Phase transitions and microstructures of: a. zirconia ceramics as: ZrO2-MxOy (M = Y, Sc, Mg, Ca, Al and mullite) systems; b. alumina and AlxMyOZ systems

The first part of this chapter deals with partially and/or fully stabilized ceramics such as ZrO2-MxOy ocides (M=Y, Sr, Mg, Ca, Al, Mullite, Sr, ...). Their physico-chemical and mechanical properties are very sensitive to subtle variations in thermal treatments (heating, cooling, quenching, aging), solute contents (% of MxOy additions), impurities (SiO2, Fe2O3, Al2O3, NiO, ...) crystalline defects (twins, microcracks, grain boundaries...)... The various phenomena (particularly the toughness), involved in phase transitions (the Martensitic Transformation for instance), are often intricate and not always clearly elucidated. It has been recently suggested that ferroelastic domain switching could be the origin of the toughness of these ceramics. We have chosen to explain all these characteristics in terms of the numerous electron microscopy results given by various research groups. Some applications will be reported. The second part provides a rapid survey of the phase transitions relative to Alumina and AlxMyOz compounds (M=Na, Mg, Li, Ba and La) which mostly depend on the thermal treatments and the M/Al ratios. (orig.)