Materials Science Forum Vol. 879

Abstract: Magnesium AZ31 alloy sheets were rolled at 100 °C at a high rolling speed of 1000 m/min. After 30% reduction, the microstructure was heavily twinned and shear banded, while a partially dynamically recrystallized and twinned microstructure was seen at the reduction of 49%. The as-rolled specimens were then annealed at 500 °C for increasing times. Microstructure and texture were characterized by optical microscopy, electron backscattered diffraction (EBSD) and X-ray diffraction (XRD). Texture weakening was found during annealing of the specimens at both reductions. However, the texture weakening was more effective in the fully twinned and shear banded specimen than the partially DRXed and twinned specimen. Effects of as-rolled microstructure on static recrystallization characteristics and texture evolution during annealing were studied.

Abstract: PtRu or Pt catalysts were supported on four types of carbon nanomaterials with different shapes, sizes, and graphitic and electrical properties, and their resulting catalytic activities were evaluated by electrochemical methods. The carbon nanomaterials used included two types of particles: Arc Black (AcB) and Vulcan XC-72R (Vulcan), and two types of nanofibers: carbon nanocoils (CNC) and VGCF-X. Pt and Ru were loaded onto the nanomaterials by a reduction method using sodium borohydride. Transmission electron microscopy and X-ray diffraction (XRD) revealed the PtRu catalyst particles to be 4–6 nm in diameters. The shifts in the Pt (111) XRD peaks of the catalysts on CNC and VGCF-X were larger than those on AcB and Vulcan, indicating a higher degree of alloying between Pt and Ru. The diameters of the CNC-supported Pt and PtRu catalyst particles had the narrowest distributions and were constant within the range of catalyst loadings investigated. Electrochemical studies of the catalysts during methanol oxidation were carried out using cyclic voltammetry. The catalyst particles supported on CNC and VGCF-X exhibited higher catalytic activity than those on AcB and Vulcan. The effect of the surface area of the carbon nanomaterials on the catalytic activity is discussed.

Abstract: Steady state thermal conductivity measuring device was designed to measure the effective thermal conductivity of composites. Computer simulations of thermal conduction revealed that the designed device over estimates the effective thermal conductivity, and the correction coefficient was suggested. With this designed device, the effective thermal conductivities of Al/SiC particle-dispersed composites were measured by changing the size of SiC particles from 0.3 μm to 3 μm. The critical element size which could determine the optimal size of reinforcements have been suggested, and validity of the critical element size for Al/SiC composites was confirmed. The thermal conductivity of the composites including small SiC particles was degraded by the interfacial thermal resistance between the matrix and the reinforcement. On the other hand, the thermal conductivity of the composites including large SiC particles was not affected by the interfacial thermal resistance. These results suggest that consideration of the critical element size is valid.

Abstract: AZ91D magnesium alloy for casting is significantly corroded in chloride environment. In addition, Al-rich-α phase, which is formed during cooling process around β-Mg₁₇Al₁₂ phase, is one of the important factors on corrosion behavior in AZ91D. It is expected that pitting formation is dominant in the corrosion behavior in magnesium alloys. To clarify the influence of the Al-rich-α phase to pitting formation and corrosion behavior in AZ91D alloy, we have investigated the relationship between Microstructure and pitting formation in this study. We have carried out polarization curve measurement and scanning electron microscope (SEM) observation before and after the corrosion-tests. SEM micrograph of as-cast samples showed AZ91D alloy consist of α phase, Al-rich-α phase which shows bright Z-contrast due to high Al contents and β phase. Al-rich-α phase formed around β phase. After the corrosion examination, holes were observed at Al-rich-α phase around the β phase, and it is thought that small pits grow up to be holes, because β phase is remained, while the Al-rich-α phase around the β phase preferentially dissolves with time. The result indicates that Al-rich-α phase around β phase enhance the formation of pitting and preferentially dissolve it in initial stage of corrosion in AZ91D alloy.

Abstract: New low-nickel Cr18Ni5Mn9Mo2N and Cr19Ni6Mn10Mo2N steels can be used up to-170 °C and differs in the highest level of durabilities in the hot-rolled and tempered from austenitic area state that provides its effective application in climatic conditions of the Arctic and Antarctic. Excess of durability over level, characteristic for traditional stainless steel of the Cr18Ni9 type, is provided due to additional solid solution hardening. Alloying with nitrogen to 0,18÷0,22% usual Cr18Ni9 steel has the smaller, but also high level of mechanical properties, differs in smaller thermal and mechanical stability of austenite and can be applied in less rigid on temperature and loadings service conditions. Corrosion resistance of austenitic stable steels free from contaminations is also higher compared to steel with regular cleanliness.

Abstract: Dynamic explicit finite element (FE) analysis of the Charpy impact test was conducted in this study to investigate the inertial effect on the stress field ahead of the V-notch in a Charpy specimen. The deformation behavior of the Charpy specimen and the constraint effect on the stress field in the plastic zone near the V-notch were numerically simulated using three-dimensional FE analysis, while considering the contact of the specimen with the striker and anvil. The effect of the strain rate on the flow stress and the increase in temperature during impact loading were included in the dynamic analysis. This analysis shows that the impact load exhibits oscillation and the contact stiffness between the specimen and the striker affects the oscillation of the impact load. The analysis was validated by comparison with experimental results obtained using an instrumented Charpy impact testing machine, which measured the impact load and the load point displacement. The oscillation of the load–time curve was recorded. The magnitude and period of the peak inertia load obtained by the FE analysis were almost consistent with the experimental results. The contact stiffness between the specimen and the striker affected the stress field near the V-notch in the specimen. This indicates that the stress field in the Charpy specimen should be analyzed by the dynamic analysis procedure considering the contact stiffness based on the Hertzian contact theory.

Abstract: Cast iron is an iron alloy mainly composed of carbon and silicon, the amount of carbon is more than 2.1 mass%. Cast irons, gray cast iron and ductile cast iron, have been used as industrial parts and automobile parts widely because they have a good wear resistance and an excellent machinability. Graphite formation mechanism have been proposed, but, it is not established clearly yet. In this study, the microstructure of flake graphite was investigated to reveal the graphite formation mechanisms using FC250 alloy. Transmission electron microscopy (TEM) samples were prepared using focused ion beam (FIB). In the case of a cross section of flake graphite taken perpendicular to its elongated direction using TEM, internal microstructure of flake graphite was observed layered structure. In the case of a cross section of flake graphite taken parallel to its elongated direction, clear microstructure was not observed. Selected area electron diffraction (SAED) from flake graphite showed <0001> direction of graphite are mostly parallel to their thickness.

Abstract: The superior elastic modulus, stiffness and wear resistance of particulate-reinforced metal composites (MMCs) have drawn much attention in various industries ranging from defence, aerospace and automobile industries. Here, friction stir processing (FSP) has successfully dispersed carbon nanotubes (CNTs) and significantly reduced cavities in selective laser melting (SLM) fabricated AlSi10Mg-CNTs composites. Further grain refinement, was achieved via FSP with the addition of CNTs. This is mainly attributed to the dynamic recrystallization and Zener pinning effect. The addition of CNTs to AlSi10Mg resulted in significant improvement in hardness of SLM fabricated aluminium composites. However, FSP of these samples resulted in reductions in the Vicker’s microhardness. This could be due to the dissolution of hardening precipitates and the absences of fine dendritic network present in SLM fabricated parts.

Abstract: Metal-based electrodes, despite being the most widely used for biomedical applications, are limited by a poor reliable skin-surface interface and patients suffer from comfort issues. The most common problems/inconveniences are caused by stiff electrodes, skin irritation, allergic reaction or corrosion. In order to overcome these problems, we produced and tested flexible electrodes involving biopolymer nanocomposite materials. Conductive polymers have been intensively studied and applied in the field of organic photovoltaics and flexible organic electronics. Recently, the use of conductive biopolymer nanocomposite has also emerged as an interesting and promising material for biomedical applications. In this study, we have designed and characterized electrodes made of a flexible and conductive nanocomposite material using a biocompatible and biodegradable polymeric matrix of poly (3-hydroxyalkanoate) (PHA, in particular poly (3-hydroxybutyrate), PHB) containing conductive nanowires. The biopolymer nanocomposites and their electrical conductivities were investigated by optical microscopy, scanning electron microscopy (SEM) and electrical four-point probing. The electrical conductivities obtained in the different PHA-polymer nanocomposites containing different concentrations of conductive additives is discussed in relation to the nanocomposite structure at the microscopic level. Finally, our developed biopolymer nanocomposite prototype electrodes have successfully been tested for transcutaneous electrical nerve stimulation (TENS) and electrocardiography ECG applications in comparison to conventional electrodes.

Abstract: In view of the demand for electrified long-range vehicles, the performance of traction batteries has to be improved. To achieve a high power density, the battery cells must be interconnected with low electrical resistance within the joints. The joining process has to fulfill specific requirements, as the battery cells may only be exposed to very low mechanical and thermal impacts. Joining by using reactive aluminum-nickel nanofoils represents an innovative technology meeting the abovementioned requirements. These foils are multilayered systems consisting of several hundred alternating monolayers of aluminum and nickel, each with a thickness in the nanometer range. Their unique ability is that they can react with temperatures up to 1500 °C for a duration of a few milliseconds upon external ignition. This thermal reaction energy serves as a heat source in the joining process, melting the materials in the interfacial surface. Subsequently, the joining partners solidify and form an adhesive bond when compressed properly. However, these advantageous characteristics are contrasted by complex reaction mechanisms and an unknown interaction of the process parameters. For the industrial application of the joining technology, the requirements for initiating the exothermic reaction must be known. Therefore, the process window and the mechanism of ignition have to be scrutinized. For this purpose, an experimental test setup was developed to generate and monitor short circuit currents to ignite the nanofoils. The amperage as well as the layer composition of the nanofoils were varied within a parameter study. Two independent process windows for a stable ignition were identified for all analyzed nanofoils.