Pressure‐induced phase changes in polycrystalline Y 2 O 3 : Influence on structure and properties

The micro‐ and nano‐structural effects of varying pressure and holding time at 1000°C during the high‐pressure processing of high‐purity, fine‐grained polycrystalline cubic yttrium oxide (c‐Y 2 O 3 ) are investigated. The critical pressure required to induce the phase transformation from c‐Y 2 O 3 to monoclinic‐Y 2 O 3 (m‐Y 2 O 3 ) ranges from 2.5 to 3.0 GPa, depending on holding time. At pressures below 2.5 GPa, where no phase transformation occurs, the original equiaxed grain structure of c‐Y 2 O 3 undergoes significant coarsening. At 3.0 GPa, a coarse‐grained columnar structure of m‐Y 2 O 3 begins to develop at the sample's surface, propagating uniformly into the interior and ultimately throughout the entire sample volume. Between 4.0 and 6.0 GPa, there is a gradual transition from surface nucleation and growth of columnar grains to bulk nucleation and growth of elongated grains across the sample. At 8.0 GPa, rapid transformation occurs simultaneously throughout the sample, resulting in nanocrystalline m‐Y 2 O 3 . When the nanocrystalline m‐Y 2 O 3 undergoes reprocessing at 1.0 GPa and 1000°C, a reverse transformation back to c‐Y 2 O 3 occurs while retaining the nanocrystalline structure. However, if the reverse transformation is conducted at lower temperatures (i.e., 700–800°C), it remains incomplete, resulting in the formation of a c/m‐Y 2 O 3 nanocomposite. The forward transformation from c‐Y 2 O 3 to m‐Y 2 O 3 results in a hardness increase of approximately 35%. Although the reverse transformation back to c‐Y 2 O 3 reduces hardness by about 15%, there is still an overall increase in hardness of around 20% when considering both processes because of the final grain‐size reduction of c‐Y 2 O 3 from micro‐ to nano‐meter dimensions.