al / Acta m ceramics are thermoelastic transformations, mainly based of 10 and deposited with carbon. These thin foils were on the tem observation mentioned above and X-ray then suitable for tEM observation measurements(3) When Sme behavior in some zirconia formed using containing ceramics was discovered, Reyes-Morel et al. an H-800 instrument with an attached heater. The temper 3]attributed it to the thermoelasticity in 12Ce-TZP ature of the foil could be increased up to 800C. By care- Here it should be noted that the monoclinic laths are fully adjusting current and illumination conditions of the thermal stress-induced martensite resulting from the illumi- electron beam, the local thermomechanical stress generated nation by the electron beam rather than the thermally by beam heating is expected to induce t-m martensitic induced martensite of the in situ TEM experiments men- transformations tioned above. Nevertheless it is well known that a transfor- mation can traditionally be defined as a thermoelastic 3. Crystallographic calculation process if the interface can adjust its position freely with a change of temperature. In addition, no in situ TEM The crystallography of the t-m transformation in observations for CeOy-ZrO, ceramics have been reported Ce-Y-TZP is calculated on the basis of the Wechsler- so far. Hence, for zirconia-containing ceramics, the results Lieberman-Read (WLR) theory [26]. The WLR theor in the literature have not provided convincing evidence for can be expressed in terms of (3 x 3)matrices as follow the thermoelastic nature, and the nature of the transforma- F= rBS tion in the newly developed ternary Ce-Y-TZP SMC also needs further investigation where F is the shape deformation, B the Bain strain, S the The crystallography of t-m martensitic transforma- simple shear and R the rigid body rotation. tions has been evaluated by a phenomenological theory The inputs required by the phenomenological theory are for many zirconia-containing ceramics [15-23]. The agree- the crystal structures, the lattice parameters of the parent ment between the experimental results and theoretical pre- and product phases, the lattice correspondence (Lc) diction demonstrates that the theory can be applied to between the two structures, and the elements of lattice make reliable, quantitative predictions for the martensitic invariant shear(LIS). In the case of the t-m transforma- transformation in these ceramic systems [24] The shape tion in zirconia-containing ceramics, there are three possi strain associated with the t-m transformation, being ble LCs denoted as LCA, LCB and LCC, which depend on extremely difficult to measure experimentally, is the key the axis of the monoclinic structure being parallel to the issue in understanding transformation toughening and unique c axis of the parent tetragonal phase. This simpl the SME in zirconia-containing ceramics. Thus, the calcu- notation was extended to provide a more comprehensive lation of the shape strain via the phenomenological theory system describing the different LCs between tetragonal is useful and provides an insight into the nature of this and monoclinic zirconia by Hayakawa et al. [15-17]. To transformation allow the different variants of a single correspondence to In the present work, the microstructural evolution is be identified and labeled, they assumed that the two crys studied by means of in situ TEM observations for both tallographically equivalent a, axes are distinguishable from thermal stress-induced and thermally induced martensitic one another and arbitrarily denoted by the axes at and bt transformations in order to investigate the nature of the for example, the notation CAB means that at, b. and ct t-m martensitic transformation in Ce-Y-TZP SMC. become cm, am and bm, respectively The phenomenological theory is applied, and some features of this transformation are discussed 2. Experimental The materials studied here were ternary TZP ceramics with two different Y2O3 contents, i.e., &Ce-0.25Y-TZP (8 mol% CeOx-025 mol% Y2O3-ZrO2) and &Ce-0.50Y TZP Superfine powders were prepared by co-precipitation 51. The composite powders were afterwards compacte into biscuits by uniaxial compression at 200 MPa, and then the biscuits were sintered at 1500 oC for 6 h. The m. values for bulk specimens were measured using an LK-02 dilatometer Discs of 50 um x 3 mm diameter were cut from bulk specimens, then glued on a molybdenum ring with ethoxy- line resin, which served as a supporting skeleton protecting the foils from breakage during manipulation. The well- Fig. 1. Thermally induced martensite(a)and selected area diffraction glued specimens were further ion-thinned at a small angle pattern(b)for 8Ce-0.25Y-TZPceramics are thermoelastic transformations, mainly based on the TEM observation mentioned above and X-ray measurements. (3) When SME behavior in some zirconia￾containing ceramics was discovered, Reyes-Morel et al. [3] attributed it to the thermoelasticity in 12Ce-TZP. Here it should be noted that the monoclinic laths are thermal stress-induced martensite resulting from the illumi￾nation by the electron beam rather than the thermally induced martensite of the in situ TEM experiments men￾tioned above. Nevertheless, it is well known that a transfor￾mation can traditionally be defined as a thermoelastic process if the interface can adjust its position freely with a change of temperature. In addition, no in situ TEM observations for CeO2–ZrO2 ceramics have been reported so far. Hence, for zirconia-containing ceramics, the results in the literature have not provided convincing evidence for the thermoelastic nature, and the nature of the transforma￾tion in the newly developed ternary Ce–Y-TZP SMC also needs further investigation. The crystallography of t ! m martensitic transforma￾tions has been evaluated by a phenomenological theory for many zirconia-containing ceramics [15–23]. The agree￾ment between the experimental results and theoretical pre￾diction demonstrates that the theory can be applied to make reliable, quantitative predictions for the martensitic transformation in these ceramic systems [24]. The shape strain associated with the t ! m transformation, being extremely difficult to measure experimentally, is the key issue in understanding transformation toughening and the SME in zirconia-containing ceramics. Thus, the calcu￾lation of the shape strain via the phenomenological theory is useful and provides an insight into the nature of this transformation. In the present work, the microstructural evolution is studied by means of in situ TEM observations for both thermal stress-induced and thermally induced martensitic transformations in order to investigate the nature of the t ! m martensitic transformation in Ce–Y-TZP SMC. The phenomenological theory is applied, and some features of this transformation are discussed. 2. Experimental The materials studied here were ternary TZP ceramics with two different Y2O3 contents, i.e., 8Ce–0.25Y-TZP (8 mol% CeO2–0.25 mol% Y2O3–ZrO2) and 8Ce–0.50Y￾TZP. Superfine powders were prepared by co-precipitation [25]. The composite powders were afterwards compacted into biscuits by uniaxial compression at 200 MPa, and then the biscuits were sintered at 1500 C for 6 h. The Ms values for bulk specimens were measured using an LK-02 dilatometer. Discs of 50 lm · 3 mm diameter were cut from bulk specimens, then glued on a molybdenum ring with ethoxy￾line resin, which served as a supporting skeleton protecting the foils from breakage during manipulation. The well￾glued specimens were further ion-thinned at a small angle of 10 and deposited with carbon. These thin foils were then suitable for TEM observations. In situ observations and analyses were performed using an H-800 instrument with an attached heater. The temper￾ature of the foil could be increased up to 800 C. By care￾fully adjusting current and illumination conditions of the electron beam, the local thermomechanical stress generated by beam heating is expected to induce t ! m martensitic transformations. 3. Crystallographic calculation The crystallography of the t ! m transformation in Ce–Y-TZP is calculated on the basis of the Wechsler– Lieberman–Read (WLR) theory [26]. The WLR theory can be expressed in terms of (3 · 3) matrices as follows: F ¼ RBS; where F is the shape deformation, B the Bain strain, S the simple shear and R the rigid body rotation. The inputs required by the phenomenological theory are the crystal structures, the lattice parameters of the parent and product phases, the lattice correspondence (LC) between the two structures, and the elements of lattice invariant shear (LIS). In the case of the t ! m transforma￾tion in zirconia-containing ceramics, there are three possi￾ble LCs denoted as LCA, LCB and LCC, which depend on the axis of the monoclinic structure being parallel to the unique c axis of the parent tetragonal phase. This simple notation was extended to provide a more comprehensive system describing the different LCs between tetragonal and monoclinic zirconia by Hayakawa et al. [15–17]. To allow the different variants of a single correspondence to be identified and labeled, they assumed that the two crys￾tallographically equivalent at axes are distinguishable from one another and arbitrarily denoted by the axes at and bt; for example, the notation CAB means that at, bt and ct become cm, am and bm, respectively. Fig. 1. Thermally induced martensite (a) and selected area diffraction pattern (b) for 8Ce–0.25Y-TZP. 1290 Y.L. Zhang et al. / Acta Materialia 54 (2006) 1289–1295