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Title: Hydrogen segregation to inclined Σ3 < 110 >twin grain boundaries in nickel

Abstract

Low-mobility twin grain boundaries dominate the microstructure of grain boundary-engineered materials and are critical to understanding their plastic deformation behaviour. The presence of solutes, such as hydrogen, has a profound effect on the thermodynamic stability of the grain boundaries. This work examines the case of a Σ3 grain boundary at inclinations from 0° ≤ Φ ≤ 90°. The angle Φ corresponds to the rotation of the Σ3 (1 1 1) < 1 1 0 > (coherent) into the Σ3 (1 1 2) < 1 1 0 > (lateral) twin boundary. To this end, atomistic models of inclined grain boundaries, utilising empirical potentials, are used to elucidate the finite-temperature boundary structure while grand canonical Monte Carlo models are applied to determine the degree of hydrogen segregation. In order to understand the boundary structure and segregation behaviour of hydrogen, the structural unit description of inclined twin grain boundaries is found to provide insight into explaining the observed variation of excess enthalpy and excess hydrogen concentration on inclination angle, but the explanatory power is limited by how the enthalpy of segregation is affected by hydrogen concentration. At higher concentrations, the grain boundaries undergo a defaceting transition. In order to develop a more completemore » mesoscale model of the interfacial behaviour, an analytical model of boundary energy and hydrogen segregation that relies on modelling the boundary as arrays of discrete 1/3 < 1 1 1 > disconnections is constructed. Lastly, the complex interaction of boundary reconstruction and concentration-dependent segregation behaviour exhibited by inclined twin grain boundaries limits the range of applicability of such an analytical model and illustrates the fundamental limitations for a structural unit model description of segregation in lower stacking fault energy materials.« less

Authors:
 [1];  [1]
  1. Sandia National Lab. (SNL-NM), Albuquerque, NM (United States)
Publication Date:
Research Org.:
Sandia National Lab. (SNL-NM), Albuquerque, NM (United States)
Sponsoring Org.:
USDOE National Nuclear Security Administration (NNSA)
OSTI Identifier:
1343269
Report Number(s):
SAND-2016-12892J
Journal ID: ISSN 1478-6435; 650389
Grant/Contract Number:
AC04-94AL85000
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Philosophical Magazine (2003, Print)
Additional Journal Information:
Journal Name: Philosophical Magazine (2003, Print); Journal Volume: 96; Journal Issue: 26; Journal ID: ISSN 1478-6435
Publisher:
Taylor & Francis
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; twinning; grain boundary structure; nickel; hydrogen in metals; embrittlement; molecular dynamics; Monte Carlo; structural unit model

Citation Formats

O’Brien, Christopher J., and Foiles, Stephen M. Hydrogen segregation to inclined Σ3 < 110 >twin grain boundaries in nickel. United States: N. p., 2016. Web. doi:10.1080/14786435.2016.1217094.
O’Brien, Christopher J., & Foiles, Stephen M. Hydrogen segregation to inclined Σ3 < 110 >twin grain boundaries in nickel. United States. doi:10.1080/14786435.2016.1217094.
O’Brien, Christopher J., and Foiles, Stephen M. Thu . "Hydrogen segregation to inclined Σ3 < 110 >twin grain boundaries in nickel". United States. doi:10.1080/14786435.2016.1217094. https://www.osti.gov/servlets/purl/1343269.
@article{osti_1343269,
title = {Hydrogen segregation to inclined Σ3 < 110 >twin grain boundaries in nickel},
author = {O’Brien, Christopher J. and Foiles, Stephen M.},
abstractNote = {Low-mobility twin grain boundaries dominate the microstructure of grain boundary-engineered materials and are critical to understanding their plastic deformation behaviour. The presence of solutes, such as hydrogen, has a profound effect on the thermodynamic stability of the grain boundaries. This work examines the case of a Σ3 grain boundary at inclinations from 0° ≤ Φ ≤ 90°. The angle Φ corresponds to the rotation of the Σ3 (1 1 1) < 1 1 0 > (coherent) into the Σ3 (1 1 2) < 1 1 0 > (lateral) twin boundary. To this end, atomistic models of inclined grain boundaries, utilising empirical potentials, are used to elucidate the finite-temperature boundary structure while grand canonical Monte Carlo models are applied to determine the degree of hydrogen segregation. In order to understand the boundary structure and segregation behaviour of hydrogen, the structural unit description of inclined twin grain boundaries is found to provide insight into explaining the observed variation of excess enthalpy and excess hydrogen concentration on inclination angle, but the explanatory power is limited by how the enthalpy of segregation is affected by hydrogen concentration. At higher concentrations, the grain boundaries undergo a defaceting transition. In order to develop a more complete mesoscale model of the interfacial behaviour, an analytical model of boundary energy and hydrogen segregation that relies on modelling the boundary as arrays of discrete 1/3 < 1 1 1 > disconnections is constructed. Lastly, the complex interaction of boundary reconstruction and concentration-dependent segregation behaviour exhibited by inclined twin grain boundaries limits the range of applicability of such an analytical model and illustrates the fundamental limitations for a structural unit model description of segregation in lower stacking fault energy materials.},
doi = {10.1080/14786435.2016.1217094},
journal = {Philosophical Magazine (2003, Print)},
number = 26,
volume = 96,
place = {United States},
year = {Thu Aug 04 00:00:00 EDT 2016},
month = {Thu Aug 04 00:00:00 EDT 2016}
}

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  • Grain boundary engineered materials are of immense interest for their resistance to hydrogen embrittlement. This work builds on the work undertaken in Part I on the thermodynamic stability and structure of misoriented grain boundaries vicinal to the Σ3 (111) <11¯0> (coherent-twin) boundary to examine hydrogen segregation to those boundaries. The segregation of hydrogen reflects the asymmetry of the boundary structure with the sense of rotation of the grains about the coherent-twin boundary, and the temperature-dependent structural transition present in one sense of misorientation. This work also finds that the presence of hydrogen affects a change in structure of the boundariesmore » with increasing concentration. The structural change effects only one sense of misorientation and results in the reduction in length of the emitted stacking faults. Moreover, the structural change results in the generation of occupied sites populated by more strongly bound hydrogen. The improved understanding of misoriented twin grain boundary structure and the effect on hydrogen segregation resulting from this work is relevant to higher length-scale models. To that end, we examine commonly used metrics such as free volume and atomic stress at the boundary. In conclusion, free volume is found not to be useful as a surrogate for predicting the degree of hydrogen segregation, whereas the volumetric virial stress reliably predicts the locations of hydrogen segregation and exclusion at concentrations below saturation or the point where structural changes are induced by increasing hydrogen concentration.« less
  • In nanocrystalline materials, structural discontinuities at grain boundaries (GBs) and the segregation of point defects to these GBs play a key role in defining the structural stability of a material, as well as its macroscopic electrical/mechanical properties. In this study, the segregation of oxygen vacancies near the Σ3 (1 1 2) [¯110] tilt GB in SrTiO 3 is explored using density functional theory. We find that oxygen vacancies segregate toward the GB, preferring to reside within the next nearest-neighbor layer. This oxygen vacancy segregation is found to be crucial for stabilizing this tilt GB. Furthermore, we find that the migrationmore » barriers of oxygen vacancies diffusing toward the first nearest-neighbor layer of the GB are low, while those away from this layer are very high. Furthermore, the segregation and trapping of the oxygen vacancies in the first nearest-neighbor layer of GBs are attributed to the large local distortions, which can now accommodate the preferred sixfold coordination of Ti. These results suggest that the electronic, transport, and capacitive properties of SrTiO 3 can be engineered through the control of GB structure and grain size or layer thickness.« less
  • To study anisotropic hydrogen segregation and diffusion in nickel polycrystalline, Secondary Ion Mass Spectrometry (SIMS) and Electron Back Scattered Diffraction (EBSD) are integrated to investigate hydrogen distribution around grain boundaries. Hydrogen distribution in pre-charged samples were correlated with grain boundary character by integrating high-resolution grain microstructure from EBSD inverse pole figure map and low-resolution hydrogen concentration profile map from SIMS. This multimodal imaging instrumentation shows that grain boundaries in nickel can be categorized into two families based on behavior of hydrogen distribution crossing grain boundary: the first one includes random grain boundaries with fast hydrogen diffusivity, showing a sharp gapmore » for hydrogen concentration profile cross the grain boundaries. The second family are special Σ3n grain boundaries with low hydrogen diffusivity, showing a smooth gradient of hydrogen concentration cross the grain boundary. Heterogeneous hydrogen distributions due to grain boundary family revealed by SIMS/EBSD on mesoscale further validate the recent hydrogen permeation data and anisotropic ab-initio calculations in nanoscale. The results highlight the fact that grain boundaries character impacts hydrogen distribution significantly.« less
  • Nanocrystalline metals are polycrystalline metals with a grain size in the sub-micrometer range, and, therefore, the volume fraction of grain boundaries is large giving rise to an overall increase of solubility of impurities due to grain boundary segregation. The effect grain boundaries have on the behavior of hydrogen in nickel is somewhat controversial. Electrolytic doping of nickel samples with hydrogen is frequently used to initiate intergranular fracture, which is supposed to occur due to hydrogen segregation at grain boundaries. However, the effect of the boundaries on H-permeation appears to be negligible unless the grain size is 100 nm or less.more » Recent calculations of the H-segregation energy at a [Sigma] tilt boundary by Moody and Foiles using the embedded atom technique revealed a broad spectrum of energies ranging from 0.55 to 0.04 eV. These calculations and the measurements in nanocrystalline palladium /1/ are both evidence for a spectrum of segregation energies rather than a single value. In the present study the authors report on measurements of hydrogen solubility in nanocrystalline nickel at high pressures which they describe by assuming a Gaussian distribution of segregation energies in the grain boundaries. Thus values of the width of the distribution and its average energy are evaluated.« less