Grain Boundary Solute Segregation in Vanadium

Abstract

Vanadium alloys are a candidate structural material in nuclear fusion applications, where the presence of grain boundaries can improve mechanical properties and act as a sink for radiation- induced defects. Solutes with a thermodynamic preference to segregate to grain boundaries can stabilize them, making this a prime consideration for alloy design, but there are limited quantitative solute segregation data for vanadium. Based on results from an ab-initio computational framework for predicting the spectrum of grain boundary segregation energies across the periodic table, select nanocrystalline vanadium-based binary alloy systems were synthesized via mechanical alloying for targeted experiments characterizing differences in segregation strength. Scanning transmission electron microscopy and energy-dispersive x-ray spectroscopy measurements of solute concentrations in the grain boundary and bulk validate computational predictions of the average segregation strengths for different solutes, while showing inhomogeneous solute distributions along the grain boundary network that confirm the necessity for a spectral model that captures the behavior of site-specific segregation energies.

After establishing the segregation behavior of different solutes in vanadium, the effects of solute segregation on other properties are examined. Heating experiments demonstrate that vanadium alloys containing strongly segregating species retain smaller grain sizes upon thermal annealing, indicating better grain boundary stability. The powder metallurgical route used produce these vanadium alloys requires a subsequent sintering step to densify powders into bulk parts for engineering applications, and dilatometry experiments reveal that that the addition of strongly segregating solutes also dramatically suppresses the sintering behavior. A kinetic analysis of the dilatometry data suggests that rapid grain boundary diffusion pathways that are necessary for effective sintering are obstructed by solute segregant, which has important repercussions for the processability of these alloys. Finally, microstructural characterization and nanohardness testing after ion-irradiation experiments demonstrate that the alloys with solute-stabilized grain boundaries are more resistant to nanovoid formation and radiation hardening. The work in this thesis advances our understanding of solute segregation and its effects in vanadium alloys, and highlights an approach for controlling grain boundaries that may facilitate future alloy design efforts for improved microstructural stability and radiation damage tolerance.