STRESS-INDUCED TRANSFORMATIONS IN COMPOSITE ZIRCONIA CERAMICS.

A martensitic transformation may be induced in confined tetragonal zirconia particles by the stress field in front of a crack tip, resulting in an increase in toughness. The microstructure, transformation mechanisms, elastic strain distribution, and in-situ studies of the interaction of the confined particles with the stress field of the crack tip have been studied by means of high voltage transmission electron microscopy techniques. Facetted particles below a critical size d//c are retained metastably down to room temperature. The transformation is predominantly controlled by a nucleation barrier. A model of nucleation by inhomogeneity of internal strains is postulated, in which the total resolved strain must exceed 0. 25 of the unconstrained transformation strain over the volume of the nucleus for initiation of the martensitic transformation. The ″critical size effect″ may then be explained in terms of irregular-shaped particles inducing an inhomogeneous internal strain field which scales with particle size and has a well-defined d//c.