The influence of Si on precipitation phenomena and age hardening of a beta Ti alloy

The phase transformations occurring in a quenched and aged β-phase (bcc) Ti-40 at. pct V+1 at. pct Si alloy have been investigated using transmission electron microscopy. Upon aging at 570°C, the most probable precipitation sequence is the following: bcc super-saturated solid solution»bcc zones»(Ti, V)_xSi_y (hexagonal)»(Ti, V)₃Si (tetragonal). The bcc precipitates may be considered Si-rich GP zones which are homogeneously nucleated and coherent with the matrix. The (Ti, V)_xSi_y particles are a rod-shaped transition phase aligned along the <100> matrix directions. The precipitation reaction causes two peaks in the dependence of the yield stress or hardness upon aging time. The particle diameter and interparticle spacings of the hexagonal silicides indicate that these precipitates are responsible for the second hardness peak. The bcc zones evidently cause the first hardness peak at short aging times at 570°C.     

Precipitation at interphase boundaries

Three problems in precipitation at interphase boundaries are examined. 1) The classu argument as to the particular phase in which such a precipitate nucleates is shown to be irrelevant; except in a special situation, the critical nucleus must normally penetrate both phases forming the interphase boundary. 2) The relative penetrations into the two phases achieved during growth can be very different than those expected during nucleation; hence, deductions about the nucleation process based upon observations on growth morphologies can be quite misleading. 3) The observations of Honeycombe and others that the nucleation of carbides at austenite: ferrite boundaries occurs predominantly at the low energy, immobile broad faces of ledges rather than at the higher energy, mobile risers of ledges are accounted for theoretically on the basis of the high velocities of the risers preventing nucleation. Example calculations on a Ti-Ni alloy indicate that precipitation at the risers of ledges may become possible in substitutional systems, but only at lower homologous temperatures, and if the migration of these boundaries is still controlled by volume diffusion while nucleation is controlled by interfacial diffusion.     

Compression of map images by multilayer context tree modeling

We propose a method for compressing color map images by context tree modeling and arithmetic coding. We consider multicomponent map images with semantic layer separation and images that are divided into binary layers by color separation. The key issue in the compression method is the utilization of interlayer correlations, and to solve the optimal ordering of the layers. The interlayer dependencies are acquired by optimizing the context tree for every pair of image layers. The resulting cost matrix of the interlayer dependencies is considered as a directed spanning tree problem and solved by an algorithm based on the Edmond's algorithm for optimum branching and by the optimal selection and removal of the background color. The proposed method gives results 50% better than JBIG and 25% better than a single-layer context tree modeling.     

The comparison of structure and properties in DC magnetron sputtered and HiPIMS W-C:H coatings with different hydrogen content

The comparison of the structure, composition, hardness and friction behavior of DCMS and HiPIMS W-C:H coatings made by hybrid reactive deposition in the presence of C₂H₂ and H₂ in Ar atmosphere revealed a transition from textured to columnar-like structures with the increased addition of acetylene in DCMS coatings whereas only amorphous-like structures were produced by HiPIMS. The hybrid sputtering involves acetylene and hydrogen dissociation and recombination reactions in the plasma resulting in the generation of carbon as well as –CH₃ and =CH₂ radicals which are chemisorbed at the growing coating surface. The concentration of hydrogen in carbon matrix seems to be controlled by –CH₃ and =CH₂ radicals and not by the amount of molecular hydrogen. The main advantage of HiPIMS compared to DCMS technology includes coefficients of dry friction below 0.1 attributed to the formation of lubricious transfer layer at moderate level of hardness (around 20?GPa) over the studied acetylene additions range. Hybrid HiPIMS technology seems to offer possibilities for substantial COF decrease without hardness degradation which results in widening of the potential of W-C:H coatings for practical engineering applications.     

The effect of different ternary mixtures of sodium phosphates on hardness of processed cheese spreads

The aim of this study was to describe the dependence of hardness of processed cheeses on the proportion of disodium hydrogenphosphate (DSP), tetrasodium diphosphate (TSPP) and/or sodium salts of polyphosphate (POLY) in ternary mixtures of emulsifying salts. The samples were observed during a 30‐day storage period (at 6 °C). On the second day of storage, hardness of the samples with the individual DSP, TSPP or POLY were in the range of 1.65–1.83 N, 2.42–2.81 N and 5.98–6.53 N, respectively. Within zero or a very low proportion of POLY in the mixture, hardness of the processed cheeses increased rapidly (up to 14 N) at a specific ratio of DSP to TSPP in range of 1:1–3:4. Hardness of the samples containing the above‐mentioned specific ratio was decreasing with the rising content of POLY (up to 60%) in the ternary mixtures. Within the prevailing content of POLY in the ternary mixtures (more than 60%), the phenomenon of a specific ratio of DSP to TSPP was no longer observed. With the increasing storage period (up to 30 days), hardness of the processed cheeses was slightly rising (in range of 2–4 N).     

Micromechanical multiscale model for alkali activation of fly ash and metakaolin

The process of alkali activation of fly ash and metakaolin is examined in the view of micromechanics. Elasticity is predicted via semi-analytical homogenization methods, using a combination of intrinsic elastic properties obtained from nanoindentation, evolving volume fractions and percolation theory. A new quantitative model for volume fraction is formulated, distinguishing the evolution of unreacted aluminosilicate material, solid gel particles of N-A-S-H gel, and open porosity, which is partially filled with the activator. The stiffening of N-A-S-H gel is modeled by increasing the fraction of solid gel particles. Their packing density and intrinsic elasticity differ in N-A-S-H gels synthesized from both activated materials. Percolation theory helps to address the quasi-solid transition at early ages and explains a long setting time and the beneficial effect of thermal curing. The low ability of N-A-S-H gel to bind water chemically explains the high porosity of Ca-deficient activated materials. Micromechanical analysis matches well the elastic experimental data during the activation and elucidates important stages in the formation of the microstructure.     

Effect of gamma irradiation on properties of hardened cement paste

Carbonation of hydrated cement paste (HCP) under γ-irradiation is researched in this paper. It is supposed that radiation-induced carbonation takes part simultaneously with “natural” carbonation. “Natural” carbonation is governed by diffusion and therefore takes part in the material close to surface only, whereas radiation-induced carbonation manifests in all irradiated material. The hypothesis has been tested by means of three sets of experiments. Within these experiments the following changes were investigated: (a) changes of composition of hydrated cement paste (transformation of portlandite to calcite), (b) changes of porous space [changes of average pore diameter (APD)], (c) changes of mechanical properties (hardness). HCP was analysed in the surface layer as well as in the centre of samples. The results show clearly that radiation at least accelerates carbonation driven by diffusion and support a hypothesis that besides natural carbonation there is also independent carbonation caused by γ-irradiation.     

Microstructure and Properties of HVOF-Sprayed WC-(W,Cr)<Subscript>2</Subscript>C-Ni Coatings

The composition WC-(W,Cr)₂C-Ni is one of the standard compositions used for the preparation of thermally sprayed coatings by high velocity oxy-fuel (HVOF) spraying. Surprisingly, this composition has been poorly investigated in the past. Frequent use of commercial designations WC-‘CrC’-Ni, WC-Cr₃C₂-Ni, and WC-NiCr indicates the insufficient knowledge about the phase compositions of these powders and coatings. The properties of these coatings differ significantly from those of WC-Co and WC-CoCr coatings. In this paper, the results of different series of experiments conducted on HVOF-sprayed WC-(W,Cr)₂C-Ni coatings are compiled and their specific benefits pointed out. The focus of this study is on the analysis of the microstructures and phase compositions of the feedstock powders and coatings. Unlike WC-Co and Cr₃C₂-NiCr, WC-(W,Cr)₂C-Ni is not a simple binary hard phase—binder metal composite. The phase (W,Cr)₂C with unknown physical and mechanical properties appears as a second hard phase, which is inhomogeneously distributed in the feedstock powders and coatings. As examples of coating properties, the oxidation resistance and dry sliding wear properties are compared with those of WC-10%Co-4%Cr coatings.