Metastable Crystallization, Phase Partitioning, and Grain Growth of ZrO ₂ ─Gd ₂ O ₃ Materials Processed from Liquid Precursors

Abstract

Aqueous mixtures of Zr(C 2 H 3 O 2 ) 4 and Gd(NO 3 ) 3 were used to prepare chemically homogeneous ZrO 2 —Gd 2 O 3 solid solutions containing 0 to 9 mol% Gd 2 O 3 . The application of kinetically limited equilibria concepts explains why a metastable tetragonal phase crystallized subsequent to precursor pyrolysis from compositions containing <6.5 mol% Gd 2 O 3 and a metastable cubic phase crystallized from precursors with >6.5 mol% Gd 2 O 3 . Partitioning experiments for a composition containing 3 mol% Gd 2 O 3 showed that the maximum equilibrium solid‐solubility of Gd 2 O 3 in tetragonal ZrO 2 is 1.0 ± 0.1 mol% and the minimum equilibrium solid‐solubility in cubic ZrO 2 is 8.0 ± 0.2 mol% at 1400°C. Although phase partitioning appeared to initiate during heating, complete partitioning at 1400°C required ∼ 200 h. Large compositional variations (≤±1 mol% Gd 2 O 3 ) were observed within tetragonal grains and from grain to grain during the first 50 h at 1400°C, whereas the composition of cubic grains produced during partitioning were relatively invariant. Experiments showed that this compositional variability was not present in material rapidly heated to 800°C, but was a result of partitioning. At 1400°C, growth of the tetragonal grains was very sluggish; it increased only to 3 μm after 500 h, whereas the number of larger cubic grains increased during this same period. The implications of these observations are related to the processing of rare‐earth‐stabilized ZrO 2 transformation‐toughened materials.