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[en] In this study, the mechanical properties and corrosion resistance of LA143 alloy specimens produced by severe plastic deformation (SPD) were investigated. It was found that SPD was an effective way to simultaneously improve the Vickers hardness and corrosion resistance of LA143 alloy specimens in 3.5 mass% NaCl aqueous solution.
[en] The present study has used response surface methodology (RSM) and central composite design (CCD) for modelling, using wear parameters to predict the wear performance of an Al 8011/6.0 vol.% AlB2 composite. The effect of applied load and sliding velocity was studied at five levels for a fixed sliding distance. To understand wear behaviour, sliding wear tests were planned according to CCD and performed on a pin-on-disc apparatus at ambient temperature. An analysis of variance (ANOVA) was conducted to show the relative significance of the parameters. A second-order regression model was developed to predict the wear loss and to establish the relationships between wear parameters. Response surface and contour plots were drawn to analyse the wear results. Worn surfaces were examined under scanning electron microscope (SEM), and energy dispersive spectroscopy (EDS) was used to interpret the operative wear mechanisms. Validation tests results show good agreement between experimental and predicted data. As an initial step of this study, AlB2 particles were reinforced in Al 8011 alloy by an in situ technique to synthesise an Al 8011/6.0 vol.% AlB2 composite. During synthesis an in situ reaction takes place between molten alloy and inorganic salt KBF4 at 850 °C, which leads to the formation of AlB2 particles. The composite was analysed by x-ray diffractometer (XRD) to detect the phases present, while optical and scanning electron microscopy (OM and SEM) were carried out to ascertain morphology and particle distribution. Hardness was evaluated by a Vickers hardness testing machine. (paper)
[en] Geopolymer is an incredible alternative green cementitious material which has ceramic-like properties, but does not require calcining that leads to reduction in processing energy usage. The purpose of this research is to study the correlation between kaolin: liquid ratio with the performance of kaolin geopolymer. Kaolin, a prominent raw geopolymer material was used to prepare enhanced geopolymer paste by mixing with alkaline activator solution. Interrelationship of kaolin to alkaline liquid ratio with hardness and flexural strength was the focus of this work. Therefore kaolin geopolymer paste with varying solid to liquid ratio ranging from 0.7 to 1.1 was prepared. Geopolymer paste was coated on low grade wood substrate prior to Vickers hardness and flexural strength. X-ray diffraction was conducted on geopolymer paste itself after 7 days to analyze the change in phase identification at early age. Kaolin geopolymer coating on wood with solid/liquid(S/L) ratio of 0.7 shows the most promising hardness and flexural strength of 15.3 Hv and 94.73MPa. X-ray diffraction test showed high existence of kaolinite on higher S/L ratio where as sodalite was observed in S/L ratio of 0.7. Microstructural studies also compliments our finding which further proves the positive dependency of S/L ratio and kaolin geopolymer strength. (paper)
[en] Problems of partitioning a finite set of Euclidean points (vectors) into clusters are considered. The criterion is to minimize the sum, over all clusters, of (1) squared norms of the sums of cluster elements normalized by the cardinality, (2) squared norms of the sums of cluster elements, and (3) norms of the sum of cluster elements. It is proved that all these problems are strongly NP-hard if the number of clusters is a part of the input and are NP-hard in the ordinary sense if the number of clusters is not a part of the input (is fixed). Moreover, the problems are NP-hard even in the case of dimension 1 (on a line).
[en] Pure hydroxyapatite(HAP) and 0.5,1,2,3,5wt% carbon nanotubes(CNTs) reinforced HAP which mixed by means of magnetic stirring method were consolidated using a spark plasma sintering(SPS) technique at SPS temperature 1273K, pressure 40MPa, and holding time 5min. The mechanical properties of pure HAP and the composites, such as hardness, flexural strength, and fracture toughness were measured. It is demonstrated that 1wt% of CNTs showed the best performance, which density was 95.78%, the Vickers hardness values was 462HV, flexural strength was 69.2MPa. Enhanced strength and toughness are attributed to the pull-out of CNTs and interfacial bonding mechanism between which and HAP during crack propagation. The flexural strength of the composites with 1 to 3wt% of CNTs were at a high level. When the content of CNTs is excessive, the mechanical properties of the material, especially flexural strength, will decrease quickly. (paper)
[en] Friction stir welding is essentially a solid state joining process and this method is being used for joining variety of metals and alloys including Al alloys, Mg alloys and steels. This joining process eliminates several welding defects such as hot cracking and porosity in the welding zone. It is well reported that the there is a large variation in the grain size and microhardness in the friction stir welded zone and these parameters adversely affect the mechanical properties of the welded joint. The objective of the present work is to determine the nature of microstructure, microtexture and variation of composition in the welding direction. The paper presents details of microstructural changes at various regions in the weld zone
[en] Highlights: • Nanograined Cu–Cr composite was produced by a method combining MM, MA and SPS. • Pre-milling Cr can efficiently decrease the size of grain and reinforcement. • Pre-milling Cr can lead to remarkable strengthening in Cu–Cr composite. • Yield strength and hardness of the composite are 1049 MPa and 327 HV respectively. • Mechanisms of microstructural changes and strengthening of composites were studied. - Abstract: Nanograined Cu–8 at.% Cr composite was produced by a combination of mechanical milling (MM), mechanical alloying (MA) and spark plasma sintering (SPS). Commercial Cu and Cr powders were pre-milled separately by MM. The milled Cu and Cr powders were then mechanically alloyed with as-received Cr and Cu powders respectively. After milling, the powder mixtures were separately subjected to SPS. It was found that pre-milling Cr can efficiently decrease the size of grain and reinforcement, resulting in remarkable strengthening. The grain size of Cu matrix was about 82 nm after SPS. The Vickers hardness, compressive yield strength and compression ratio of the composite were 327 HV, 1049 MPa and 10.4%, respectively. The excellent mechanical properties were primarily attributed to dispersion strengthening of the Cr particles and fine grain strengthening of the Cu matrix. The strong Cu/Cr interface and dissolved Cr atoms can also contribute to strengthening of the composite.
[en] The mechanical properties of severely hot rolled Mg–1.6Gd (wt.%) alloys, such as hardness, ultimate strength, yield strength and ductility have been studied in the context of biodegradable implant materials. Unidirectional rolling (UR) and cross rolling (CR)were applied to prepare Mg–1.6Gd alloy ingots. In general, the mechanical properties of the alloy greatly improved after hot rolling due to a refinement of the microstructures. The relationship between the microstructure and mechanical properties is discussed in detail. Hot rolled samples were prepared in the temperature range of recrystallization(400–550 °C)with a speed of 10 mm min−1 and a total reduction of 95% at 23.75% per pass, with an aim to characterize mechanical properties of the Mg–1.6Gd alloy such as hardness, ultimate strength yield strength and ductility. Tension and hardness testing were carried out and theVickers hardness values for the hot rolled samples were about 40–50 HV. The Vickers hardness increased with increasing hot rolling temperature. The maximum tensile strength and yield strength observed forUR samples were 197 MPa and 157 MPa, respectively and for CR samples, the values were 164 MPa and 107 MPa, respectively. Further, the maximum elongation for UR samples was 26% and, 17% for CR samples. Samples hot rolled with 95% reduction showed a higher ultimate tensile strength and ultimate yield strength than samples hot rolled with 30% reduction. The maximum elongation also differed between the two rates of reduction, where 95% reduction yielded an elongation of 26%, while a 30% reduction resulted in a maximum of 15%. The mechanical properties reported in this work highlight the benefits of Mg–1.6Gd as material for use in degradable implants. (paper)
[en] The instrumented indentation method requires the sample surface to be flat and smooth; thus, hardness and elastic modulus values are affected by the roughness. A model that accounts for the isotropic surface roughness and can be used to correct the data in two limiting cases is proposed. Suggested approach requires the surface roughness parameters to be known. (paper)
[en] Highlights: • We proposed a new REBCO coil structure for Maglev in post study. • In actual use, the magnets receive electromagnetic agitation and mechanical force. • We conducted the repeated bending tests of these REBCO pancake coils. • The coils have no deterioration even if the repeated bending deformation was applied. • We confirmed the durability of the coils manufactured by the method that we proposed. - Abstract: In the past study, two manufacturing methods were developed that can manufacture pancake coils by using REBCO coated conductors. It was confirmed that the conductors have no electric degradation that caused by the manufacturing method. The durability evaluation tests of the pancake coils were conducted as the final evaluation of the coil manufacturing method in this study. The repeated bending deformation was applied to manufactured pancake coils in the tests. As the results of these tests, it was confirmed that the pancake coils that were manufactured by two methods had the durability for the repeated bending deformation and the coils maintained the appropriate mechanical performance and electric performance. We adopted the fusion bonding method as the coil manufacturing method of the HTS magnet Furthermore, using the prototype pancake coil that was manufactured by the fusion bonding method as a test sample, the repeated bending test under the exited condition was conducted. Thus it was confirmed that the coil manufactured by the fusion bonding method has no degradation of the electricity performance and the mechanical properties even if the repeated bending deformation was applied under the exited condition.