Materials Science Forum Vols. 584-586

Abstract: Here we report about the microstructure of a metal-polymer composite that was processed by severe plastic deformation. The composite was prepared by compaction of a sandwich made of Al foils and polyethylene films. This aluminum-polyethylene composite was processed by high pressure torsion and the microstructure was characterized by optical microscopy and scanning electron microscopy. Our experimental data clearly show that in the early stage, the deformation is not homogeneous within the sample, indicating that significant softening occurred. However, at larger number of revolution the deformation progressively reaches the sample centre and the final material exhibits an ultrafine grained composite structure.

Abstract: Equal channel angular pressing (ECAP) is a material processing method for developing an ultrafine-grained (UFG) structure by introducing severe plastic deformation (SPD) in a bulk material with no changes in its cross-section. Numerous analytical and numerical studies on equal channel angular pressing have been performed in recent years. The present work focuses on the effects of die geometry width is defined by the angle between two channels Φ, angle on outer corner of die Ψ (or radius R) and angle within internal corner (or radius r) of die on average effective strain after one pass route. Next, there are analyses of strength properties, plastic properties, fracture mechanism, as well as analyses of Cu structure evolution after SPD by ECAP technology, in the paper. The sixteen passes through the ECAP matrix were realized using route C. The following experimental results and their analyses, the biggest increase of strength and microhardness was proved already after 4th pass. Valuation of fracture surfaces shows that after 12th pass plastic fracture is transformed from transcrystalline ductile mixed fracture. After 4th pass, the avarage grain size decreased from initial approximate size 7 µm to 200 nm, whereby the average grain size was changeless after subsequent deformations. Possible mechanism of high-angle boundary nanograins evolution consists of formation of cell structure, subgrains that transform with the increase of deformation into nanograins with big-angle misorientation.

Abstract: In the present investigation, a bimodal structured alloy with ultrafine-grained (UFG) eutectoid matrix embedded with micrometer-grained pre-eutectoid phase was introduced into the hypo-eutectoid Cu-10.8wt.%Al and Cu-11.3wt.%Al alloys by means of pre-pressing heat-treatment, equal-channel-angular pressing (ECAP) and subsequent annealing. Different size of micrometer grained pre-eutectoid phase was obtained by controlling the cooling rate during pre-pressing heat-treatment of the hypo-eutectoid alloy. The tensile deformation behavior of the developed microstructures is characterized by a maximum tensile yield strength up to 800MPa, which is three times higher than that of the un-treated alloy. It is found that the size of the micrometer grained pre-eutectoid phase is critical to the improvement of the bimodal structured alloy. With larger micrometer grained pre-eutectoid phase, no obvious improvement in plastic elongation was observed with the increase of volume fraction of the pre-eutectoid phase from 20% to 40%, but a decrease in the yield tensile strength was observed. An optimal combination of strength and ductility was obtained particularly in those samples embedded with small-sized micrometer-grained pre-eutectoid phase, which provide extra strain gradient hardening effect.

Abstract: Asymmetric rolling has been considered as a possible way to obtain severe plastic deformation (SPD) since it will give an extra shear deformation to the processed materials during rolling. Previous researches have confirmed such a shear deformation. Very recently, the method of inserting-block is used to characterize the shear deformation through direct observation, but when the reduction is more than 70%, the lineation scratched on the side face of internal mark becomes vague and illegible. In order to directly observe the shear deformation of metallic material with large reduction, the internal mark method is employed in this research and asymmetric rolling was performed with pure aluminum and iron at room temperature. In severe plastic deformation, the shear deformation caused by asymmetric rolling was clearly observed and measured through employing internal mark method. Remarkable extra shear deformation during asymmetric rolling was confirmed. Very high equivalent strains were achieved when sheet samples were asymmetrically rolled to high reduction ratio. These strain values fall into the range of SPD.

Abstract: The microstructures of pure Cu processed by equal channel angular pressing (ECAP) from 4 to 24 passes were investigated. It was found that the microstructures of Cu samples with a small number of ECAP passes (4-8) were not inhomogeneous and the fraction of high-angle grain boundary (HAGB) was low (25~43%). While for the samples with many number of ECAP passes (12-24), the grains became more equiaxed-like and the GB misorientations exhibited double-peak distribution with high fraction (51~64%) of HAGB. It was dislocation cells formed in large grains of the few-pass samples, but subgrains in the many-pass samples. These characterizations suggested that ultrafine-grained (UFG) microstructures in the few-pass samples were not fully accomplished, while it was obtained after many passes (>12). It is believed that dynamic recovery during processing for many passes was attributed to the formation of UFG microstructures.

Abstract: Distinctive structure features of rods produced by hot and cold radial forging of a Zr- 2.5%Nb alloy were revealed. Most experimental data were obtained by X-ray methods. Among considered aspects, connected with the undulatory mechanism of energy transfer, there are the uniformity of bulk deformation, structure perfection under hot radial forging, the demolition of secondary phases and the oversaturation of α-Zr with Nb under cold radial forging, the role of phase transformations in deformation, and the suppression of recrystallization by annealing. Structure, texture and properties of products, produced by radial forging from Zr-based alloys, allow to consider this technology as an acceptable alternative to that usable nowadays.

Abstract: Commercially Pure Magnesium initially hot rolled and having a basal texture was deformed by Equal Channel Angular Extrusion (ECAE). ECAE was carried out upto 8 passes in a 90° die following routes A and Bc through a processing sequence involving two temperatures, namely 523 and 473 K. Texture and microstructure formed were studied using electron back scatter diffraction (EBSD) technique. In addition to significant reduction in grain size, strong <0002> fiber texture inclined at an angle ~ 45o from the extrusion axis formed in the material. Texture was also analyzed by orientation distribution function (ODF) and compared vis-à-vis shear texture. A significant amount of dynamic recrystallization occurred during ECAE, which apparently did not influence texture.

Abstract: High purity (99.99, wt %) niobium ingots were received by double electron beam melting technology. The specimens of pure metals with cast microstructure were processed using cold equal-channel angular pressing (ECAP) followed by a heat treatment at around 1080 °C in vacuum furnace for 2 h. Shear bands and microstructure evolution was studied in shear region at first pass and at next passes of ECAP. Microstructure processing characterization was performed using light optical and field emission scanning electron microscopy. Accompanied to microstructure the changes of mechanical properties were determined by micro- and universal hardness testing. The shear bands forming, evolution of large crystals to elongated laminar and dislocation fine-grain structures during processing was characterized and discussed in view of metal hardening-softening and viscoplastic behavior under subsequent hard cyclic straining.

Abstract: Pure palladium (99.95%) was hydrogenated, subsequently deformed by High Pressure Torsion (HPT) and analyzed by differential scanning calorimetry (DSC). For comparison some hydrogen-free HPT processed samples were also investigated. In case of the hydrogenated HPT Pd, the concentration of single / double vacancies is noticeably higher. The importance of hydrogen for the formation and stabilization of vacancy type defects and dislocations is discussed.

Abstract: Annealed pure copper was subjected to equal-channel angular pressing (ECAP) by route Bc for different passes number. Tensile test specimens were manufactured and subjected to hard cyclic viscoplastic (HCV) deformation by means of the materials testing installation Instron 8516 in strain control regime at room temperature. The specimens were cyclically deformed with a frequency of 0.5Hz at different strain amplitudes, step-by-step increased from ±0.05 to ±2.5% for 30 cycles, up to seven test series in this study. The microstructure of ECAP and HCV deformed samples were characterized by optical- and transmission electron microscope, X-ray diffraction, tensile- and hardness testing methods. The ECAP processed metal has mainly elongated subgrains with low-angle grain boundaries and texture, oriented in direction of metal flow during latest pressing. We demonstrate that during HCV deformation the dislocations density of ECAP processed UFG copper was decreased. The ECAP texture was reoriented under cyclic load applied as elongated subgrains were jointed to small pieces under this same angle to axis as texture before. The grain- and crystallite sizes were decreased, which were accompanied with dislocation ribbons forming nearby new formed high-angle grain boundaries. This paper builds on knowledge that the combined treatment by ECAP and followed HCV deformation enable to improve UFG microstructure and ductility with lowering the strength and hardness of UFG metals due to the lower dislocation density while coarse grained copper exhibits increasing the strength and hardness.