Microstructure Evolution of ZrO 2 ‐(Fe 2 O 3 , Al 2 O 3 ) Materials Synthesized with Solution Precursors

Aqueous Zr‐nitrate solutions containing appropriate amounts of Fe(NO 3 ) 3 ·6H 2 O and Al(NO 3 ) 3 ·6H 2 O were used to synthesize ZrO 2 ‐Fe 2 O 3 compositions (up to 40 mol% Fe 2 O 3 ) and one ZrO 2 ‐Al 2 O 3 ‐Fe 2 O 3 composition. An amorphous phase was produced after pyrolysis, which subsequently crystallized to a single‐phase Zr(Fe)O 2 solid solution (or Zr(Fe,Al)O 2 ss) that appeared cubic by X‐ray diffraction, but tetragonal ( c/a → 1) by electron diffraction. The crystallization temperature increased with Fe 2 O 3 content. At higher temperatures, the single phase partitioned to two phases, tetragonal‐ZrO 2 +γ‐Fe 2 O 3 . The γ‐ to α‐Fe 2 O 3 transformation occurred at still higher temperatures to produce a two‐phase microstructure composed of two interpenetrating phases (ZrO 2 +α‐Fe 2 O 3 ), each with equiaxed grains. This microstructure was relatively stable to grain coarsening, even to temperatures just below the apparent eutectic temperature. With the exception of the lower transition temperatures and the equiaxed grains observed for both phases, these observations are similar to that previously reported for the ZrO 2 ‐Al 2 O 3 system. The microstructure development reported here is another example for the synthesis via solution processing that involves diffusion‐limited crystallization. Namely, a two‐phase material can be produced where the grain size of each can be very small, and can be prevented from growing rapidly at high temperatures due to the constraint of each phase on one another.