《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_Martensitic transformation in zirconia containing ceramics

●" Science Direct Current Opinion in Solid state Materials science ELSEVIER Current Opinion in Solid State and Materials Science 9(2005)313-318 Martensitic transformation in zirconia containing ceramics and its applications School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, China An introduction to tetragonal(t)- monoclinic(m) martensitic transformation in zirconia containing ceramics, especially tetragonal zirconia polycrystalline (TZP) was presented. Thermodynamics, crystallographic and kinetics of t-m martensitic transformation in TZP were emphasized. Transformation toughening and shape memory effect (SME) associated with t- m martensitic transformation in the TzP were reviewed. Perspective of future challenges was briefly mentioned at the end o 2006 Published by Elsevier Ltd Keywords: Martensitic transformation; TZP, Transformation toughening: Shape memory effect 1. Introduction Undoped zirconia exhibits the following phase trans tions under ambient during thermal cycling[7] Zirconia containing ceramics are materials of imparting toughness(known as transformation toughening [l] while m-ZrO2#t-Zro 2370oC c- ZrO2 liquid maintaining strength and chemical inertness, and of exhib- 950°C ing new functions such as shape memory effect [2] by It has been well documented that the t-m transformation manipulating new microstructure. These properties are is a athermal martensitic transformation, associated with a mainly dominated by the structure transformation from large temperature hysteresis(several hundred K), a volume tetragonal(t) to monoclinic(m) change or dilation component of transformation strain (4- Gale irconia containing ceramics can be classified into three 5%)and a large shear strain %or9)[6-*8 tegories: tetragonal zirconia polycrystalline(TZP), par- leads to disintegration of sintered undoped zirconia parts tially stabilized zirconia(PSZ) and zirconia toughened/ Dopants(yttria, ceria, etc. are added to stabilize the high dispersed ceramics(ZTC/ZDC). Tetragonal zirconia poly- temperature tetragonal and/or cubic phase in the sintered crystalline (TZP)is a material with nearly 100% t-ZrO2 microstructure 3] phase, stabilized by yttria or ceria additions [3]. An alterna In the view of the potential commercial applications [9] tive way to stabilize the tetragonal phase is to decrease the typically room temperature)of high temperature polymor grain size of tetragonal phase to nanoscale[*4, **5]. Grain phs (tetragonal and cubic)of ZrO2, the issues associated sizes of TZP ceramics are typically in the range of 0.2-1 um with t-m martensitic transformation, related mechanism [6]. These ceramics are often designated with the prefix with of transformation toughening and stabilization of metasta- Ce- or Y-to denote ceria- or yttria-stabilized, for example, gonal phase at lower temp 8Ce-0.5Y-TZP represents a 8 mol%CeO2 and 0.5 mol% much attention in both ceramic research and martensitic Y2O3 stabilized zirconia transformation worlds for three decades [1-**11]. In the present review, the author discusses characteris- tics of tetragonal(t)- monoclinic(m)martensitic trans- E-mailaddress:jin(@sjtu.edu.cn formation in TZP, the shape memory effect and the 1359-0286/s- see front matter 2006 Published by Elsevier Ltd. doi:l0.1016 cossms.2006.02012

Martensitic transformation in zirconia containing ceramics and its applications Xue-Jun Jin School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, China Abstract An introduction to tetragonal (t) ! monoclinic (m) martensitic transformation in zirconia containing ceramics, especially tetragonal zirconia polycrystalline (TZP) was presented. Thermodynamics, crystallographics and kinetics of t ! m martensitic transformation in TZP were emphasized. Transformation toughening and shape memory effect (SME) associated with t ! m martensitic transformation in the TZP were reviewed. Perspective of future challenges was briefly mentioned at the end. 2006 Published by Elsevier Ltd. Keywords: Martensitic transformation; TZP; Transformation toughening; Shape memory effect 1. Introduction Zirconia containing ceramics are materials of imparting toughness (known as transformation toughening [1]) while maintaining strength and chemical inertness, and of exhib￾iting new functions such as shape memory effect [2] by manipulating new microstructure. These properties are mainly dominated by the structure transformation from tetragonal (t) to monoclinic (m). Zirconia containing ceramics can be classified into three categories: tetragonal zirconia polycrystalline (TZP), par￾tially stabilized zirconia (PSZ) and zirconia toughened/ dispersed ceramics (ZTC/ZDC). Tetragonal zirconia poly￾crystalline (TZP) is a material with nearly 100% t-ZrO2 phase, stabilized by yttria or ceria additions [3]. An alterna￾tive way to stabilize the tetragonal phase is to decrease the grain size of tetragonal phase to nanoscale [*4,**5]. Grain sizes of TZP ceramics are typically in the range of 0.2–1 lm [6]. These ceramics are often designated with the prefix with Ce- or Y- to denote ceria- or yttria-stabilized, for example, 8Ce–0.5Y–TZP represents a 8 mol% CeO2 and 0.5 mol% Y2O3 stabilized zirconia. Undoped zirconia exhibits the following phase transi￾tions under ambient during thermal cycling [*7]: m-ZrO2 )*1170 C 950 C t-ZrO2 )*2370 C c-ZrO2 )*2680 C liquid It has been well documented that the t ! m transformation is a athermal martensitic transformation, associated with a large temperature hysteresis (several hundred K), a volume change or dilation component of transformation strain (4– 5%) and a large shear strain (14–15% or 9) [6–*8]. This leads to disintegration of sintered undoped zirconia parts. Dopants (yittria, ceria, etc.) are added to stabilize the high temperature tetragonal and/or cubic phase in the sintered microstructure [3]. In the view of the potential commercial applications [9] (typically room temperature) of high temperature polymor￾phs (tetragonal and cubic) of ZrO2, the issues associated with t ! m martensitic transformation, related mechanism of transformation toughening and stabilization of metasta￾ble tetragonal phase at lower temperatures have drawn much attention in both ceramic research and martensitic transformation worlds for three decades [1–**11]. In the present review, the author discusses characteris￾tics of tetragonal (t) ! monoclinic (m) martensitic trans￾formation in TZP, the shape memory effect and the 1359-0286/$ - see front matter 2006 Published by Elsevier Ltd. doi:10.1016/j.cossms.2006.02.012 E-mail address: jin@sjtu.edu.cn Current Opinion in Solid State and Materials Science 9 (2005) 313–318

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

The thermoelastic behavior and crystallography of th 8mol%Ceo-0 5mol%Y.o-Zro t-m martensitic transformation in Ce-Y-TZP ceramics were investigated by means of in situ TEM observation and Wechsler-Lieberman-Read (W-L-R)phenomenolog ical theory. In situ TEM observations [17] showed that in Ce-Y-TZP the t/m interface can move freely with the change of thermal stress generated by beam illumination shown in Fig. 2, whereas it was not found in thermal 3 T cycles. Based on the features of reversibility of interface 2 motion, large thermal hysteresis and high critical driving -2000 Y-TZP was suggested as a semi-thermoelastic one. the habit plane and the lattice correspondence were determined as( 30)t and ([001]t[O10]m), which is in ,(ca)=832.5K agreement with the calculated results by the phenomeno- M(ca)=2499K logical theory. 200300400500600700 Temperature/K 3. Thermodynamics of tetragonal (t-monoclinic(m) martensitic transformation Fig. 3. Thermodynamic evaluation of Gibbs free energies for tetragonal and monoclinic phases of 8Ce-0.5Y-TZP The change in total Gibbs free energy associated with the athermal martensitic transformation t-+ m can be △G1 AGah+△Gstr+△Gsu-△Gext △G1-m=V(-△G+△Gsm)+S△Gmr, △G-△Gext+△ Gb where subscripts"ch", "str", and"sur"refer to the chem- where Gext is the interaction energy density due to the ical free energy, the strain energy including both shear and external stress; Gbarrier is the sum of the changes in surface dilatational energy, and the surface energy including the and strain free energ Considering the phenomenology of stress induced phase surface free energy, twinning energy and micro-cracking transformation, a critical transformation stress may be energy, respectively. V refers to the volume and S, the area associated with the transformation. The equilibrium tem- defined as perature between the t→ m transformation, To, is the tem.o=(-△G+△ Barrier)/e rature at which AGch =0, and the Ms is defined as the temperature at which AGt-m=0 where g is the resultant dilational transformation strain The chemical energy difference between two phases localized in the transformation zone around the crack tip (AGch) of a multi-element system can be calculated from Application of only the resultant dilational transformation models [22]. The other required parameters in the right the controversy(see Section 5[*8]). It is clear that critical hand side of Eq. (1) can be derived through estima- stress is reduced as the temperature approaches Ms, since tion either from some available data or by experiments. the difference of chemical Gibbs free energy between t Thus the Ms temperature can be calculated accordin and m phases increases and contribute more to the driving Eq.(1) The difference of Gibbs free energy between tetragonal Also, from Eq (3), the total free energy change can be and monoclinic phases in ZrO2-CeOxY2O3 as a function increased and t-ZrO2 is retained by, decreasing the chemi of composition and temperature was thermodynamically cal free energy change by stabilizing with the addition of calculated from the three related binary systems [21]. In dopant oxide(e.g. yttria, ceria) increasing the strain free 8 mol% CeO2-05 mol%Y2O3-TZP, the equilibrium tem- energy change by dispersing the tetragonal phase in a con- perature between tetragonal and monoclinic phases, To, straining elastic matrix (e.g. alumina, cubic zirconia) was evaluated as 832.5K and the Ms temperature of this increasing the surface free energy (e.g. by reducing the alloy with a mean grain size of 0.90 um was calculated as tetragonal grain size)[**ll 249.9 K using the approach, which is in good agreement with the experimental one of 253 K by dilation measure- 4. Kinetics of t-m martensitic transformation in TZP (Fig. 3) Related to the application of external stress in the case Generally, athermal diffusionless t-m martensitic of transformation toughening(see Section 5), the total tree transformation takes place quickly, with the motion of energy change per unit volume required for constrained phase boundary as high as the sound speed [*23]. The over transformation [18] can be expressed as I transformation proceeds in two major stages [24]. First

The thermoelastic behavior and crystallography of the t ! m martensitic transformation in Ce–Y–TZP ceramics were investigated by means of in situ TEM observation and Wechsler–Lieberman–Read (W–L–R) phenomenolog￾ical theory. In situ TEM observations [17] showed that in Ce–Y–TZP the t/m interface can move freely with the change of thermal stress generated by beam illumination shown in Fig. 2, whereas it was not found in thermal cycles. Based on the features of reversibility of interface motion, large thermal hysteresis and high critical driving force for Ce–Y–TZP, the t ! m transformation in Ce– Y–TZP was suggested as a semi-thermoelastic one. The habit plane and the lattice correspondence were determined as (1 3 0)t and ([0 0 1]tk[0 1 0]m), which is in agreement with the calculated results by the phenomeno￾logical theory. 3. Thermodynamics of tetragonal (t) ! monoclinic (m) martensitic transformation The change in total Gibbs free energy associated with the athermal martensitic transformation t ! m can be expressed as [18–21] DGt!m ¼ V ðDGch þ DGstrÞ þ SDGsur; ð1Þ where subscripts ‘‘ch’’, ‘‘str’’, and ‘‘sur’’ refer to the chem￾ical free energy, the strain energy including both shear and dilatational energy, and the surface energy including the surface free energy, twinning energy and micro-cracking energy, respectively. V refers to the volume and S, the area associated with the transformation. The equilibrium tem￾perature between the t ! m transformation, T0, is the tem￾perature at which DGch = 0, and the Ms is defined as the temperature at which DGt!m = 0. The chemical energy difference between two phases (DGch) of a multi-element system can be calculated from the related binary systems by means of thermodynamic models [22]. The other required parameters in the right hand side of Eq. (1) can be derived through estima￾tion either from some available data or by experiments. Thus the Ms temperature can be calculated according to Eq. (1). The difference of Gibbs free energy between tetragonal and monoclinic phases in ZrO2–CeO2–Y2O3 as a function of composition and temperature was thermodynamically calculated from the three related binary systems [21]. In 8 mol% CeO2–0.5 mol% Y2O3–TZP, the equilibrium tem￾perature between tetragonal and monoclinic phases, T0, was evaluated as 832.5 K and the Ms temperature of this alloy with a mean grain size of 0.90 lm was calculated as 249.9 K using the approach, which is in good agreement with the experimental one of 253 K by dilation measure￾ment (Fig. 3). Related to the application of external stress in the case of transformation toughening(see Section 5), the total tree energy change per unit volume required for constrained transformation [18] can be expressed as DGt!m ¼ DGch þ DGstr þ S V DGsur DGext ¼ DGch DGext þ DGbarrier; ð2Þ where Gext is the interaction energy density due to the external stress; Gbarrier is the sum of the changes in surface and strain free energy. Considering the phenomenology of stress induced phase transformation, a critical transformation stress may be defined as rc ¼ ðDGch þ DGbarrierÞ=e t ; ð3Þ where e t is the resultant dilational transformation strain, localized in the transformation zone around the crack tip. Application of only the resultant dilational transformation strain in explaining the transformation toughening is still in the controversy (see Section 5 [*8]). It is clear that critical stress is reduced as the temperature approaches Ms, since the difference of chemical Gibbs free energy between t and m phases increases and contribute more to the driving force. Also, from Eq. (3), the total free energy change can be increased and t-ZrO2 is retained by, decreasing the chemi￾cal free energy change by stabilizing with the addition of dopant oxide (e.g. yttria, ceria); increasing the strain free energy change by dispersing the tetragonal phase in a con￾straining elastic matrix (e.g. alumina, cubic zirconia); increasing the surface free energy (e.g. by reducing the tetragonal grain size) [**11]. 4. Kinetics of t ! m martensitic transformation in TZP Generally, athermal diffusionless t ! m martensitic transformation takes place quickly, with the motion of phase boundary as high as the sound speed [*23]. The over￾all transformation proceeds in two major stages [24]. First, Fig. 3. Thermodynamic evaluation of Gibbs free energies for tetragonal and monoclinic phases of 8Ce–0.5Y–TZP. X.-J. Jin / Current Opinion in Solid State and Materials Science 9 (2005) 313–318 315

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-TZP

transition 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

X.. Jin Current Opinion in Solid State and Materials Science 9(2005)313-316 1500 Fig. 5. Morphology and shape memory effect of 8Ce-05Y-TZP ceramics manufactured by superfine particle and sintered at ambient, which shows 1.2% recovery strain upon pre-strained at room temperature followed by heat ove550°C 7. Stabilization of metastable tetragonal phase Table 1 Interfacial energies (Jm-) for monoclinic and tetragonal ZrO2under metastable tetragonal phase stabilization in zirconia was Interface o nditions reported by Garvie[4, "5) recently presented by Shukla and Seal [*5] Incoherent 146 The tetragonal phase can be stabilized at room tempera Partially coherent 0.73 0.55 ture in an isolated, single, strain free nanoparticle below crit- Coherent 0.29 0.22 ical size of 10 nm on account of the surface energy difference etween tetragonal and monoclinic phases(Table 1) besides of tetragonal structure in an isolated, strain free, spherically to a large specific surface area on the nanoscale[*4,**5, 37) shaped ZrOz nanocrystallite is because of the generation of size can be schematically shown as Fig. 6. The critical size size effecr en ion vacancies as a result of the"nanoparticle An schematic phase diagram including the effect of grain excess o the mechanism of tetragona increases to 33 nm owing to aggregation of ZrO2 nanocrys- phase stabilization in nanocrystalline ZrOz appears to be tallies [4. Various factors such as hydrostatic stra the same as that in doped ZrO2 at roo nergy, non-hydrostatic strain energy, structural similarity, undoped ZrOz at higher temperature". The excess oxygen ion vacancies may correlate with the excess volume,a significantly affect the tetragonal phase stability at room parameter used in the evaluation the contribution of surface temperature [38-42]. It is suggested that the stabilization layer to the whole Gibbs free energy by dilation model [44, 45]. The contribution becomes significant as the grain size of ZrO, nanoparticle below the critical value. The con cept that the presence of vacancies are essential for stabilize tion of the tetragonal phase was not accepted by all and more evidence is required [7] Tetragonal 8. Conclusions Characteristics(thermodynamics, kinetics and crystal Monoclinic graphics) of t-m martensitic transformation, related mechanism of transformation toughening and stabilization of metastable tetragonal phase at lower temperatures have been briefly reviewed. Combination of toughness and new functions makes TZP very attractive. Attention will be con- 1/2nm tinuously paid to the mechanism of t- m transformation in bulk as well as in nanocrystalline such as the role of Fig. 6. Schematic presentation of the effect of nanocrystallinite size on the vacancies in the stabilization of metastable tetragonal t- m martensitic transformation temperature under ambient. phase and the structural similarities between the evolving

7. Stabilization of metastable tetragonal phase A detail review about mechanisms of room temperature metastable tetragonal phase stabilization in zirconia was recently presented by Shukla and Seal [**5]. The tetragonal phase can be stabilized at room tempera￾ture in an isolated, single, strain free nanoparticle below crit￾ical size of 10 nm on account of the surface energy difference between tetragonal and monoclinic phases (Table 1) besides to a large specific surface area on the nanoscale [*4,**5,37]. An schematic phase diagram including the effect of grain size can be schematically shown as Fig. 6. The critical size increases to 33 nm owing to aggregation of ZrO2 nanocrys￾tallines [*4]. Various factors such as hydrostatic strain energy, non-hydrostatic strain energy, structural similarity, foreign surface oxides, water vapor and anionic impurities, significantly affect the tetragonal phase stability at room temperature [38–42]. It is suggested that the stabilization of tetragonal structure in an isolated, strain free, spherically shaped ZrO2 nanocrystallite is because of the generation of excess oxygen ion vacancies as a result of the ‘‘nanoparticle size effect’’ [43]. Therefore, ‘‘the mechanism of tetragonal phase stabilization in nanocrystalline ZrO2 appears to be the same as that in doped ZrO2 at room temperature and undoped ZrO2 at higher temperature’’. The excess oxygen ion vacancies may correlate with the excess volume, a parameter used in the evaluation the contribution of surface layer to the whole Gibbs free energy by dilation model [44,45]. The contribution becomes significant as the grain size of ZrO2 nanoparticle below the critical value. The con￾cept that the presence of vacancies are essential for stabiliza￾tion of the tetragonal phase was not accepted by all and more evidence is required [*7]. 8. Conclusions Characteristics (thermodynamics, kinetics and crystallo￾graphics) of t ! m martensitic transformation, related mechanism of transformation toughening and stabilization of metastable tetragonal phase at lower temperatures have been briefly reviewed. Combination of toughness and new functions makes TZP very attractive. Attention will be con￾tinuously paid to the mechanism of t ! m transformation in bulk as well as in nanocrystalline such as the role of vacancies in the stabilization of metastable tetragonal phase and the structural similarities between the evolving 500 400 300 200 100 Strain (%) Temperature (˚C) 0 1 2 3 4 5 0 500 1000 1500 2000 Stress (MPa) Fig. 5. Morphology and shape memory effect of 8Ce–0.5Y–TZP ceramics manufactured by superfine particle and sintered at ambient, which shows 1.2% recovery strain upon pre-strained at room temperature followed by heating above 550 C. 1443 K 300 K 1/12 nm-1 Temperature 1/D Tetragonal Monoclinic Fig. 6. Schematic presentation of the effect of nanocrystallinite size on the t ! m martensitic transformation temperature under ambient. Table 1 Interfacial energies (J m2 ) for monoclinic and tetragonal ZrO2 under different conditions reported by Garvie [*4,**5] Interface Monoclinic Tetragonal Incoherent 1.46 1.1 Partially coherent 0.73 0.55 Coherent 0.29 0.22 X.-J. Jin / Current Opinion in Solid State and Materials Science 9 (2005) 313–318 317