Near‐Intrinsic Grain‐Boundary Mobility in Dense Yttria

Abstract

Grain‐boundary mobilities and grain growth activation energies were measured on high‐purity and dense yttria samples annealed in both air and 5%H2–N2 at temperatures ranging from 1200° to 1900°C. The grain‐boundary mobility for samples annealed in 5%H2–N2 was higher in comparison with air‐annealed materials. The measured grain growth activation energy for samples annealed in air was 356 ± 35 kJ/mol. For samples annealed in 5%H2–N2, a transition temperature of 1579°C was identified where the grain growth activation energy changed dramatically. The measured activation energies were 373 ± 60 and 770 ± 2 kJ/mol for samples annealed at temperatures below and above 1579°C, respectively. The increase in activation energy was tentatively attributed to a reduction‐induced disordering of the grain boundary corresponding to the temperature at which yttrium metal liquid forms at low oxygen pressures. The data represent the best attempt to measure the near‐intrinsic grain‐boundary mobility for high‐purity highly dense yttria. High‐resolution transmission electron microscopyand high‐angle annular dark field‐scanning transmission electron microscopy confirmed that the grain boundaries were clean and free of any observable second phases.