Materials Science Forum Vols. 584-586

Abstract: Application of electroplastic deformation (EPD) by rolling to bulk long-sized samples of Ti-50.7 at.%Ni alloy allows increasing of the deformation strain without macrofailure by 1.5 to 3 times in comparison to cold rolling without electrical current application. Structure formation and functional properties were studied after various EPD regimes: current density (84 to 168 А/mm2) and impulse duration (80 and 160 ms). When the stage of mixed nanocrystalline and amorphous structure formation is reached as a result of EPD, a post-deformation annealing at 400 °С leads to a nanocrystalline structure formation in austenite and highest recovery stress values generation by the martensite.

Abstract: The corrosion behaviour of the aluminium alloy, AA6082, processed by equal-channel angular pressing (ECAP) after different passes (route E, room temperature) was studied in comparison to the coarse-grained counterpart. The results of the electrochemical investigations (cyclovoltammetry; electrochemical impedance spectroscopy, EIS) are presented in correlation with the microstructure before and after the corrosion examinations. Both, chemical (precipitations, phases) and physical (dislocations, high-angle grain boundaries, grain size, low-angle grain boundaries) inhomogeneities characterize the microstructure of this commercially used Al-Mg-Si alloy. Results indicate an improved resistance against pitting of the ECAP material expressed by a reduced pitting density of up to 50 % and lower pit depths. EIS measurements and microstructural examinations (scanning electron microscopy, transmission electron microscopy, 3D topography measurement) confirm that ECAP modifies the number, size and distribution of these inhomogeneities, which leads to a more favourable corrosion behaviour.

Abstract: The influence of various processing routes (casting, extrusion, ECAP) on microstructure evolution and electrochemical properties of the magnesium alloy AZ80 were investigated. Both extrusion and ECAP were found to result in significant grain refinement (by a factor of 100 in the ECAP alloy) of the as cast AZ80 alloy. The electrochemical impedance spectroscopy was used to characterize the electrochemical properties of the surface of different microstructure states of the alloy. After the exposure to the corrosion environment containig aggressive chloride ions (0.1M NaCl) a complex polarization layer consisting of two different layers was formed on the specimen surface. The first compact layer directly adjoined to the specimen surface while the second porous layer was formed on the first layer. Microstructure changes due to extrusion and equal channel angular pressing were found to influence the stability of these layers. In the as cast alloy the compact layer was broken already during the first 24 hours of the exposure, while the compact layer in the extruded specimen remained unchanged up to 72 hours of the exposure. In the ECAP specimen both layers remained stable even after 96 hours of the exposure and no degradation occurred. Enhanced electrochemical properties were therefore found in the ultrafine-grained ECAP specimen.

Abstract: In the present work high pressure torsion (HPT) was imposed on commercial purity (CP) tungsten at different temperatures of 450 °С and 490 °С to achieve different microstructures and grain boundary misorientation spectra (GBMS). After HPT at 450°С, ultrafine grained microstructure with an average grain size of ~150 nm was developed in the metal. HPT at 490 °С results in an elongated structures with average width of ~500nm. EBSD investigations showed that over 92% are HAGB type in microstructure HPT-processed at 450°С, and in contrast, over 50% of grain boundaries are LAGB in sample processed at 490°С. Annealing at 900°С for 1h, of the sample with homogeneous UFG structure resulted from HPT at 450°C, leads to only limited decrease (~20%) in microhardness.

Abstract: Prepared were p-type Bi2Te3-based thermoelectric semiconductors, having a grain-refined microstructure and a preferred orientation of anisotropic crystallographic structure. Disks with a nominal composition Bi0.5Sb1.5Te3.0 were cut from an ingot grown by the vertical Bridgman method (VBM) and deformed at 473 K under a pressure of 6.0 GPa by high pressure torsion (HPT). The crystal orientation was characterized with X-ray diffraction. The microstructures were characterized by using optical microscopy and scanning electron microscopy. It was found that the HPT disks had a fine and preferentially oriented grain compared to that of the VBM disks. Further, the power factor of the HPT disks was about twice as large as that of the VBM disks. These results indicate that HPT is effective for improving the thermoelectric properties of Bi2Te3-based thermoelectric semiconductors.

Abstract: The paper provides an overview of recent results of the radiotracer investigations of short-circuit diffusion in ultra fine grained (UFG) materials produced by severe plastic deformation (SPD). Different material classes (copper of different purity levels and Cu alloys) are considered. The study is focused on the existence of non-equilibrium grain boundaries after SPD. Although a dominant contribution of common high-angle grain boundaries with very similar diffusivities as those in the corresponding coarse-grained material is established, much faster diffusion rates are also observed experimentally. The nature and kinetic properties of these “high mobility” paths in different materials are investigated and critically discussed.

Abstract: In most of the simulation studies of equal channel angular pressing (ECAP) it has been assumed that materials obey isotropic hardening law. However, in the case of precipitation hardenable alloys, an accurate prediction of the deformation behavior requires incorporation of kinematic hardening model. In this study, the influences of kinematic, isotropic and combined hardening laws on deformation behavior have been investigated. For this purpose, an ECAP die consisting of two 120° channels has been selected, and the effect of hardening law on the strain profile and ram pressure at the final exit channel has been studied. The simulation results showed that the hardening mechanism does not affect the strain profiles extensively; but, when kinematic hardening mechanism was considered the ram pressure decreases significantly due to less hardening of the material during reverse loading in the final exit channel.

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: Ti-50.26at.%Ni shape memory alloy samples were subjected to cold rolling (CR) with true strains encompassing from moderate (logarithmic strain e=0.25) to severe (e=2.1) deformation. СR with e = 0.5 and more initiated a partial austenite amorphization. The evaluation of structural changes in the material during its long-term storage was performed using Vickers microhardness (HV) technique. It was shown that during storage at room temperature up to 9 months, microhardness varied following a dome-shaped trend, thus reflecting commonly encountered interaction between two concurrent time-dependent phenomena, the first responsible for the material hardening, and the second, for the material softening. To represent such phenomena, a simple mathematical model was proposed and experimentally validated.