Investigations on the Thermal Cycling Stability of SiFeCr Coated NbtZr

 Refractory metal alloys play an important role as highly thermostable materials. Due to their lack of oxidation stability under elevated temperatures it is necessary to protect them with coatings. The thermal cycling stability of Si20Cr20Fe-coatings on Nb10W2.5Zr was investigated by means of imaging secondary ion mass spectrometry showing the inward-diffusion of oxygen and subsequent preferential precipitation of zirconium oxides at the grain boundaries in the bulk. An unintentional interface layer of high boron content that most likely plays a major role in oxidation protection was found. EDX measurements have been performed to investigate on the phases formed in this complex system.     

SIMS Characterisation of Aluminum-Alloyed Hot Isostatic Pressed Steel

 Recently, interest in steel alloyed with∼1% Al increased. These materials are expected to show a higher resistance against oxidation at high temperatures which will improve their performance when used as high-speed-steels. Furthermore, efforts have been made to combine the advantages of Al-steels with those of powder-metallurgic methods like hotisostatic-pressing (hip). This should result in a better macro- and microscopic homogeneity of the distribution of all compounds, yielding less waste-production and a longer life-time of products. In order to gain deeper insights in the process of “hipping” of the considered material and to see the distribution of both main as well as trace components, 2D and 3D-SIMS measurements of the material were performed before and after having undergone the hip process. The measurements showed enrichments of Na, K, Mg, Ca, and S at the particle boundaries before as well as after the hip-process. In the hipped material, the enrichments are significantly stronger in the spatial domains in-between larger particles. They also reveal, that the concentration of C is not equal in all particles and that the particles are covered with an Al₂O₃-layer which is not dispersed by the hip-process. There were also found small (∼50–100 nm) nitride-precipitates before as well as after the hip-process.     

Quantitative investigation of boron incorporation in polycrystalline CVD diamond films by SIMS

Polycrystalline diamond films have been produced on pre-treated silicon substrate by CVD hot filament method, with B(C₂H₅)₃ added to the gas phase. However, under identical surface conditions, boron incorporation is not homogeneous. In {111} growth sectors, the boron concentration is found to be about 5 times higher than in {100} growth sectors. Moreover, a marked increase in contaminating elements such as aluminium and sodium in regions with higher boron concentrations is detected. Under SIMS fine focus conditions it can be shown that the interface between these two different facet regions is smaller than 0.5 μm. With 3D-depth profile images it can also be shown that the carbon distribution in the diamond layer is not totally homogeneous.     

Important aspects in the formulation of solid–fluid debris-flow models. Part I. Thermodynamic implications

This article points at some critical issues which are connected with the theoretical formulation of the thermodynamics of solid–fluid mixtures of frictional materials. It is our view that a complete thermodynamic exploitation of the second law of thermodynamics is necessary to obtain the proper parameterizations of the constitutive quantities in such theories. These issues are explained in detail in a recently published book by Schneider and Hutter (Solid–Fluid Mixtures of Frictional Materials in Geophysical and Geotechnical Context, 2009), which we wish to advertize with these notes. The model is a saturated mixture of an arbitrary number of solid and fluid constituents which may be compressible or density preserving, which exhibit visco-frictional (visco-hypoplastic) behavior, but are all subject to the same temperature. Mass exchange between the constituents may account for particle size separation and phase changes due to fragmentation and abrasion. Destabilization of a saturated soil mass from the pre- and the post-critical phases of a catastrophic motion from initiation to deposition is modeled by symmetric tensorial variables which are related to the rate independent parts of the constituent stress tensors.     

Size quantized formation and self-assembly of gold encased silver nanoparticles

Mixing aqueous dispersions of thiocyanate ion coated small (< 3.5 nm diameter) gold nanoparticles and EDTA covered larger (> 22 nm diameter) silver nanoparticles, results in the formation of robust gold encased silver nanoparticles; in contrast to using larger (> 11 nm diameter) gold nanoparticles which forms chained structures.     

2D- and 3D SIMS investigations on hot-pressed steel powder HS 6-5-3-8

Processing of steel with powder metallurgical methods such as sintering or hot-pressing have proven to be a powerful tool for the production of industrial parts and for components in the automotive industry. Series of steel-powders (HS 6-5-3-8) produced by gas atomization has been hot-pressed in a graphite tube at temperatures from 820 degrees C to 1050 degrees C. The samples have been characterized with a Secondary Electron Microscope (SEM) due to their porosity and then investigated with 2D- and 3D- SIMS. The spatial distribution of the non-metallic impurities and the covering oxide layer of the single particles has been traced dependent to the pressing temperature.Powders pressed at temperatures higher than 880 degrees C exhibited different precipitation behavior of the impurities and an excessive loss of the covering oxide layer of the single powder particles.     

Pattern formation in granular avalanches

Three new experiments are described which exhibit strong pattern formation in the deposits left by successive granular avalanches. At low flow rates continuous deposition, erosion or rotation gives rise to intermittent avalanche release. Once in motion kinetic sieving of a bi-disperse granular mixture creates a two-layer shear band in which the larger particles overlie the smaller particles. When this is brought abruptly to rest by the upslope propagation of a shock wave a pair of str ipes is “frozen” into the deposited material. Successive releases create a large scale pattern, which strongly reflects the history of the granular flow. At faster deposition, erosion and rotation rates a new flow regime is entered in which intermittency and shock formation ceases, and the associated patterns change. Received Aug. 25, 1997     

Original Article On the frame dependence and form invariance of the transport equations for the Reynolds stress tensor and the turbulent heat flux vector: its consequences on closure models in turbulence modelling

The present paper shows that the transport equations governing second order turbulent closures are form invariant, but remain frame dependent through the emergence of the body force; thus they do not fulfil the principle of material frame indifference as formulated by Truesdell & Noll (1965). However, this frame dependence corresponds to that first discussed by Müller (1972) and today developed in the framework of the new concept of extended thermodynamics. Following this new concept, these relations are consequently incorporated as additional basic balance laws. The results are: 1) in the case of the Reynolds-stress-transport equation, this eliminates the so-called constraints imposed in [15–17, 19] on turbulence models; 2) to ensure the closure of the new set of basic balance laws, closure assumptions can then be considered as proper constitutive equations which must be restricted by the well known constitutive theory principles in extended thermodynamics. Received: April 4, 1996     

A thermodynamic model of turbulent motions in a granular material

This paper is devoted to a thermodynamic theory of granular materials subjected to slow frictional as well as rapid flows with strong collisional interactions. The microstructure of the material is taken into account by considering the solid volume fraction as a basic field. This variable is of a kinematic nature and enters the formulation via the balance law of the configurational momentum, including corresponding contributions to the energy balance, as originally proposed by Goodman and Cowin [1], but modified here. Complemented by constitutive equations, the emerging field equations are postulated to be adequate for motions, be they laminar or turbulent, if the resolved length scales are sufficiently small. On large length scales the sub-grid motion may be interpreted as fluctuations, which manifest themselves in correspondingly filtered equations as correlation products, like in the turbulence theory. We apply an ergodic (Reynolds) filter to these equations and thus deduce averaged equations for the mean motions. The averaged equations comprise balances of mass, linear and configurational momenta, energy, and turbulent kinetic energy as well as turbulent configurational kinetic energy. They are complemented by balance laws for two internal fields, the dissipation rates of the turbulent kinetic energy and of the turbulent configurational kinetic energy. We formulate closure relations for the averages of the laminar constitutive quantities and for the correlation terms by using the rules of material and turbulent objectivity, including equipresence. Many versions of the second law of thermodynamics are known in the literature. We follow the Müller-Liu theory and extend Müllers entropy principle to allow the satisfaction of the second law of thermodynamics for both laminar and turbulent motions. Its exploitation, performed in the spirit of the Müller-Liu theory, delivers restrictions on the dependent constitutive quantities (through the Liu equations) and a residual inequality, from which thermodynamic equilibrium properties are deduced. Finally, linear relationships are proposed for the nonequilibrium closure relations.Received: 21 March 2003, Accepted: 1 September 2003, Published online: 11 February 2004PACS: 05.70.Ln, 61.25.Hq, 61.30.-vCorrespondence to: I. Luca     

Solvent effects on electronic properties from Wannier functions in a dimethyl sulfoxide/water mixture

We present an efficient implementation for the calculation of maximally localized Wannier functions (MLWFs) during parallel Car-Parrinello molecular dynamics simulations. The implementation is based on a block Jacobi method. The calculation of MLWFs results in only a moderate (10%-20%) increase in computer time. Consequently it is possible to calculate MLWFs routinely during Car-Parrinello simulations. The Wannier functions are then applied to derive molecular dipole moments of dimethyl sulfoxide (DMSO) in gas phase and aqueous solution. We observe a large increase of the local dipole moment from 3.97 to 7.39 D. This large solvent effect is caused by strong hydrogen bonding at the DMSO oxygen atom and methyl groups. Decomposing the dipole moment into local contributions from the S-O bond and the methyl groups is used to understand the electrostatic response of DMSO in aqueous solution. A scheme is given to derive charges on individual atoms from the MLWFs using the D-RESP methodology. The charges also display large solvent effects and give insight into the transferability of recent force field models for DMSO.