Papers by Keyword: Homogeneity

Abstract: As the ultra high way and long distance pumping concrete became increasingly common, the engineers not only required the concrete have large liquidity but also increasingly concerned about the homogeneity of concrete. However, without effective and convincing method, currently the engineers had to assess and judge the homogeneity just by their sight, feel and experience. In order to test the homogeneity rapidly, exactly and quantitatively, a new device and method was developed relaying on the theory that the barycentre of the concrete which was filled in regular-shape container would deflect away from the shape-centre of the container more or less if the components with different density in concrete separated to stratifications. C30 and C40 concrete whose slump≥180mm and slump flow≥480mm were tested by this method and their homogeneity modulus (C_v) were figured out by the supporting formula. The result showed that the homogeneity of concrete was accredited if 0.90<C_v<1.10, and C_v was closer to 1.00, the homogeneity was better.

Abstract: A number of nonequilibrium nanocrystalline fcc solid solutions Cu_1-x-Ag_x (x = 0,1; 0,2; ... 0,9; 1,0) were obtained by mechanical alloying of powders by the use of high pressure torsion. Chemical homogeneity, microstructure, mechanical properties and thermostability were studied. The obtained alloys were found to be characterized by a positive excess over Vegard law, ~ 20 nm grain sizes, a microhardeness of 4.5-6 times higher than the Cu and Ag one and brittle type of the fracture surface. In situ shear stress vs. strain measurements and energy-power parameters estimation were performed. A decomposition of nonequilibrium solid solution induced by a thermal influence begins from close to room temperatures and finishes completely at heating up to 500°С accompanied by collective recrystallization development.

Abstract: Influence of a nearly room and cryogenic severe deformation of a Cu-Zn powder mixture on a structural and phase transitions and kinetics of mechanical alloying was investigated. A sufficient retardation of structural and phase transformations and low homogeneity of solid solution made at 80K were established in comparison to processing at 273K. A lowering of solution formation kinetics rate is supposed to be due to lock of plastic deformation mechanisms, activated by thermal impact.

Abstract: Al₂O₃/Ni nanocomposite powder was obtained by high-energy mechanical milling starting from a mixture of Al₂O₃ and Ni commercially powders. The Al₂O₃+15%vol. Ni mixture was homogenized for 15 minutes in the Turbula-type blender and then was milled in a planetary ball mill (Fritsch, Pulverisette 4) under argon atmosphere up to 120 min. Several milling times were used: 10, 30, 60, 90 and 120 minutes respectively. The evolution of the powders during milling and the stability of the composite phases were investigated by X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersive X-ray microanalysis (EDX). The SEM and OM images show a high level of homogenization of the Ni and Al₂O₃ phases for milling times larger than 90 minutes. The X-ray studies indicate no mixing between the two phases. The crystallite grain size is decreasing with the milling time.

Abstract: Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) is fine-grained cement-based material characterized by high compressive strength (exceeding 150 MPa) and high modulus of rupture (over 15 MPa). The results of strengths depend on the size of the tested element, the loading rate and the boundary conditions during a testing. This type of material is used in a lot of countries (e.g. Germany, France, the USA, Japan, Austria, and the Netherlands) but it is not commonly used in the Czech Republic. The bridge over R10 road from Prague to Mlada Boleslav near Benatky nad Jizerou was the first structure where UHPFRC materials were used for lost shuttering slabs. Due to inhomogeneity of the steel fiber distribution the maximum attained force ranged between 9.6 kN and 25.7 kN for different lost shuttering slabs. The amount of steel fibers was very low at a tension zone at slabs with the lower load-bearing capacity. Steel fibers in these slabs were at the bottom of the formwork. The inhomogeneity of the steel fiber distribution was the foundation for producing of functionally layered beams with controlled inhomogeneity due to the mixtures with different fibers volumes. The results and behaviors of the layered beams and the homogeneous beams are presented in this paper.

Abstract: This paper describes the mixing process and homogeneity analysis of a newly developed binder system based on waste polystyrene (PS) and palm kernel oil (PKO) to produce feedstock for metal injection molding (MIM). Since mixing is a critical step in MIM process, hence the mixture of powder and binder should be homogeneous and injectable. In this study, water atomised Stainless Steel powder was mixed with a new binder system consisting of waste polystyrene and palm kernel oil in a Brabender Plastograph EC rotary mixer. Several tests were performed to assess the homogeneity of the feedstock that was produced at 60 vol.% powder loadings. The 60 vol.% was chosen because the Critical Powder Volume Concentration (CPVC) of the SS316L powder was found to be 64.8 vol.%. The tests conducted were density, binder burn-out and SEM morphology observation. It was found that the feedstock shows good homogeneity and suitable for further processing in MIM.

Abstract: High-pressure torsion (HPT) was conducted on disks of commercial purity Ti under applied pressures of 3 and 6 GPa. Measurements of the Vickers microhardness showed improving hardness homogeneity with increasing numbers of HPT turns. Transmission electron microscopy demonstrated that a higher HPT pressure leads to a smaller grain size after straining and these grains contain a high dislocation density with arrays of twins. This is consistent with the higher hardness of the Ti samples processed by HPT under 6 GPa pressure.

Abstract: Equal-channel angular pressing (ECAP) has been established as an effective method to achieve ultrafine-grained materials and ECAP combined with the Conform process (ECAP-Conform) is considered a promising candidate to realize continuous processing by ECAP. This paper describes experiments conducted on samples of an Al-6061 alloy in order to evaluate the homogeneity development of the alloy processed by ECAP and ECAP-Conform. The samples of the Al-6061 alloy were processed by ECAP and ECAP-Conform at room temperature up to 4 passes via route B_C. The microhardness measurements were recorded both in the cross-sectional planes and along the longitudinal directions of the samples. The results show the alloy obtained significant strengthening using both processes. There was reasonable plastic isotropy after 4 passes of ECAP-Conform in both the cross-sectional plane and the longitudinal direction and the alloy showed a homogeneous microhardness distribution in the cross-sectional plane after 4 passes of conventional ECAP.

Abstract: Applying continuous casting to magnesium alloys is one of the recent issues due to relatively less defects inside the as-cast billet. In this study, homogeneity in the chemical composition of a commercial continuously-casted ZK60A billet was investigated. Density distribution depending on the distance from the center of the billet was measured, and the difference in density was analyzed based on the measured chemical composition.

Abstract: Halbach magnet array has advantages of its small size and yokeless, so it has been widely used in various areas. In this paper, two Halbach permanent magnet arrays for small-size NMR with cube and octagonal shape, respectively, were compared in parameters of average magnetic field strength, field homogeneity and volume with the same material consumption. It was found that the octagonal shape magnet has better performance than the cube one. Then the field homogeneity optimization for octagonal magnet was performed. By analysis of magnetic field strength, the best homogeneity in 5mm DSV (Diameter of Spherical Volume) with 133.4ppm was achieved when the height of magnetic blocks is 16.25mm. After optimization, the magnetic field homogeneity has been significantly improved when the shape of magnet has small volume. This result laid a foundation for the design and manufacture of the octagonal Halbach permanent magnet array in small size.