Displacive Transformations and their Applications in Structural Ceramics

The current status of knowledge of first order, potentially displacive transformations in ceramics is reviewed. Those proven to be martensitic in bulk, single phase materials are the tetragonal to monoclinic transformation in zirconia (ZrO2) and the cubic to tetragonal symmetry change in lead titanate (PbTiO3). Numerous transformations in a variety of other ceramic materials have been reported and the incompleteness of the data is here indicated. The relevance of displacive and martensitic transformations to toughening mechanisms for reducing brittleness has been demonstrated. The mechanisms are : (i) transformation toughening of composites ; (ii) transformation weakening of critical interphases in composites ; and (iii) mechanical rearrangement of domains arising from ferroelastic transformations in monoliths. Another potential application lies in their use as large force actuators in adaptive (smart) systems. Transformations mechanisms need to be elucidated in the context of the sequential loss or gain of symmetry on cooling or heating. The coupling of mechanical forces to induce nucleation, as well as their interaction with the crystallographic volume and shape changes accompanying transformation, need to be understood and controlled. This fundamental knowledge will guide the microstructural tailoring of monoliths and composites to achieve useful coupling, and thus enhanced mechanical properties