Hindrance of Grain Growth in Al ₂ O ₃ by ZrO ₂ Inclusions.

Abstract

Alumina and Al 2 O 3 /ZrO 2 (1 to 10 vol%) composite powders were mixed and consolidated by a colloidal method, sintered to >98% theoretical density at 1550°C, and subsequently heat‐treated at temperatures up to 1700°C for grain‐size measurements. Within the temperature range studied, the ZrO 2 inclusions exhibited sufficient self‐diffusion to move with the Al 2 O 3 4‐grain junctions during grain growth. Growth of the ZrO 2 , inclusions occurred by coalescence. The inclusions exerted a dragging force at the 4‐grain junctions to limit grain growth. Abnormal grain growth occurred when the inclusion distribution was not sufficiently uniform to hinder the growth of all Al 2 O 3 grains. This condition was observed for compositions containing ≤2.5 vol% ZrO 2 , where the inclusions did not fill all 4‐grain junctions. Exaggerated grains consumed both neighboring grains and ZrO 2 , inclusions. Grain‐growth control (no abnormal grain growth) was achieved when a majority (or all) 4‐grain junctions contained a ZrO 2 inclusion, viz., for compositions containing ≥5 vol% ZrO 2 . For this condition, the grain size was inversely proportional to the volume fraction of the inclusions. Since the ZrO 2 inclusions mimic voids in all ways except that they do not disappear, it is hypothesized that abnormal grain growth in single‐phase materials is a result of a nonuniform distribution of voids during the last stage of sintering.