete annihilation of double ended pile-ups which are generated by the same source is presented. Another striking evidence of radical annihilation of dislocations within uniformly aligned dislocation groups of the same sign is also discovered. Mechanisms acceptable for explaining such results are proposed respectively, i.e., the mutual annihilation of dislocations of opposite signs, and the double cross-slip mechanism. It is suggested that the characteristics of rearrangements of dislocations against stress reversal are probably connected with the latter mechanism, which would be responsible for cyclic strain hardening

Dynamic Strain Aging and the Bauschinger Effect During Cyclic Deformation in Polycrystalline Silver and Copper Base Solid Solutional Alloys

In order to investigate the characteristics of the P-L effect and to obtain additional information on the production of vacancies during tensile and fatigue deformation, tensile and tension-compression tests were performed in Ag-6.3 at.% Al and α-brass polycrystals. It was found that the P-L effect in Ag-Al alloy is well explained by the dynamic strain aging model by Cottrell. The effective activation energy for the migration of a vacancy in Ag-Al alloy is estimated to be 0.6±0.02eV. It was also confirmed, from the experiment of annealing the excess vacancies, that the P-L effect is strongly affected by the accumulation of vacancies produced by strain. The strain exponent m, of a vacancy concentration produced by strain, was determined to be m＝1.35. This is equal to that obtained from the static strain aging experiment, assuming the strain exponent of total dislocation density to be β＝1. In the cyclic straining conducted under the prescribed strain amplitude, the cumulative strain to the onset of the P-L effect is greater for a lower strain amplitude. However, the values subtracted by the sum of a Bauschinger strain β₁ in each half cycle from the cumulative strains give an approximately constant value independent of the magnitude of the strain amplitude. The values are larger than the critical strain for the onset of the P-L effect in unidirectional deformation by an amount of 20% in both alloys. From the results, it can be concluded that vacancies are produced only in the complete plastic strain region subsequent to the Bauschinger strain β₁ during cyclic straining. Hence, the efficiency of the vacancy production would decrease with an increase in the number of cycles. Namely, the saturation of the vacancy production is to be directly correlated to the saturation of the cyclic strain hardening. Furthermore, it is suggested that the efficiency of the vacancy production during the cyclic straining is lower by 20%, even in the complete plastic strain region, probably because of the vacancy-interstitial annililation