Understanding the influence of grain boundary thickness variation on the mechanical strength of a nickel-doped tungsten grain boundary

• Thickness dependent grain boundary strength in a nickel (Ni)-doped tungsten (W) GB is examined using ab initio simulations.
• Thickness dependent GB mechanical strength strongly correlates to lower acoustic mode phonon frequencies.
• Lower GB thicknesses at unsaturation are metastable.
• Ni atomic fraction dependent strength directly correlates to electron density of states changes.
• An analytical relation to express GB strength in terms of single crystal W strength is proposed.

Grain boundaries (GBs) in nickel (Ni)-doped tungsten (W) are found to have thickness as a function of the level of saturation of W atoms with respect to Ni atoms in the GBs. While the unsaturated Ni-doped W GBs have average thickness of approximately 0.3 nm, the saturated Ni-doped W GBs have twice the average thickness (∼0.6 nm). The present work examines (1 1 0)–(2 1 0) W GB mechanical strength as a function of thickness using an ab initio calculation framework based on Car–Parrinello molecular dynamics (CPMD) simulations. The atomic fraction of Ni atoms is varied to understand the influence of Ni addition and its correlation with thickness variation on the GB mechanical strength. In the case of GBs with 0.3 nm thickness, the variation of peak tensile strength as a function of Ni atomic fraction variation from 5% to 50% is negligible. However, in the case of 0.6 nm GB, the changes in the peak tensile strength are significant with the maximum peak tensile strength observed in the case of 58% Ni atomic fraction. Analyses examine electron density of states and phonon dispersion relations to delineate the role of atomic level bond strength in thickness dependent GB mechanical strength. The thickness dependent GB strength variation is found to be strongly correlated to the thickness dependent change in lower acoustic mode phonon frequencies. At the same time Ni atomic fraction dependent change in GB strength is found to be strongly correlated to the corresponding changes in electron density of states. Based on the analyses performed, an analytical relation to predict GB peak tensile strength as a function of atomic cohesive energy, GB thickness (level of saturation), and the Ni atomic fraction is proposed.