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Abstract

Grain boundaries play an important role during plastic deformation and failure of polycrystals.
The presence of point defects, line defects or segregated second phase particles at
the grain boundaries affects their mechanical properties, which in turn alter the hardness or
fracture toughness of the poly-crystals favorably or adversely. In case of the body centered
cubic (bcc) transition metals, which are the materials of interest for high temperature
applications, grain boundaries and segregated impurities at grain boundaries play a vital role
as well [1]. Therefore, a sigma-5 (310)[001] symmetrically tilt grain boundary (STGB) present
in molybdenum (Mo) has been investigated during this study at the atomistic level using
density functional theory (DFT).

The simulations have been performed using the Vienna Ab-initio Simulation Package
(VASP) [2] employing the generalized gradient approximation (GGA) to DFT. After the initial
convergence tests for the optimization of k-point meshing and energy cut-off of the plane
wave basis set, the favorable position of a carbon (C) atom in Mo was determined to be the
interstitial octahedral position. Thereafter, a sigma-5 (310)[001] grain boundary structure was
constructed and relaxed using super-cells of different sizes. Stable translation states in the
three directions [310], [-130] and [001] were obtained. Subsequently a carbon atom as an
impurity was introduced at and near the grain boundary on different octahedral positions.
With this model tensile tests of sigma-5 (310)[001] Mo grain boundaries with different C
concentrations have been performed using DFT. From these results traction separation data
was derived that can be used to parameterize cohesive zone models for fracture simulations
of a Mo bi-crystals on the continuum level.