J. R. Cowan

Bio: J. R. Cowan is an academic researcher. The author has contributed to research in topics: Dislocation & Lattice diffusion coefficient. The author has an hindex of 2, co-authored 2 publications receiving 34 citations.

Role of dislocation density and structure in grain boundary segregation of phosphorus and carbon in model Fe-P-C alloy

Abstract: The present study has investigated the effects of dislocation density, produced by tensile prestraining, on the grain boundary segregation of phosphorus and carbon in a Fe-0.06P-0.002C (wt-%) alloy during stress free isothermal annealing at 500°C for periods up to 1800 h. Changes in grain boundary segregation were followed using Auger spectroscopy, while changes in dislocation density and structure were observed using transmission electron microscopy techniques. The segregation of phosphorus (but not carbon) was enhanced, compared with unstrained specimens, during initial aging. Analysis of diffusion rates required to cause the observed increase in phosphorus segregation suggested that the kinetics of phosphorus segregation was enhanced by pipe diffusion. At intermediate aging times, desegregation of phosphorus was observed, an effect attributed to a reduction in intragranular solute levels resulting from phosphorus precipitation on dislocations. In the case of carbon this process continued to the...

Thermodynamics of Grain Boundary Segregation and Applications to Anisotropy, Compensation Effect and Prediction

Abstract: Interfacial segregation has long been studied because of its impact on various properties of materials. Usually, theoretical descriptions are based on thermodynamic terms such as segregation enthalpy and entropy. Comparisons of published papers of different authors reveal fundamental differences in our understanding and application of these thermodynamic characteristics. To clarify this situation, the report reviews several types of segregation isotherms and their derivations. Individual sets of thermodynamic state functions appearing in segregation isotherms—Gibbs energy, enthalpy and entropy—are interpreted in detail. We identify the sources of controversial interpretations and the physical meaning of particular sets of thermodynamic state functions. It is shown that for a correct interpretation an unambiguous and systematic use of the term excess in thermodynamic terminology is necessary, as well as a correct evaluation scheme for data obtained with Auger Electron Sepctroscopy (AES). Successful applica...

Quantifying the energetics and length scales of carbon segregation to α-Fe symmetric tilt grain boundaries using atomistic simulations

Abstract: Segregation of impurities to grain boundaries (GBs) plays an important role in both the stability and macroscopic behavior of polycrystalline materials. The research objective in this work is to better characterize the energetics and length scales involved with the process of solute and impurity segregation to GBs. Molecular statics simulations are used to calculate the segregation energies for carbon within multiple substitutional and interstitial GB sites over a database of 125 symmetric tilt GBs in Fe. The simulation results show that there are two energetically favorable GB segregation processes: (1) an octahedral C atom in the lattice segregating to an interstitial GB site and (2) an octahedral C atom and a vacancy in the lattice segregating to a grain boundary substitutional site. In both cases, lower segregation energies than appear in the bulk lattice were calculated. Moreover, based on segregation energies approaching bulk values, the length scale of interaction is larger for interstitial C than for substitutional C in the GB (?5?? compared to ?3?? from center of the GB). A subsequent data reduction and statistical representation of this dataset provides critical information about the mean segregation energy and the associated energy distributions for carbon atoms as a function of distance from the grain boundary, which quantitatively informs higher scale models with energetics and length scales necessary for capturing the segregation behavior of alloying elements and impurities in Fe. The significance of this research is the development of a methodology capable of ascertaining segregation energies over a wide range of GB character (typical of that observed in polycrystalline materials), which herein has been applied to carbon segregation to substitutional and interstitial sites in a specific class of GBs in ?-Fe.

The two odors of iron when touched or pickled: (skin) carbonyl compounds and organophosphines

Abstract: Humans are perplexed by the metallic odor from touchingiron metal objects, such as tools, cutlery, railings, doorhandles, firearms, jewelry, and coins. Phosphorus-containingiron which is under acid attack gives rise to a different“carbide”or“garlic”odorwhichmetallurgistshaveattributedto the gas phosphine (PH

Quantifying the Energetics and Length Scales of Carbon Segregation to Fe Symmetric Tilt Grain Boundaries Using Atomistic Simulations

Abstract: Segregation of impurities to grain boundaries plays an important role in both the stability and macroscopic behavior of polycrystalline materials. The research objective in this work is to better characterize the energetics and length scales involved with the process of solute and impurity segregation to grain boundaries. Molecular dynamics simulations are used to calculate the segregation energies for carbon within multiple grain boundary sites over a database of 125 symmetric tilt grain boundaries in Fe. The simulation results show that the majority of atomic sites near the grain boundary have segregation energies lower than in the bulk. Moreover, depending on the boundary, the segregation energies approach the bulk value approximately 5-12 \AA\ away from the center of the grain boundary, providing an energetic length scale for carbon segregation. A subsequent data reduction and statistical representation of this dataset provides critical information such as about the mean segregation energy and the associated energy distributions for carbon atoms as a function of distance from the grain boundary, which quantitatively informs higher scale models with energetics and length scales necessary for capturing the segregation behavior of impurities in Fe. The significance of this research is the development of a methodology capable of ascertaining segregation energies over a wide range of grain boundary character (typical of that observed in polycrystalline materials), which herein has been applied to carbon segregation in a specific class of grain boundaries in iron.