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

Dislocation Arrays and the Bauschinger Effect in Copper 9 at. % Aluminum Bicrystals

Dislocation arrays in lightly deformed Cu-9 at.% Al bicrystals, whose component crystals have a crystallographic mirror symmetry with respect to the grain boundary, are observed by the etch-pitting technique. Then, the influence of the grain boundary on the Bauschinger effect is discussed. It is shown that the induced secondary slips near the boundary can be explained quantitatively in terms of stress concentration due to the piled-up dislocations of adjacent crystal. Some of them can be also explained by the microscopic incompatibility owing to the mismatch of primary slip bands in each component crystal at the boundary. From the observations of reversed bicrystals, it becomes evident that the induced secondary disloca tions near the boundary, which exist mostly by forming double ended pile-ups between the primary slip bands, are unstable against reverse stress. Namely, most of them are annihilated in the first stage of reverse stressing, and are subsequently recovered by a further reverse stress. It is pointed out that although primary dislocations near the boundary can move even a small reverse stress backwards by the aid of high back stress on dislocations because of the latent hardening by multiple slips and pile-up dislocations, the mean free path of the primary dislocations is thought to be small due to the interference of secondary dislocations. Consequently, the Bauschinger effect in a multiple slip layer is presumably smaller than that in the center of each component crystal

Can the Power of Platforms be Harnessed for Governance?

The platform concept examines how strategic leadership and institutional and technological resources enable multiple distributed activities to innovate, adapt, and change. The central question addressed in this paper is: Can this potentially powerful organizing logic be harnessed for public purposes? Since governance platforms are still largely experimental, we cannot fully settle this question at present. However, we can begin to address the issue to help scholars and practitioners explore the potential of platforms. We start with a general statement about what governance platforms might offer to the public sector, before probing the concept more deeply. We then investigate the institutional mechanisms that purportedly make platforms powerful and propose a typology of governance platforms. Finally, we investigate the challenges and successes they have encountered

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

Plastic Deformation of <111> Oriented Aluminum Single Crystals

It is known that an aluminum single crystal having tensile orientation shows an initial rapid hardening, and that its flow stress increases gradually until failure. However, many problems pertaining to the deformation mechanism of the oriented single crystal remain unsolved. In the present study, to clarify the deformation mode of aluminum single crystals having multiple slip orientations, tensile test were performed at various temperatures on oriented single crystals. At room temperature, the oriented single crystal deformed only by fine multiple slips, and the flow stress increased with an increase of strain until failure. On the other hand, the coarse wavy slips composed of {111} and {100} slips occurred at high temperatures, such as 473K. In the single crystal having a tensile orientation deviated from by a few degrees, the clustered slips were observed in addition to the fine multiple slips. It is concluded that the deformation mode and flow stress of the single crystal are very sensitive to the tensile oriention in the vicinity of

Effect of Deformation Temperature on the Stage IV Deformation in <100> Oriented Aluminum Single Crystal

It is known that at room temperature the tensile stress-strain curve of an aluminum single having tensile orientation becomes flat after about a two percent elongation. (This flat region of the stress-strain curve is named Stage IV). In the previous paper, the propagation of a clustered slip accompanied by a prominent cross slip was observed in the Stage IV region of curve. In the present study, oriented aluminum single crystals were tested in tension at various temperatures so as to clarify the influence of temperature on the deformation mechanisms in Stage IV. At 203K, a few clustered slip lines accompanied by a prominent cross slip occurred, but they did not propagate in the entire region of the specimen. The stress-strain curve became flat only from about a ten percent elongation. In the crystal stretched at 77K, the cluster did not propagate at all from either end of the specimen, and so the stress-strain curve did not become flat. On the other hand, a wavy coarse slip was observed and the curve became flat from about a 0.5 percent elongation at 473K. It was suggested that these wavy slip lines were produced by a frequent repetition of the cross slip on two {111} planes and also possidly on a {110} plane having the same slip direction. It was confirmed that the occurrence of Stage IV is caused by the of the clustered slip accompanied by a prominent cross slip, and is much influenced by the temperature of deformation due to the easiness of the cross slip