316 X. Jin Current Opinion in Solid State and Materials Science 9(2005)313-318 transition of the lattice structure from tetragonal to mono- by shearing displacement of zir The second stage involves migration of oxygen ions to oxy- gen sites in the monoclinic lattice. The displacement of the i oxygen ions from the ideal fluorite positions along the c- O metastable t-ZrO, axis has been investigated by X-ray diffraction (XRD) [25]. It was proposed that, while the rapid shear displace-. K transformed m-Zro2 ment of the zirconium ions is the rate-controlling factor for 桑 crack tip stress field nucleation and longitudinal growth of the monoclinic plates, the migration of the oxygen ions controls the lateral growth of the plates. In the reverse m-t transformation migration of the zr and o- ions to their respective posi- Fig 4. Schematic presentation of stress induced phase transformation of tions is diffusion controlled. Strongly time dependent metastable tetragonal zirconia particles in crack tip stress field;arrow behavior was also observed in Ce-TZP [26] indicate generation of compressive residual stress by transformation It was also documented that the length of compressed induced volume expansion and microstructural constrain Bly Ce-Y-TZP specimens increases continuously in the aging at room temperature [27], resulting from gradual reverse m-t transformation. This unusual anelasticity may be The characteristics of an ideal transformation tough A ggested as pseudo-anelasticity phenomenon associated ened ceramic such as TZP are summarized as ["85. ith transformation, differing from normal anelasticity. Martensitic transformation is suppressed with transfor- mation start temperature Ms just below operating room 5. Transformation toughening temperature and metastable parent phase will be stress induced transformed at the crack tip resulting in a posi e. Zirconia containing ceramic is one of only two classes of Live volume change( dilation) aterials exhibiting transformation toughening. The other .The shape strain has a relatively large shear component one is transformation induced plasticity/TRIP steels great importance to that the transfo The martensitic t-,m transformation can be induced mation is easy to stress-induce at a crack tip and to have by cooling or by external loading under isothermal condi- the shear accommodated by means of the formation of tions[1, 29]. Both transformation routes are of importance correlated variants **11]."While thermally induced transformation will The underlying physical mechanisms of transformation ontrol the amount of tetragonal phase that can be toughening can be conveniently considered to involve retained after thermal cycling, the stress induced martens- itic transformation enhances the toughness of zirconia either a process zone or a bridging zone [6]. ceramics Martensitic transformation exhibits high speed and a 6 Shape memory effect change of shape of the transformed volume, both of which are essential for transformation toughening. Transforma Shape memory behavior originated from martensitic ion toughening occurs when metastable retained t-ZrO2 and its reverse transformation, tetragonal (t)++mono- transforms to the stable m-zrO2 phase in the tensile stress clinic(m), was first found in zirconia ceramics partially field around a propagat ting crac volume expan- stabilized with magnesia (Mg-PSZ) in 1986 [2] and sion(4-5%)characteristic of the t-m martensitic trans- observed in ceria-TZP [34] as well as ceria-yittria-TZP formation introduces a net compressive stress in the stabilized tetragonal zirconia polycrystals several years process zone around the crack tip [30,31]. This reduces later [35]. Though the relative low recoverable strain the local crack tip stress intensity and hence the driving i.e., <1%, and the brittleness limit their practical applica force for crack propagation, so increasing the effective tion, the high operating temperature(a few hundred toughness of the ceramics(Fig 4) degrees higher than Nitinol shape memory alloys), high Following [32, 33]. PM Kelly and LR Francis Rose sug- strength and chemical inertness make ZrO2 containing gested[8]a model of'decoupling the nucleation strain shape memory ceramics attr from the final strain--the net transformation strain and By comparison of the shape memory effect (SME)and allowing the final transformation strains to include a related properties among specimens with different con- shear component. Nucleation strain determines whether tents of(8-12 mol%)CeO2 and(0.25-0.75 mil%)Y203 or not the stress-induced martensitic transformation can fabricated by different processes, it was found that the occur at the tip of a potentially dangerous crack. "It is 8Ce-05Y-TZP sintered for 6 h at 1773 K demonstrated the net transformation strain left behind in the trans- excellent SME [36], i.e., a complete shape recovery rate formed region that provides toughening by hindering with a strain of w1. 2% shown in Fig. 5. No microcracks crack growth were found after shape recovery in &Ce-0.5Y-TZPtransition of the lattice structure from tetragonal to mono￾clinic occurs by shearing displacement of zirconium ions. The second stage involves migration of oxygen ions to oxy￾gen sites in the monoclinic lattice. The displacement of the oxygen ions from the ideal fluorite positions along the c￾axis has been investigated by X-ray diffraction (XRD) [25]. It was proposed that, ‘‘while the rapid shear displace￾ment of the zirconium ions is the rate-controlling factor for nucleation and longitudinal growth of the monoclinic plates, the migration of the oxygen ions controls the lateral growth of the plates’’. In the reverse m ! t transformation, migration of the Zr4+ and O2 ions to their respective posi￾tions is diffusion controlled. Strongly time dependent behavior was also observed in Ce–TZP [26]. It was also documented that the length of compressed Ce–Y–TZP specimens increases continuously in the aging at room temperature [27], resulting from gradual reverse m ! t transformation. This unusual anelasticity may be suggested as pseudo-anelasticity phenomenon associated with transformation, differing from normal anelasticity [28]. 5. Transformation toughening Zirconia containing ceramic is one of only two classes of materials exhibiting transformation toughening. The other one is transformation induced plasticity/TRIP steels. The martensitic t ! m transformation can be induced by cooling or by external loading under isothermal condi￾tions [1,29]. Both transformation routes are of importance [**11]. ‘‘While thermally induced transformation will control the amount of tetragonal phase that can be retained after thermal cycling, the stress induced martens￾itic transformation enhances the toughness of zirconia ceramics’’. Martensitic transformation exhibits high speed and a change of shape of the transformed volume, both of which are essential for transformation toughening. Transforma￾tion toughening occurs when metastable retained t-ZrO2 transforms to the stable m-ZrO2 phase in the tensile stress field around a propagating crack [29]. The volume expan￾sion (4–5%) characteristic of the t ! m martensitic trans￾formation introduces a net compressive stress in the process zone around the crack tip [30,31]. This reduces the local crack tip stress intensity and hence the driving force for crack propagation, so increasing the effective toughness of the ceramics (Fig. 4). Following [32,33], PM Kelly and LR Francis Rose sug￾gested [*8] a model of ‘decoupling’ the nucleation strain from the final strain—the net transformation strain and allowing the final transformation strains to include a shear component. Nucleation strain determines whether or not the stress-induced martensitic transformation can occur at the tip of a potentially dangerous crack. ‘‘It is the net transformation strain left behind in the trans￾formed region that provides toughening by hindering crack growth’’. The characteristics of an ideal transformation tough￾ened ceramic such as TZP are summarized as [*8]: • Martensitic transformation is suppressed with transfor￾mation start temperature Ms just below operating room temperature and metastable parent phase will be stress￾induced transformed at the crack tip resulting in a posi￾tive volume change (dilation). • The shape strain has a relatively large shear component, which is of great importance to ensure that the transfor￾mation is easy to stress-induce at a crack tip and to have the shear accommodated by means of the formation of correlated variants. The underlying physical mechanisms of transformation toughening can be conveniently considered to involve either a process zone or a bridging zone [6]. 6. Shape memory effect Shape memory behavior originated from martensitic and its reverse transformation, tetragonal (t) M mono￾clinic (m), was first found in zirconia ceramics partially stabilized with magnesia (Mg–PSZ) in 1986 [2] and observed in ceria–TZP [*34] as well as ceria–yittria–TZP stabilized tetragonal zirconia polycrystals several years later [35]. Though the relative low recoverable strain, i.e., <1%, and the brittleness limit their practical applica￾tion, the high operating temperature(a few hundred degrees higher than Nitinol shape memory alloys), high strength and chemical inertness make ZrO2 containing shape memory ceramics attractive. By comparison of the shape memory effect (SME) and related properties among specimens with different con￾tents of (8–12 mol%) CeO2 and (0.25–0.75 mil%) Y2O3 fabricated by different processes, it was found that the 8Ce–0.5Y–TZP sintered for 6 h at 1773 K demonstrated excellent SME [36], i.e., a complete shape recovery rate with a strain of 1.2% shown in Fig. 5. No microcracks were found after shape recovery in 8Ce–0.5Y–TZP. Fig. 4. Schematic presentation of stress induced phase transformation of metastable tetragonal zirconia particles in crack tip stress field; arrow indicate generation of compressive residual stress by transformation induced volume expansion and microstructural constrain [31]. 316 X.-J. Jin / Current Opinion in Solid State and Materials Science 9 (2005) 313–318