From Solid State Diffusion to Configurational Kinetics

1998 ◽  
Vol 527 ◽  
Author(s):  
G. Martin ◽  
M. Athènes ◽  
C. Desgranges ◽  
M. Nastar ◽  
F. Soisson
Keyword(s):  
Diffusion Mechanism ◽  
Transport Coefficients ◽  
Mean Field ◽  
Kinetic Properties ◽  
Monte Carlo Techniques ◽  
Complex Diffusion ◽  
Specific Effects ◽  
State Diffusion ◽  
Diffusion Mechanisms ◽  
Jump Frequencies

ABSTRACTA single model is proposed to describe the cohesive energy and the vacancy jump frequencies as a function of the alloy configuration at the atomic level. The very same model therefore yields both equilibrium and kinetic properties, close and far away from equilibrium. The model is handled at two levels of approximation: Monte Carlo techniques and mean-field type approximations. The model yields equilibrium properties (including the transport coefficients and complex diffusion mechanisms in ordered compounds close to equilibrium) as well as the kinetic path for phase separation, with or without ordering. This allows to identify specific effects of the vacancy diffusion mechanism on the kinetic path for alloy decomposition. Here, we summarise the main results of the above approach.

scholarly journals Comparison of Jump Frequencies of 111In/Cd Tracer Atoms in Sn3R and In3R Phases Having the L12 Structure (R = Rare-Earth)

2011 ◽  
Vol 311 ◽  
pp. 159-166 ◽  
Author(s):  
Megan Lockwood Harberts ◽  
Benjamin Norman ◽  
Randal Newhouse ◽  
Gary S. Collins
Keyword(s):  
Rare Earth ◽  
Perturbed Angular Correlation ◽  
Diffusion Mechanism ◽  
Correlation Spectroscopy ◽  
Perturbed Angular Correlation Spectroscopy ◽  
Dominant Mechanism ◽  
Boundary Composition ◽  
Light Lanthanide ◽  
Diffusion Mechanisms ◽  
Jump Frequencies

Measurements were made of jump frequencies of 111In/Cd tracer atoms on the Sn-sublattice in rare-earth tri-stannides having the L12 crystal structure via perturbed angular correlation spectroscopy (PAC). Phases studied were Sn3R (R= La, Ce, Pr, Nd, Sm and Gd). Earlier measurements on isostructural rare-earth tri-indides showed that the dominant diffusion mechanism changed along that series [4]. The dominant mechanism was determined by comparing jump frequencies measured at opposing phase boundary compositions (that is, more In-rich and more In-poor). Jump frequencies were observed to be greater at the In-rich boundary composition in light lanthanide indides and greater at the In-poor boundary composition in heavy-lanthanide indides. These observations were attributed to predominance of diffusion via rare-earth vacancies in the former case and indium vacancies in the latter. Contrary to results for the indides, jump frequencies found in the present work are greater for the Sn-poor boundary compositions of the stannides, signaling that diffusive jumps are controlled by Sn-vacancies. Possible origins of these differences in diffusion mechanisms are discussed.

scholarly journals Diffusion and Equilibration of Site-Preferences Following Transmutation of Tracer Atoms

2018 ◽  
Vol 19 ◽  
pp. 61-79 ◽  
Author(s):  
Gary S. Collins
Keyword(s):  
Diffusion Mechanism ◽  
Nuclear Quadrupole Interaction ◽  
Site Preference ◽  
Jump Frequency ◽  
Heavy Lanthanide ◽  
Site Preferences ◽  
Lanthanide Elements ◽  
Light Lanthanides ◽  
Diffusion Mechanisms ◽  
Jump Frequencies

Using the method of perturbed angular correlation of gamma rays, diffusional jump-frequencies of probe atoms can be measured through relaxation of the nuclear quadrupole interaction. This was first shown in 2004 for jumps of tracer atoms that lead to reorientation of the local electric field-gradient, such as jumps on the connected a-sublattice in the L12 crystal structure. Studies on many such phases using the 111In/Cd PAC probe are reviewed in this paper. A major finding from a 2009 study of indides of rare-earth elements, In3R, was the apparent observation of two diffusional regimes: one dominant for heavy-lanthanide phases, R= Lu, Tm, Er, Dy, Tb, Gd, that was consistent with a simple model of vacancy diffusion on the In a-sublattice, and another for light-lanthanides, R= La, Ce, Pr, Nd, that had no obvious explanation but for which several alternative diffusion mechanisms were suggested. It is herein proposed that the latter regime arises not from a diffusion mechanism but from transfer of Cd-probes from In-sites where they originate to R-sites as a consequence of a change in site-preference of 111Cd-daughter atoms from In-sites to R-sites following transmutation of 111In. Support for this transfer mechanism comes from a study of site-preferences and jump-frequencies of 111In/Cd probes in Pd3R phases. Possible mechanisms for transfer are described, with the most likely mechanism identified as one in which Cd-probes on a-sites transfer to interstitial sites, diffuse interstitially, and then react with vacancies on b-sites. Implications of this proposal are discussed. For indides of heavy-lanthanide elements, the Cd-tracer remains on the In-sublattice and relaxation gives the diffusional jump-frequency.

Beyond the Cahn-Hilliard Equation: a Vacancy-Based Kinetic Theory

2011 ◽  
Vol 172-174 ◽  
pp. 321-330 ◽  
Author(s):  
Maylise Nastar
Keyword(s):  
Kinetic Theory ◽  
Structure Function ◽  
Diffusion Mechanism ◽  
Kinetic Equations ◽  
Mean Field ◽  
Early Time ◽  
Present Theory ◽  
Kinetic Coupling ◽  
Amplification Rate ◽  
Cahn Hilliard Equation

A Self-Consistent Mean Field (SCMF) kinetic theory including an explicit description ofthe vacancy diffusion mechanism is developed. The present theory goes beyond the usual local equi-librium hypothesis. It is applied to the study of the early time spinodal decomposition in alloys. Theresulting analytical expression of the structure function highlights the contribution of the vacancydiffusion mechanism. Instead of the single amplification rate of the Cahn-Hillard linear theory, thelinearized SCMF kinetic equations involve three constant rates, first one describing the vacancy re-laxation kinetics, second one related to the kinetic coupling between local concentrations and paircorrelations and the third one representing the spinodal amplification rate. Starting from the same va-cancy diffusion model, we perform kineticMonte Carlo simulations of a Body Centered Cubic (BCC)demixting alloy. The resulting spherically averaged structure function is compared to the SCMF pre-dictions. Both qualitative and quantitative agreements are satisfying.

Modeling complex diffusion mechanisms in L1 2 -structured compounds

2010 ◽  
Vol 197 (1-3) ◽  
pp. 219-222 ◽  
Author(s):  
M. O. Zacate ◽  
M. Lape ◽  
M. Stufflebeam ◽  
W. E. Evenson
Keyword(s):  
Complex Diffusion ◽  
Diffusion Mechanisms

THE EDWARDS MODEL: THE GLASS TRANSITION

1992 ◽  
Vol 06 (10) ◽  
pp. 1587-1594 ◽  
Author(s):  
S.F. EDWARDS
Keyword(s):  
Field Theory ◽  
Glass Transition ◽  
Liquid Crystal ◽  
Transport Coefficients ◽  
Mean Field Theory ◽  
Mean Field ◽  
Essential Singularity ◽  
Liquid Crystal Polymers ◽  
New Class ◽  
Edwards Model

The structure of the transport coefficients in the approach to the glass transition is characterized by an essential singularity in their behaviour as functions of the temperature, variously known as the Vogel-Fulcher or Dolittle or Williams-Landel-Ferry laws. This behaviour can be derived directly in certain simple models notably a glass of liquid crystal polymers where mean field theory may be applied. In this paper a new class of closed expressions for transport coefficients are derived and it is shown that they can be used to derive the Vogel-Fulcher law.

The influence of palladium impurities on vacancy diffusion in cubic silicon carbide

2010 ◽  
Vol 1264 ◽  
Author(s):  
Guido Roma
Keyword(s):  
Silicon Carbide ◽  
First Principles ◽  
Kinetic Properties ◽  
Vacancy Diffusion ◽  
First Principles Calculations ◽  
Low Concentrations ◽  
Diffusion Mechanisms ◽  
Cubic Silicon Carbide ◽  
As Solubility ◽  
Correlation Factors

AbstractThe basic properties of palladium impurities in silicon carbide, such as solubility or diffusion mechanisms, are far from being well understood. In a recent paper I presented a systematic study of stability and kinetic properties of Pd in cubic silicon carbide using first principles calculations. In this paper I focus on the effect of the presence of palladium in silicon carbide, even in very low concentrations, on the kinetic properties of carbon vacancies. I apply a odel of Pd diffusion through a vacancy mechanism on the carbon sublattice and extract the correlation factors leading to an enhancement of vacancy migration, due to the coupling of iffusion fluxes between vacancies and palladium impurities.

Determination of Ionic Diffusion Mechanisms in Solids

1998 ◽  
Vol 527 ◽  
Author(s):  
John Corish
Keyword(s):  
Molecular Dynamics ◽  
Diffusion Coefficient ◽  
Atomistic Simulation ◽  
Atomistic Simulations ◽  
Ionic Diffusion ◽  
Simulation Studies ◽  
Lattice Statics ◽  
Monte Carlo Techniques ◽  
Diffusion Mechanisms

ABSTRACTThe experimental and atomistic simulation methodologies by which microscopic diffusion mechanisms can be determined in solids are described. Measurement of the Haven Ratio requires evaluation of the diffusion coefficient and of the ionic conductivity for the species in pure and doped specimens and is, in practice, limited to simpler materials. Atomistic simulations using lattice statics, molecular dynamics and Monte Carlo techniques can yield very detailed information on the pathways followed by migrating ions and are being utilised more extensively for this purpose. Examples of such experimental and simulation studies are discussed.

Cu Precipitation Dynamics in an Fe-1.18%Cu Alloy

2012 ◽  
Vol 509 ◽  
pp. 22-27 ◽  
Author(s):  
Hui Ping Ren ◽  
Hai Yan Wang ◽  
Zong Chang Liu
Keyword(s):  
Diffusion Mechanism ◽  
Structural Features ◽  
Precipitation Process ◽  
Thermally Activated ◽  
Cu Alloy ◽  
Transmission Electron ◽  
Wide Range ◽  
Means Of Transmission ◽  
Evolution Of Microstructure ◽  
Jump Frequencies

The precipitation of copper during aging at 600oC in high-purity Fe-Cu alloy was examined by means of transmission electron microscopy (TEM).Nano-scale copper-rich clusters with a B2-like structure were observed during heat treatment. These micro structural features play an important role in precipitation strengthening. In addition, the precipitation process has been analyzed in terms of the evolution of microstructure by a Monte Carlo method. A description of the coherent precipitation of copper in iron, based on a vacancy diffusion mechanism, thermally activated jump frequencies and cohesive energy is discussed in order to deal with simultaneous precipitation of metastable and stable phases in Cu-containing steel during aging. This analysis gives an estimation of the precipitation dynamics, as well as the evolution of Cu precipitates across a wide range of temperatures.

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