OVERVIEW .45. ON THE NUCLEATION OF THE MARTENSITIC-TRANSFORMATION IN ZIRCONIA (ZRO2)

The kinetics of the martensitic tetragonal (t) yields monoclinic (m) transformation are nucleation-controlled, and classical and non-classical models are considered. Transmission electron microscopy studies do not reveal any obvious defects that could act as classical heterogeneous nuclei. This is true for t-ZrO//2 as a coherent precipitate phase, as an incoherent dispersed phase, as a fine-grain matrix phase, or as an incoherent precipitate phase in internally-oxidized alloys. On the other hand, nucleation via a Clapp-type localized soft mode mechanism at strain concentrations is consistent with all observations. Nucleation is always stress-assisted, even when the transformation occurs spontaneously. In these cases, stresses arise from thermal expansion mismatch or thermal expansion anisotropy and are enhanced by stress concentrations provided by particle or grain facets and corners.