ABSTRACT
In contrast to reconstructive transformations, displacive transformations do not involve the breaking of bonds, but rather occur by the displacement of atomic planes relative to one another, as illustrated in Fig. 8.lb. These reactions occur quite rapidly, and the resulting microstructures are usually heavily twinned. In these transformations, the role of thermal entropy is important since the enthalpies of the phases on either side of the transformation temperature are quite comparable. It follows that the transformation usually results in the formation of more open (less dense) structures at higher temperatures, for reasons that were touched upon in Chap. 5, namely that the more open structures have higher thermal entropies. 121
(a) (b)
Zirconia
Monoclinic _____., tetragonal _____., cubic _____., liquid 1170°C 2370°C 2680°C
It exhibits three well-defined polymorphs: a monoclinic phase, a tetragonal phase, and a cubic phase. The low-temperature phase is monoclinic, stable to 1170oC at which temperature it changes reversibly to the tetragonal phase, which in turn is stable to 2370°C. Above that temperature the cubic phase becomes stable up to the melting point of 2680°C. The tetragonalto-monoclinic ( t =? m) transformation is believed to occur by a diffusionless
shear process that is similar to the formation of martensite in steels. This transformation is associated with a large volume change and undergoes extensive shear which is the basis for transformation toughening of zirconia, addressed in greater detail in Chap. 11.
