Effects of Unidirection/Bidirection Torsional Thermomechanical Processes on Grain Boundary Characteristics and Plasticity of Pure Nickel

The “torsion and annealing” grain boundary modification of pure nickel wires with different diameters was carried out in this paper. The effects of torsional cycles as well as unidirectional/bidirectional torsion methods on grain boundary characteristic distribution and plasticity were investigated. The fraction of special boundaries, grain boundary characteristic distributions and grain orientations of samples with different torsion parameters were detected by electron backscatter diffraction. Hardness measurement was conducted to characterize the plasticity. Then, the relationship between micro grain boundary characteristics and macro plasticity was explored. It was found that the special boundaries, especially Σ3 boundaries, are increased after torsion and annealing and effectively broke the random boundary network. The bidirectional torsion with small torsional circulation unit was the most conducive way to improve the fraction of special boundaries. The experiments also showed that there was a good linear correlation between the fraction of special boundaries and hardness. The plasticization mechanism was that plenty of grains with Σ3 boundaries, [001] orientations and small Taylor factor were generated in the thermomechanical processes. Meanwhile, the special boundaries broke the random boundary network. Therefore, the material was able to achieve greater plastic deformation. Moreover, the mechanism of torsion and annealing on the plasticity of pure nickel was illustrated, which provides theoretical guidance for the pre-plasticization of nickel workpieces.

Investigation on the Microstructures and Mechanical Properties of Friction Stir Processed 2A14 Aluminum Alloy Fabricated by Different Initial Precipitation States

6 mm thick 2A14 Al alloy plates were friction stir processed with different initial precipitation states of as-cast, homogenization, rolling and T6. The results indicated that FSP can dramatically reduce the grain and particles size and promote the formation of fine recrystallized grains with random orientation in the stirred zone (SZ). No significant influences of the initial precipitation states of base metal (BM) on the grain size, grain boundary characteristic, texture component and texture intensity of SZ were perceived. Rather, the initial precipitation states can evidently affect the morphology and distribution of precipitates and dislocations. The dominant C and A1* texture components developed in the SZ are correlated with the shear deformation and dynamic recrystallization. And the weakened texture intensity created in the SZ after FSP also signifying that discontinuous dynamic recrystallization might be involved. Compared with the corresponding BM, the SZ fabricated by the BM under as-cast and homogenized states were strengthened arises from the obvious refined grains, uniform dispersed particles and elimination of casting defects caused by FSP, while the softening of SZ were observed for the BM under rolled and T6 states, which are mainly dependent on the reduction of dislocation density and dissolution/coarsening of fine precipitates during FSP.

Taxonomy, ecology and analysis of type material of some small Encyonopsis with description of new species in Ireland

The Encyonopsis microcephala-complex occurs in oligo-mesotrophic freshwaters and is often abundant in the periphyton of low nutrient lakes of high conservation value. It is globally distributed and increasing in apparent diversity with the discovery of additional species. To better differentiate this taxonomic aggregate, we re-examined the original type material of E. alpina, E. angusta and E. tavirana with light (LM) and scanning electron (SEM) microscopy. We investigated the morphology, ultrastructure and ecology of Encyonopsis populations from Ireland, which included previously unreported taxa and morphodemes of E. krammeri, E. neerlandica and E. subminuta. The apparently cosmopolitan E. microcephala was not found in collections from Ireland but two new species were described and compared to others in the complex. Encyonopsis carraensis sp. nov. was associated with a marl-precipitating community in low nutrient lakes, while Encyonopsis hibernica sp. nov. occurred in more productive waterbodies. Several Encyonopsis species had environmental optima near the oligo-mesotrophic boundary characteristic of the microcephala-complex. However, the niches of others were distributed along a wider gradient of water quality. Therefore, when Encyonopsis populations are prevalent in epilithic communities, the use of a more refined taxonomy is recommended for lake ecological assessment.