Effect of Coarse‐Powder Portion on Abnormal Grain Growth during Hot Pressing of Commercial‐Purity Alumina Powder

By classification, two powder portions, one consisting of coarse particles and the other consisting of fine particles, were separated from a MgO‐doped (1000 ppm) commercial‐purity Al ₂ O ₃ powder. Examinations of microstructure evolution during hot pressing showed that extensive abnormal grain growth occurred for the coarse portion. For the fine portion, although there was an indication that grain‐size distribution deviated from normal distribution on prolonged hot pressing, such extensive abnormal grain growth did not occur. Extensive abnormal grain growth also occurred when the coarse portion was mixed into a high‐purity powder that exhibited no abnormal grain growth alone. Chemical analyses revealed that the coarse portion contained the higher concentration of impurities but lower concentration of magnesium than the fine portion. It was discussed that particle aggregates in the coarse portion might have been responsible for the higher concentration of impurities but lower concentration of magnesium and, thus, for the extensive abnormal grain growth.