Hydrogen segregation to inclined 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 grain boundary at inclinations from . The angle corresponds to the rotation of the (coherent) into the (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 disconnections is constructed. Furthermore, 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.

Keywords:

• Twinning
• grain boundary structure
• nickel
• hydrogen in metals
• embrittlement
• molecular dynamics
• Monte Carlo
• structural unit model

Acknowledgements

The authors would like to thank D.L. Medlin of Sandia National Laboratories for very helpful discussions. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the US Department of Energy’s National Nuclear Security Administration [contract number DE-AC04-94AL85000].

Notes

No potential conflict of interest was reported by the authors.

1 Repeating C structural units make up the symmetric tilt grain boundary. Rittner and Seidman [31] showed that the DC combination represents the lateral twin boundary. A single structural unit is unable to account for the translation of the two grains at equilibrium and the possible dissociation of the D structural unit in lower stacking fault energy materials..