DISPLACIVE TRANSFORMATIONS AND THEIR APPLICATIONS IN STRUCTURAL CERAMICS

The current status of knowledge; of first order, potentially displaciv e transformations in ceramics is reviewed. Those proven to be martensi tic in bulk, single phase materials are the tetragonal to monoclinic t ransformation in zirconia (ZrO2) and the cubic to tetragonal symmetry change in lead titanate (PbTiO3). Numerous transformations in a variet y of other ceramic materials have been reported and the incompleteness of the data is here indicated. The relevance of displacive and marten sitic transformations to toughening mechanisms for reducing brittlenes s has been demonstrated. The mechanisms are: (i) transformation toughe ning of composites; (ii) transformation weakening of critical interpha ses in composites; and (iii) mechanical rearrangement of domains arisi ng from ferroelastic transformations in monoliths. Another potential a pplication lies in their use as large force actuators in adaptive (sma rt) systems. Transformations mechanisms need to be elucidated in the c ontext of the sequential loss or gain of symmetry on cooling or heatin g. The coupling of mechanical forces to induce nucleation, as well as their interaction with the crystallographic volume and shape changes a ccompanying transformation, need to be understood and controlled. This fundamental knowledge will guide the microstructural tailoring of mon oliths and composites to achieve useful coupling, and thus enhanced me chanical properties.