Grain‐Boundary Segregation and Final‐Stage Sintering of Y 2 O 3

The addition of ThO 2 to Y 2 O 3 inhibits grain growth during sintering and allows the sintering process to proceed to theoretical density by maintaining a high diffusion flux of vacancies from the pores to the grain boundaries. The inhibition of grain growth is accomplished by the segregation of ThO 2 solute at the grain boundaries, causing a decrease in the grain‐boundary mobility. The segregation of ThO 2 at the grain boundaries can be inferred from the results of the microhardness and grain‐growth studies presented. Further evidence for segregation is provided by quenching experiments and surface activity experiments.