Defect and Diffusion Forum Vol. 429

Abstract: Considering that velocity of diffusion in a system solute-porous material-solvent depends on several factors, (among them, the concentration differential between “solute” in the porous material and “solute” in the solvent) the diffusion process will finish only when maximum entropy is achieved. Thus, the solute concentration will be equal in the matrix and in the solvent (equilibrium concept). On the other hand, if the velocity of diffusion depends on the differential of concentration, then, the amount of materials transferred per time unit (diffusion rate) will diminish as the process goes on. Moreover, when the final concentration of solute in the porous material is desired to be lower than that of the one-stage-equilibrium, then n-more stages must be added. Thus, the decision to choose a process with one or more stages, as well as the end point in each stage (as close or as far as equilibrium) will determine processing time and the use of other resources, i.e. amount of solvent, installation size, financial investment and so on. Therefore, the objective of this study is to develop a tool that helps to optimize these decisions by using a numerical approach.

Abstract: In the present work, a numerical investigation of the unsteady mixed convection and entropy generation of a nanofluid in an annular cylindrical space is presented using the Buongiorno’s two-phase flow model. It deals with a concentric tube heat exchanger where the inner cylinder rotates with a constant frequency and is maintained at hot temperature, while the outer cylinder is cold. The aim of the present investigation is to highlight the effects of some parameters on the hydrodynamic, thermal and mass behavior of the considered nanofluid as well as on the system irreversibility, namely: the inertia (1 ⩽ Re ⩽ 20), the buoyancy (0 ⩽ Ri ⩽ 5), the mass diffusion (0.1 ⩽ Le ⩽ 10) and the vertical positions of the inner cylinder (-0.4 ⩽ H ⩽ 0.4). Moreover, at specific parameters, an optimal position in terms of heat transfer has been determined. The flow of the nanofluid is two-dimensional and governed by the equations of continuity, momentum, energy as well as volume fraction conservation. After performing a finite element method mesh test and validation with the literature, the Nusselt number and the entropy generation are discussed. The results show that the heat transfer rate and entropy generation increase with increasing values of Richardson and Reynolds number, especially when positioning the inner cylinder in the lower part. On the other hand, the nanoparticles migration under the thermophoretic diffusion decrease with the increase of the Lewis number, which consequent decrease of the heat transfer rate.

Abstract: The purpose of this work is developing of the statistical model of hydrogen diffusion in the crystal lattice of BCC metals with an estimate of the contribution of quantum effects and deviations from the Arrhenius equation. The values of the statistical model calculations of H diffusion coefficients in Fe, V, Nb and Ta are in good agreement with the experimental data. The statistical model can also explain deviations from the Arrhenius equation at temperatures 300-500 K in Fe and Nb. The downward deviation of the diffusion coefficient at 300K can be explained by the fact that the statistical model does not consider the tunneling effect at temperatures below 300K. It was suggested that thermally activated fast tunnelling transition of hydrogen atoms through the potential barrier at temperatures below 500 K provides an almost free movement of H atoms in the α-Fe and V. Using the statistical model allows for the prediction of the diffusion coefficient for H in BCC metals at intermediate temperatures.

Abstract: In view of the fact that X-ray sources present characteristic spectra that make them unique, the spectral fitting technique has proven to play a fundamental role through the use of models that make it possible to reproduce the observed spectrum, thus making it possible to characterize the type of source that gave rise to it. A tool of paramount importance, among others that are currently gaining ground, is the XSPEC software, which is a solid and stable spectral fitting package that allows us to conduct scientific work with high standards of rigor in the analysis of data from astronomical objects in whose processes high energies are intrinsically involved, as is the case of X-rays. In this work we fit and analyze experimental data of two X-ray binary spectra: Cyg X-1 and V 0332+53, with theoretical models in XSPEC to obtain the expected statistics of the best fit through the reduced chi-square (hereafter, χ²) in both astronomical sources. From the results, it can be concluded that in both sources the best fit representing the physical processes occurring in these binaries was achieved, very close to results obtained by other authors using different techniques, contributing to the state of the art of the spectrum of astrophysical processes of high energy binaries.

Abstract: In laser cutting processes, the removal of material is achieved without the application of external force, distinguishing it from traditional machining methods. An additional advantage of laser cutting is the ability to achieve desired surface quality in a single step, eliminating the need for additional finishing processes to smoothen and clean the cutting surface. To ensure the quality of the resulting cuts, a comprehensive understanding of the thermal behavior of the cut parts, influenced by the movement of the laser beam, is essential. The article focuses on the numerical simulation of the laser cutting process of the AISI 304 steel sheets with a thickness of 2 mm to investigate the impact of laser cutting parameters on transient thermal fields and the quality of the resulting cuts. A simulation model was developed and verified through temperature measurements during an experimental laser cutting process using the Bystronic Bysprint 3015 CO₂ Laser Cutting Machine. Numerical simulations in ANSYS software were used to design a working diagram showing the relationship between laser power and cutting kerf width for three different cutting speeds: 2000 mm.min^-1, 4000 mm.min^-1, and 5000 mm.min^-1.

Abstract: In this paper, a numerical study of fluid flow through perforated panels with square holes and open-cell material with cubic cells is presented. Structures with a wide variety of porosities (0.15<φ<0.94) and Reynolds numbers (0.01<Re<6000) are studied. Among the various outcomes obtained, the results indicate that pressure gradient vs Reynolds number exhibits three different forms of variation, including linear (Re<1), nonlinear (1≤Re<4000), and one where the pressure gradient is virtually constant with the Reynolds number (Re≥4000). The results were provided in terms of loss factor, but also of intrinsic permeability and the Forchheimer coefficient. Relationships that connect porosity to the loss factor, intrinsic permeability, and Forchheimer coefficient are also presented. These findings may prove useful in better understanding the flow behaviors in perforated panels and cell metal foams, which have a wide range of applications.

Abstract: An experimental method to calculate average charge of metal ions by electrolysis at different temperatures is proposed. Aluminium undergoes dissolution to the Al³⁺ ions at all temperatures. Iron undergoes dissolution to the Fe²⁺ or the Fe³⁺ ions and copper undergoes dissolution to the Cu⁺ or the Cu²⁺. It depends on temperature and electric current density. Direct electric current value and anode mass decreasing were measured during electrolysis into concentrated NaCl solution in water (5 mol/kg or 23.1%, freezing point equals -22°C, pH 6.5–7.5) at room temperature and 100°C. The average charges of copper, iron, and aluminium ions were calculated using Faraday’s law of electrolysis at electric current density 3,000 A/m² (or 30 A/dm²): +3 for aluminium; +2 for iron; and +1 for copper at room temperature, and +3 for aluminium; +2 for iron; and +1.5 for copper at temperature 100°C. The main condition was z_Al=3. We concluded that calculations of the average metal ions charges, z_Fe and z_Cu, were correct since z_Al=3. The result is as follows: the Al³⁺, the Fe²⁺, and the Cu⁺ ions dissolve into concentrated NaCl solution in water at room temperature; the Al³⁺, the Fe²⁺, the Cu⁺ and the Cu²⁺ ions (50%/50%) dissolve into the solution at temperature 100°C. We have obtained experimentally and by mathematical modelling that aluminium anodes (cylindrical or spherical) dissolve into the solution more rapidly with temperature increasing during electrolysis accordingly to the Arrhenius law, while copper anodes (cylindrical or spherical) dissolve more slowly with temperature increasing from room temperature to temperature 180°C like “inverse Arrhenius law”. Iron electrochemical corrosion rate practically does not depend on temperature below 100°C (and, obviously, up to 180°C) like “zeroth Arrhenius law”. The spherical anode effect is greater than the cylindrical anode effect in 1.5 times.

Abstract: The present investigation has been carried out to study the microstructure evolution and microhardness of the multi-component B–C–N diffusion coatings developed on wrought AISI M2 high-speed steel substrate at 560 and 650 °C for 1 and 4 h for both temperatures. The microstructure of the coatings was studied using scanning electron microscopy, energy dispersion spectroscopy and X-ray diffraction analysis. Vickers microhardness measurements were also performed. The investigations showed that varying conditions of the thermochemical treatment resulted in a variety of coatings which exhibited varying microstructure, phase composition and microhardness.

Abstract: We report on experimental investigations of a Ni_2.36Mn_0.64Ga Heusler alloy, which transforms to tetragonal martensite at cooling below M_s ≈ 271°С. The evolution of lattice constants was tracked by in situ neutron diffraction measurements. It was found that the martensite tetragonality c/a gradually decreases during heating from room temperature to austenite transition start temperature A_s ≈ 272°С. The phenomenon of martensite stabilization was investigated by differential scanning calorimetry utilizing three different protocols of the martensite aging. It was found that the martensite aging at a constant temperature T = 255°С merely shifts the reverse transformation to higher temperatures, while the reverse transformation temperature interval (A_f – A_s) remains the same (≈ 30°C) independently of aging time. On the other hand, a multistep aging at different temperatures starting from T = 255°С not only shifts the reverse transformation temperature, but makes the transformation temperature interval narrower down to A_s – A_f ≈ 10°C.