55 research outputs found
Resampling technique applied to the characterization of microsegregation
Characterization of short-range chemical heterogeneities in metallic materials, such as the so-called microsegregation resulting from solidification, is most often performed using EDS or WDS spot measurements. The most usual way is to perform countings on points located along a regular grid. Due to experimental limitation, the grid step is generally of the same order of magnitude than the characteristic distance(s) of the chemical heterogeneities under investigation. In such a case, the measurements can not be assumed to be independent one from each other, and the resulting interferences (correlations) preclude application of simple statistics to the solute distribution obtained. In the present work, this is clearly shown by using a resampling technique applied to "chemical" images obtained by phase field modelling
Phase boundary anisotropy and its effects on the maze-to-lamellar transition in a directionally solidified Al-Al2Cu eutectic
Solid-solid phase boundary anisotropy is a key factor controlling the
selection and evolution of non-faceted eutectic patterns during directional
solidification. This is most remarkably observed during the so-called
maze-to-lamellar transition. By using serial sectioning, we followed the
spatio-temporal evolution of a maze pattern over long times in a large Al-Al2Cu
eutectic grain with known crystal orientation of the Al and Al2Cu phases, hence
known crystal orientation relationship (OR). The corresponding phase boundary
energy anisotropy (-plot) was also known, as being previously estimated
from molecular-dynamics computations. The experimental observations reveal the
time-scale of the maze-to-lamellar transition and shed light on the processes
involved in the gradual alignment of the phase boundaries to one distinct
energy minimum which nearly corresponds to one distinct plane from the family
. This particular plane is selected
due to a crystallographic bias induced by a small disorientation of the
crystals relative to the perfect OR. The symmetry of the OR is thus slightly
broken, which promotes lamellar alignment. Finally, the maze-to-lamellar
transition leaves behind a network of fault lines inherited from the phase
boundary alignment process. In the maze pattern, the fault lines align along
the corners of the Wulff shape, thus allowing us to propose a link between the
pattern defects and missing orientations in the Wulff shapeComment: 26 pages, 6 figure
3D phase-field computations of microsegregation in nodular cast iron compared to experimental data and CalPhad-based Scheil-prediction
International audienceThe redistribution of solute elements during processing of a nodular cast iron alloy was simulated for the first time comprehensively over time and 3D space. Numerical predictions had so far been limited to 1D models, neglecting local morphological aspects and commonly also diffusion and growth in solid-state. Application of the standard multi-phase-field method was hindered by the inherent simplifying assumption of equal and constant molar volume, causing artificial piling-up of solute and biased kinetics during modelling of graphite growth. A recently developed volumetric multi-phase-field approach now accounts for the changing partial molar volume of the individual elements. The Calphad-based phase-field study was benchmarked to experimental cooling and nodule density data, and the predicted as-cast distributions were validated by experimental segregation analysis. The combined numerical and experimental findings were furthermore used as a basis to discuss simplifying assumptions commonly made in 1D Scheil-type models
Resampling technique applied to statistics of microsegregation characterization
Characterization of chemical heterogeneities at the dendrite scale is of practical importance for understanding phase transformation either during solidification or during subsequent solid-state treatment. Spot analysis with electron probe is definitely well-suited to investigate such heterogeneities at the micron scale that is relevant for most solidified products. However, very few has been done about the statistics of experimental solute distributions gained from such analyses when they are now more and more used for validating simulation data. There are two main sources generating discrepancies between estimated and actual solute distributions in an alloy: i) data sampling with a limited number of measurements to keep analysis within a reasonable time length; and ii) uncertainty linked to the measurement process, namely the physical noise that accompanies X-ray emission. Focusing on the first of these sources, a few 2-D composition images have been generated by phase field modelling of a Mg-Al alloy. These images were then used to obtain "true" solute distributions to which to compare coarse grid analyses as generally performed with a microanalyser. Resampling, i.e. generating several distributions by grid analyses with limited number of picked-up values, was then used to get statistics of estimates of solute distribution. The discussion of the present results deals first with estimating the average solute content and then focuses on the distribution in the primary phase
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Calphad-based phase-field study of the interplay between spheroidal graphite growth and chemical segregation in ductile cast iron
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