X-.Jin/ Current Opinion in Solid State and Materials Science 9(2005)313-318 transformation toughening closely related to the transfor The strong covalent nature of the Zr-o bond favours a tetragonal phase, as well as perspective of challenger ble sevenfold co-ordination number, and as a result, mono- mation, room temperature stabilization of metast clinic ZrO is thermodynamically stable at lower tempera tures. whereas the co-ordination number of Z+ cations in 2. Crystal structures and crystallographic of tetragonal tetragonal and cubic-Zro2 is 8. To accommodate the ther (t- monoclinic(m) martensitic transformation mally generated oxygen ion vacancies" at higher tempera ture, the structure of ZrO changes to the structure Zirconia has three allotropes: cubic (c), tetragonal(t) having eightfold co-ordination (t or c) while it still and monoclinic(m). The tetragonal (t) and monoclinic maintains an effective co-ordination number close to 7 (m)polymorphs have distorted fluorite structures shown owing to the association of Zr" ions with the oxygen ion in Fig. I from [7] vacancies [**5, 12, 13]. The detailed crystallographic infor mation on different Zro, polymorphs can be for Ref.[*7] The crystallography of t-m martensitic trans tions has been evaluated by a phenomenological theory in lots of zirconia-containing ceramics [8, 14-17]. espe- cially a recent comprehensive review by Kelly and Fr Rose [*8]. The phenomenological theory is believed to be capable of explaining all reported microstructural and crys- tallographic features of the t-m martensitic transforma- tion in zirconia containing ceramics. The agreement between the experimental results and theoretical prediction demonstrates that the the eory can be applied to make reli- monoclinic(m) ible, quantitative predictions for the martensitic transfor mation in those ceramic systems, even better than its Fig 1. Crystal structures of tetragonal(t)and monoclinic(m)phases[7]. application in steels where it was developed Fig. 2. In situ TEM observation for stress-induced martensitic transformation in 8Ce-025Y-TZP. Reversible motion of boundary between thermal stress-induced monoclinic and tetragonal phases was observed(marked by an arrow in(a))and grows(b, c) when focusing the electron beam, shrinks(d, e) and disappears (f) when defocusing the electron beam.transformation toughening closely related to the transfor￾mation, room temperature stabilization of metastable tetragonal phase, as well as perspective of challenges. 2. Crystal structures and crystallographics of tetragonal (t) ! monoclinic (m) martensitic transformation Zirconia has three allotropes: cubic (c), tetragonal (t) and monoclinic (m). The tetragonal (t) and monoclinic (m) polymorphs have distorted fluorite structures shown in Fig. 1 from [*7]. The strong covalent nature of the Zr–O bond favours a sevenfold co-ordination number, and as a result, mono￾clinic ZrO2 is thermodynamically stable at lower tempera￾tures, whereas the co-ordination number of Zr4+ cations in tetragonal and cubic-ZrO2 is 8. ‘‘To accommodate the ther￾mally generated oxygen ion vacancies’’ at higher tempera￾ture, the structure of ZrO2 changes to the structure having eightfold co-ordination (t or c) while it still maintains an effective co-ordination number close to 7 owing to the association of Zr4+ ions with the oxygen ion vacancies [**5,12,13]. The detailed crystallographic infor￾mation on different ZrO2 polymorphs can be found in Ref. [*7]. The crystallography of t ! m martensitic transforma￾tions has been evaluated by a phenomenological theory in lots of zirconia-containing ceramics [*8,14–17], espe￾cially a recent comprehensive review by Kelly and Francis Rose [*8]. The phenomenological theory is believed to be capable of explaining all reported microstructural and crys￾tallographic features of the t ! m martensitic transforma￾tion in zirconia containing ceramics. The agreement between the experimental results and theoretical prediction demonstrates that the theory can be applied to make reli￾able, quantitative predictions for the martensitic transfor￾mation in those ceramic systems, even better than its Fig. 1. Crystal structures of tetragonal (t) and monoclinic (m) phases [*7]. application in steels where it was developed. Fig. 2. In situ TEM observation for stress-induced martensitic transformation in 8Ce–0.25Y–TZP. Reversible motion of boundary between thermal stress-induced monoclinic and tetragonal phases was observed (marked by an arrow in (a)) and grows (b,c) when focusing the electron beam, shrinks (d,e) and disappears (f) when defocusing the electron beam. 314 X.-J. Jin / Current Opinion in Solid State and Materials Science 9 (2005) 313–318