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In this work the microstructure and degradation behavior of several as-cast alloy compositions belonging to the Mg rich corner of the Mg-Si-Sr system are presented and related. The intermetallic phases are identified and analyzed... more
In this work the microstructure and degradation behavior of several as-cast alloy compositions belonging to the Mg rich corner of the Mg-Si-Sr system are presented and related. The intermetallic phases are identified and analyzed describing the microstructure evolution during solidification. It is intended in this work to obtain insight in the behavior of the ternary alloys in in vitro tests and to analyze the degradation behavior of the alloys under physiologically relevant conditions. The as-cast specimens have been exposed to immersion tests, both mass loss (ML) and potentiodynamic polarization (PDP). The degradation rate (DR) have been assessed and correlated to microstructure features, impurity levels and alloy composition. The initial reactions resulted to be more severe while the degradation stabilizes with time. A higher DR is related with a high content of the Mg17Sr2 phase and with the presence of coarse particles of the intermetallics Mg2Si, MgSiSr and MgSi2Sr. Specimens ...
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Phase-field modeling has proven to be a versatile tool for simulating microstructural evolution phenomena, such as grain growth in polycrystalline materials. However, the computing time and computing memory requirements of a phase-field... more
Phase-field modeling has proven to be a versatile tool for simulating microstructural evolution phenomena, such as grain growth in polycrystalline materials. However, the computing time and computing memory requirements of a phase-field model pose severe limitations on the number of phase-field variables that can be taken into account in a practical implementation. In this paper, a sparse bounding box algorithm is proposed that allows the use of a large number of phase-field variables without excessive memory usage or computational requirements. The algorithm is applied to a three-dimensional model for grain growth in the presence of second-phase particles.
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The evolution of fibre textured structures is simulated in two dimensions using a generalised phase field model assuming two forms for the misorientation dependence of the grain boundary energy. In each case, a steady-state regime is... more
The evolution of fibre textured structures is simulated in two dimensions using a generalised phase field model assuming two forms for the misorientation dependence of the grain boundary energy. In each case, a steady-state regime is reached after a finite amount of grain growth, where the number and length weighted misorientation distribution functions (MDF) are constant in time, and the mean grain area A as a function of time t follows a power growth law A - A 0 = kt n with n close to 1 and A 0 the initial mean grain area. The final shape of the MDF and value of the prefactor k in the power growth law clearly correlate with the misorientation dependence of the grain boundary energy. Furthermore, a mean field approach is worked out to predict the growth exponent for systems with non-uniform grain boundary energy. The conclusions from the mean field approach are consistent with the simulation results. In previous studies on grain growth in anisotropic fibre textured systems, this steady-state regime was often not reached, which resulted in wrong conclusions on the growth exponent n and evolution of the MDF.
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ABSTRACT
Magnesium and its alloys are being investigated worldwide for application in biodegradable implants, yet there are no commercially available alloys specifically developed for this application. Magnesium is an essential element in the... more
Magnesium and its alloys are being investigated worldwide for application in biodegradable implants, yet there are no commercially available alloys specifically developed for this application. Magnesium is an essential element in the human body and it would be logical then to explore the development of alloys that contain only elements which are known to be biocompatible because they are already part of the human body. In this work, Ca, Si and Sr were selected to develop magnesium alloys for biomedical application due to their good biocompatibility. A number of ternary Mg-Ca-Si and Mg-Si-Sr alloy compositions were explored. The alloys were cast in steel molds and their phase content was measured and compared with thermodynamic predictions. The effect of the characterized phases on the hardness and compressive strength was evaluated.
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ABSTRACT The solubility range has a large effect on the growth rate of intermetallic compounds (IMCs) formed at hetero-junctions, however, is extremely difficult to determine accurately experimentally. In this work, it is studied for IMCs... more
ABSTRACT The solubility range has a large effect on the growth rate of intermetallic compounds (IMCs) formed at hetero-junctions, however, is extremely difficult to determine accurately experimentally. In this work, it is studied for IMCs with a narrow solubility range how the thermodynamic properties affect their solubility and growth rate in hetero-junctions. Furthermore, a method is developed to estimate the solubility range of IMCs based on phase-field simulations combined with experimentally measured diffusion coefficients and steady-state growth rate coefficients. Application of the method to the phases -Cu3Sn and -Cu6Sn5 of the Cu-Sn system at 453 K, gives the solubility ranges of the order of = 2.0 10−2 and = 2.6 10−2 using the experimental growth rates = 4.40 10−9 and = 7.76 10−9 m s−0.5 [1] together with the diffusion coefficients = 3.62 10−16 and = 1.01 10−15 m2 s−1 [2], respectively.
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ABSTRACT The crystallization behavior of CaSiO3 in different CaO–Al2O3–SiO2 melts was comprehensively investigated in-situ with a confocal scanning laser microscope (CSLM) over a wide range of temperatures. The observations clearly... more
ABSTRACT The crystallization behavior of CaSiO3 in different CaO–Al2O3–SiO2 melts was comprehensively investigated in-situ with a confocal scanning laser microscope (CSLM) over a wide range of temperatures. The observations clearly indicate a transition from a faceted to dendritic crystal morphology with decreasing temperature. The undercooling required for dendritic growth increases with decreasing Al2O3 (under same basicity) and increasing basicity. The dendrite structure becomes finer at higher growth rates with a lower Al2O3 and higher basicity. The growth rates of different dendrites are time-independent. With increasing temperature, the growth rate first increases and then decreases. The observed dendrite tip radii are compared with those obtained from Ivantsov theory in 2D and 3D. With decreasing temperature, the growth conditions in the CSLM experiments appeared to shift from 3D (with the dendrite tip below the surface melt) close to 2D (with the dendrite tip on top of the surface melt).
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This document is part of Volume 11 `Ternary Alloy Systems: Phase Diagrams, Crystallographic and Thermodynamic Data', Subvolume E `Refractory Metal... more
This document is part of Volume 11 `Ternary Alloy Systems: Phase Diagrams, Crystallographic and Thermodynamic Data', Subvolume E `Refractory Metal Systems', of Landolt-Börnstein - Group IV `Physical Chemistry'.
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ABSTRACT The coarsening behavior of three-phase materials, such as eutectic material systems, is of high technological interest. Microstructure evolution simulations can help to understand the effect of different magnitudes of the... more
ABSTRACT The coarsening behavior of three-phase materials, such as eutectic material systems, is of high technological interest. Microstructure evolution simulations can help to understand the effect of different magnitudes of the diffusivities in the different phases. In this study, the evolution of a 3D three-phase morphology was modeled with equal interfacial energy and volume fraction and similar thermodynamic properties for the three phases, but the diffusion mobilities were taken different. It was observed that the phase with the lowest mobility has the highest growth rate and, on average, a larger number of grain faces, while the other two phases have a nearly equal growth rate and average number of grain faces. The simulation results are compared with results from experiments and simulation studies for single-phase and two-phase materials.