Defect and Diffusion Forum Vol. 445

Abstract: This work presents experimental and computational research on the influence of hot isostatic pressing (HIP) of CMSX-4, a second-generation single-crystal nickel-based superalloy, on interdiffusion behavior and the formation of Kirkendall voids in subsequently assembled diffusion couples under isothermal annealing. Two sets of Ni/CMSX-4 diffusion couples were prepared: one using CMSX-4 that had undergone HIP treatment, and the other using CMSX-4 in the as-cast condition. The diffusion couples were annealed at 1250 °C for 144 h. Experimentally measured interdiffusion profiles of all alloying elements, together with synchrotron X-ray 3D μ-tomography of Kirkendall voids, were compared with calculated diffusion profiles and phase-field simulations of void evolution. A direct comparison between the simulation and experimental results enables an explanation of the HIP effect on the density, distribution, and size of the voids, as well as on the observed shifts in the interdiffusion profiles.

Abstract: During high-pressure torsion (HPT), the sample positioned between the plungers of the experimental setup is resistant to fracturing, allowing the HPT process to be sustained almost indefinitely. Despite this, relaxation processes taking place within the sample during HPT lead swiftly to the establishment of a steady state. Factors such as hardness, grain size, the scale of second-phase precipitates, electrical conductivity, lattice spacing, among others, rapidly reach a saturation point, albeit after varying revolutions of the plunger. For instance, in the scenario of HPT involving a binary solid solution accompanied by secondary phase particles that act as sources of dissolved atoms, a dynamic equilibrium and competition emerge between the formation and decomposition of a supersaturated solid solution. Consequently, a specific equilibrium state is achieved with a designated concentration (c_ss) of the second component within the solid solution. This equilibrium state is independent of the initial one (referred to as equifinality). The steady-state concentration c_ss can be identified on the solubility limit line (solvus) of the second component in the phase diagram at an effective temperature T_eff. In copper alloys, the value of T_eff grows as the activation enthalpy for the volume diffusion of the second component increases. This amplification signifies a rise in defect concentration and an activation-driven character of mass transfer during HPT.

Abstract: According to Stokes, the time required for a particle to precipitate depends on several conditions such as sphericity and laminar flow, as well as key parameters like particle size, densities (particle and fluid) and fluid viscosity. Therefore, if any of these conditions or parameters are unknown or not met, it becomes impossible to estimate the precipitation time. Additionally, when the separation under 1-g (Earth gravity) takes days or months but an estimation is needed in just minutes, the separation time at 1-g can be approximated by relating it to conditions at other values of gravity (n-g). For example, in a centrifuge. However, this n-g value is not reached instantaneously but require to consider the acceleration, plateau, and deacceleration phases to obtain a reliable estimation. This task has been addressed before; however, the resulting models tend to be either complex for practical laboratory use, or fail to account for the relationship between distance travelled by a particle under 1-g and the distance travelled under centrifugal forces. Moreover, even during the plateau phase, centripetal acceleration is unsteady because the radial distance of the precipitating particle is constantly changing. Thus, the aim of this study is to simplify the methodology for estimating particle separation time at 1-g by using separation time obtained under the unsteady conditions of centrifugation, even when the properties of the particle and fluid are unknown, through a numerical approach.

Abstract: The objective of this study is to suggest a method for judging jumping periods, which means an atom moves significantly in a short time in liquid metal. In this study, molecular dynamics (MD) simulation of liquid Pb at 773 K was performed. The self-diffusion coefficient was calculated to confirm that the simulation adequately reproduces liquid Pb and was almost consistent with the reliable experimental data. In the evaluation of jumping period, atomic motion during jumping was considered. A method for estimating jumping period by using each atomic speed and 1st-peak of pair distribution function was suggested by using a time when speed is at a local minimum value.

Abstract: Volcanism is a fundamental planetary process, and understanding the dynamics of lava flows is critical for both hazard mitigation and geological studies. The final length and morphology of a lava flow are governed by a complex interplay between effusion rate, total volume, topography, and the lava's evolving thermo-rheological properties. While empirical power laws relating flow length to effusion rate or volume have proven useful, they do not fully capture the physical processes driving flow behavior, particularly the effects of crust formation and fragmentation. This paper presents an integrated theoretical framework that links macroscopic flow dynamics with the microscale processes of fragmentation. We begin by deriving scaling laws for flow length and width based on a Herschel-Bulkley fluid model. We then introduce a novel component to this framework by postulating that key rheological parameters evolve as a function of the flow's developing prefractal dimension, which quantifies its fragmentation and complexity. Finally, we propose a modified power law for the final flow length that accounts for energy dissipation due to both viscous shearing and the creation of new prefractal surfaces. By analyzing observational data from various basaltic to rhyolitic lava flows, we calibrate and discuss the model's empirical coefficients. The results demonstrate that highly fragmented lava flows like 'a'ā have their runout distance significantly reduced by the energetic cost of their increasing complexity, consistent with field observations. This framework provides a more physically robust foundation for forecasting lava flow behavior and interpreting their final morphologies.

Abstract: This work numerically studies the water-in-oil (W/O) droplet formation inside a flow focusing on the micro junction formed by rectangular channels with dimensions of 390 × 190 μm² using OpenFoam. An automatic algorithm was developed to assess the effect of key parameters such as water viscosity, restriction ratio and water mass flow rate ratio on the droplet size. A total of 96 simulations, with different parameter combinations, were conducted to train a Machine Learning (ML) algorithm capable of predicting the droplet dimensions based on the key parameters mentioned. The ML algorithm was also compared to a Newtonian-based optimization method, where the geometry is iteratively adjusted to produce droplets of a fixed size. Results reveal that both methods appear valid in the prediction of droplet dimensions.

Abstract: The need to decarbonize the energy matrix has pushed different sectors to seek alternative energy generation methods. Among these, biofuels stand out. This study proposes to analyze the trend of coprocessing renewables (bio-oil) in refineries using a prioritization technique called Roadmap. This methodology allowed for the creation of short-term trend maps based on an analysis of approved patents. Additional short-term maps were developed from patent applications. Finally, a third long-term map considered information extracted from articles, patents, and patent applications available on various platforms (Scopus, Science Direct, among others) during the study period. The study spanned an 11-year timeframe. The Roadmap methodology involves temporal evaluation, which helps identify associations between different institutions (companies, universities, or research centers). A taxonomy derived from reading all the material was used for analysis, allowing for an assessment of the state-of-the-art in terms of the types or developments of catalysts employed and the types of processing. The results indicate that hydrotreating and hydrocracking are promising routes, despite higher costs, mainly due to the use of hydrogen and high operating pressures. However, these are effective short-term alternatives for refineries without a Fluid Catalytic Cracking (FCC) structure. Fluid Catalytic Cracking (FCC) demonstrates the highest maturity, flexibility, and development potential, making it economically viable for bio-oil coprocessing. FCC catalysts containing zeolites in their formulation, which are extensively tested and evaluated for coprocessing, represent the most promising options, even when facing unfavorable results caused by their deactivation. It was concluded that hydrodeoxygenation (HDO) technology as a pretreatment for bio-oil obtained from fast pyrolysis biomass conversion could be the solution to reduce coke generation and prevent the rapid deactivation of zeolite-based catalysts.

Abstract: This work presents the numerical analysis of the thermal behavior of a new model of a flat solar collector. The computational model integrates embedded piping configured in a Rhomboid Tessellation Pattern (RTP), with scaling governed by allometric and fractal principles, constrained within a 3 × 3-branched fractal tree structure. The numerical analysis was performed using Computational Fluid Dynamics (CFD) with Autodesk CFD software. The operation of the collector was estimated with water mass flows ranging from 0.01 to 0.06 kg/s, with the water inlet temperature set at 20°C, and analyzed under two simulated solar radiation conditions, 850 and 650 W/m². The studied collector exhibits superior performance compared to traditional collectors. Specifically, it achieves higher fluid temperatures with similar mass flows, even under lower solar radiation conditions. The collector demonstrates thermal performance with efficiencies reaching up to 84.3% for small mass flows. On average, the collector efficiency was 78.1%. The higher thermal efficiency compared to conventional flat plate solar collectors and the reduction in pressure drop by up to 90% compared to traditional collectors make the collector model analyzed in this study a promising option for systems employing solar collectors or collector-evaporators.

Abstract: In this paper, we present a microwave characterization of some copper-based nanocomposite materials. They are composed of epoxy resin reinforced with copper powders nanostructured at different mechanical milling durations. Main purpose of this work is to probe the material properties such as complex permittivity and conductivity. For that, elementary copper powder were nanostructured via high-energy mechanical milling process. Milled powders were sampled at 3, 12, 33 and 58 hours milling and characterized via X-Ray Diffraction (XRD). The materials were then incorporated into epoxy resin to mold bulk composite structures of 1 mm thickness and a section that matches the R120 waveguide one. Microwave characterization of the bulk composite samples was carried out by a two- port measure of the scattering parameters using a vector network analyzer. The computation of complex dielectric permittivity followed the noniterative approach proposed by A.H. Boughriet. Obtained results are presented in the form of spectra. XRD spectra attest to the structural refinement during mechanical milling. Complex dielectric permittivity and microwave conductivity spectra exhibit the effect of the structural refinement on the microwave absorbing properties.