Papers by Keyword: Ultra Fine Grained Metal

Abstract: Creating a small amount of ultrafine grained metals by severe plastic deformation, for example using equal channel angular pressing, is possible in many research laboratories. However, industrial production of these materials is lagging behind because of the lack of industrially viable severe plastic deformation processes. One attempt to change this situation is based on the concept of incremental equal channel angular pressing developed by the University of Strathclyde and Warsaw University of Technology. The paper describes the path the researchers took to develop the process starting from finite element simulation, through tool design and process implementation, to material characterisation. Examples of various process configurations, which enable obtaining UFG bars, plates and sheets are given and possible future developments discussed.

Abstract: In this study, we develop a multiscale crystal plasticity model that represents evolution of dislocation structure on formation process of ultrafine-grained metal based both on dislocation patterning and geometrically necessary dislocation accumulation. A computation on the processes of ultrafine-graining, i.e., generation of dislocation cell and subgrain patterns, evolution of dense dislocation walls, its transition to micro-bands and lamellar dislocation structure and formation of subdivision surrounded by high angle boundaries, is performed by use of the present model. Dislocation patterning depending on activity of slip systems is reproduced introducing slip rate of each slip system into reaction-diffusion equations governing self-organization of dislocation structure and increasing immobilizing rate of dislocation with activation of the secondary slip system. In addition, we investigate the effect of active slip systems to the processes of fine-graining by using the pseudo-three-dimensional model with twelve slip systems of FCC metal.

Abstract: In this study, the conventional Bailey-Hirsch’s relationship is extended in order to express the increase of critical resolved shear stress due to the lack of dislocation lines in a grain. This model is introduced into a triple-scale crystal plasticity model based on geometrically necessary crystal defects and the homogenization method. A FE simulation is carried out based on the proposed model for FCC polycrystals with different grain sizes. It is numerically predicted that yield behavior of fine-grained metals depends on the initial dislocation density and the initial grain size. Furthermore, yield point drop that is observed in annealed FCC fine-grained metal can be reproduced.

Abstract: The fatigue behaviour of aluminium-magnesium alloys has been investigated in the recrystallized CG state and in an ultrafine-grained (UFG) state after equal channel angular pressing (ECAP). A strong improvement of the fatigue behaviour up to 12 ECAP passes has been found for an AlMg0.5 alloy. The results have been interpreted in consideration of the microstructure of the different states. Additionally, for an investigation of the influence of impurities on the cyclic stability of Aluminium, 3 different AlMg alloys with Magnesium contents of 0.5, 1, and 2 wt.-% have been compared. Total strain controlled fatigue tests have shown an improvement of the cyclic stability with increasing Mg content.

Abstract: The strain rate sensitivity of the aluminium alloy AA6061 has been investigated in a conventional grain sized (CG) state and in two different ultrafine grained (UFG) conditions processed by Equal Channel Angular Pressing (ECAP) for 2 and 6 passes at 100o C. Strain rate jump tests in compression were performed at different temperatures and the strain-rate sensitivity exponent m was determined. The tests were accomplished by microstructural investigations before and after compression testing in CG and UFG conditions. It is shown that all UFG microstructures exhibit strongly increased strain-rate sensitivity (SRS) compared to the CG state. The SRS increases with increasing temperature and is more pronounced for the UFG material processed using 6 ECAP passes. The microstructural investigations show a rather high stability of the grain structure for the UFG conditions up to 250o C. The results are discussed with respect to the relevant deformation mechanisms.

Abstract: It has been revealed that in Iridium influenced be severe plastic deformation (SPD) a ultrafine grained (UFG) structure is formed (the grain size of 20-30 nm), but in the bodies of grains there are practically no defects of structure, however, after irradiation a subgrain structure, (subgrain size of 3-5 nm) is formed, and in the bodies of subgrains there are defects. The subgrain structure was also revealed in UFG Nickel and Copper after SPD (subgrain size of 3-15 nm), but in the latter case the observed boundary region is broader and subgrain are highly disoriented.

Abstract: The dependence of the strain rate sensitivity (SRS) of α-Fe and Al 99.5, as typical representatives of fcc- and bcc-type metals, on the testing temperature and with respect to the microstructure is investigated. In particular, the differences between conventional grain size (CG) and ultrafine grain size (UFG) are pointed out. UFG Al 99.5 generally shows an elevated SRS compared to CG Al 99.5. In case of α-Fe the SRS of the UFG state is decreased at room temperature, but increased at 200 °C, compared to the CG state. It is shown that the SRS also influences the ductility of UFG-metals in tensile tests.

Abstract: The mechanical properties of ultrafine-grained metals processed by equal channel angular pressing is investigated by nanoindentations in comparison with measurements on nanocrystalline nickel with a grain size between 20 and 400 nm produced by pulsed electrodeposition. Besides hardness and Young’s modulus measurements, the nanoindentation method allows also controlled experiments on the strain rate sensitivity, which are discussed in detail in this paper. Nanoindentation measurements can be performed at indentation strain rates between 10-3 s-1 and 0.1 s-1. Nanocrystalline and ultrafine-grained fcc metals as Al and Ni show a significant strain rate sensitivity at room temperature in comparison with conventional grain sized materials. In ultrafine-grained bcc Fe the strain rate sensitivity does not change significantly after severe plastic deformation. Inelastic effects are found during repeated unloading-loading experiments in nanoindentations.