S. Bueno et al. /Journal of the European Ceramic Society 28 (2008)1961-1971 Table I Properties of the materials: average grain size(G), relative density (p), static Young s modulus(E) and three points bending strength(of) GA(S.D. )(um) GAT(.D)(um) p(S D )( theoretical) E(S D )(GPa) ar(SD )(MPa) 3.5(0.3) 981(0.3) 379(8) 456(29) 5.5(0.6) 981(0.5) 349(31) A10-1450 3.2(0.4 2.2(0.1) 973(0.5) 301(4) 261(6 10-1550 3.9(0.3 24(0.2) 97200.3) 272(10) 230(1) AlOAT 985 367(5) 360(31) A: alumina, AT: aluminum titanate: S D. standard deviation notch depths utilized. Reported values are the average of three(0.85-0.87)was slightly lower than the stoichiometric(0.89) determinations and errors are the standard deviations. R curves Nevertheless, results of these semi quantitative analyses are valid were determined from the load versus displacement curves cor- for comparative purposes. No Ti was detected inside the alumina responding to tests performed with a relative notch depth of 0.6 grains of the composites(Fig 2a), whose analyses were simi- of the thickness of The fracture surfaces of tested strength and SEVNB speci mens were characterized by FEG-SEM. Also small samples of the lateral faces(face dimension 50 mm x 6 mm)containing the notches and the cracks were polished and chemically etched (HF-10 vol %0-3 min) in order to observe the zones surround ing the propagating cracks to characterize the process zones. In order to complement the fractographic observations, polished surfaces of composite samples indented with a Vickers point using 50N during 10s, were also observed. 4. Results and discussion 4. Microstructure The microstructures of both aluminas were typical of mate- rials fabricated from high-purity submicron alumina powders The material sintered at 1450C was constituted by equidimen sional grains with a narrow distribution of relatively small sizes whereas that sintered at 1550C presented a coarser microstruc ture with a wide distribution of sizes and pore trapping associated 04m with exaggerated grain growth. The microstructural parameters together with the density, static Youngs modulus and strength values are reported in Table 1 The composites presented micrometer sized(2.2-2. 4 um, Fig. la and b, Table 1)aluminium titanate grains homoge neously distributed and located mainly at alumina triple points and grain boundaries and alumina grains of sizes similar to those of the monophase alumina sintered at 1450C(3. 2-3.9 um, Fig. la, Table 1). Submicrometric second phase grains were also observed inside the alumina grains and occasionally at grai boundaries(Fig. la). Additional nanometer sized grains were bserved at grain boundaries by SEM(Fig. lb) In Fig. 2 characteristic STEM observations for the ites sintered at 1550C together with EDX chemical analysis are shown. The ratios(wt %)Al/O(E1.4)and T1/O(E1.3)in the than those corresponding to the stoichiometric, 0.68 and 0.60, electron micrographs of polishedand s of the stud grains of aluminium titanate( Fig. 2a)were always well higher Fig. 1. Characteristic micr AlO composites. Scanning ermally etched surfaces. Alumina grains respectively. The Ka. B radiations emitted by light elements have appear with dark grey colour whereas micrometer sized aluminium titanat lower energies and are preferentially absorbed by carbon con- cromenc. ghter gray shade.(a)Composite A10 sintered at 1450CSubmi- tamination formed during the spot analyses. This induces an (b)Composite A10 sintered at 1550C. Detail of nanosized(arrows)aluminium underestimation of oxygen concentration. Also, the ratio Ti/al titanate grains located at the boundaries between the alumina grains.1964 S. Bueno et al. / Journal of the European Ceramic Society 28 (2008) 1961–1971 Table 1 Properties of the materials: average grain size (G), relative density (ρ), static Young’s modulus (E) and three points bending strength (σf) GA (S.D.) (m) GAT (S.D.) (m) ρ (S.D.) (% theoretical) E (S.D.) (GPa) σf (S.D.) (MPa) A-1450 3.5 (0.3) – 98.1 (0.3) 379 (8) 456 (29) A-1550 5.5 (0.6) – 98.1 (0.5) 376 (6) 349 (31) A10-1450 3.2 (0.4) 2.2 (0.1) 97.3 (0.5) 301 (4) 261 (6) A10-1550 3.9 (0.3) 2.4 (0.2) 97.2 (0.3) 272 (10) 230 (1) A10AT 98.5 (0.1) 367 (5) 360 (31) A: alumina, AT: aluminum titanate; S.D.: standard deviation. notch depths utilized. Reported values are the average of three determinations and errors are the standard deviations. R curves were determined from the load versus displacement curves cor￾responding to tests performed with a relative notch depth of 0.6 of the thickness of the samples. The fracture surfaces of tested strength and SEVNB speci￾mens were characterized by FEG-SEM. Also small samples of the lateral faces (face dimension 50 mm × 6 mm) containing the notches and the cracks were polished and chemically etched (HF-10 vol.%–3 min) in order to observe the zones surround￾ing the propagating cracks to characterize the process zones. In order to complement the fractographic observations, polished surfaces of composite samples indented with a Vickers point using 50 N during 10 s, were also observed. 4. Results and discussion 4.1. Microstructure The microstructures of both aluminas were typical of mate￾rials fabricated from high-purity submicron alumina powders. The material sintered at 1450 ◦C was constituted by equidimen￾sional grains with a narrow distribution of relatively small sizes whereas that sintered at 1550 ◦C presented a coarser microstruc￾ture with a wide distribution of sizes and pore trapping associated with exaggerated grain growth. The microstructural parameters together with the density, static Young’s modulus and strength values are reported in Table 1. The composites presented micrometer sized (2.2–2.4m, Fig. 1a and b, Table 1) aluminium titanate grains homoge￾neously distributed and located mainly at alumina triple points and grain boundaries and alumina grains of sizes similar to those of the monophase alumina sintered at 1450 ◦C (3.2–3.9m, Fig. 1a, Table 1). Submicrometric second phase grains were also observed inside the alumina grains and occasionally at grain boundaries (Fig. 1a). Additional nanometer sized grains were observed at grain boundaries by SEM (Fig. 1b). In Fig. 2 characteristic STEM observations for the compos￾ites sintered at 1550 ◦C together with EDX chemical analysis are shown. The ratios (wt.%) Al/O (∼=1.4) and Ti/O (∼=1.3) in the grains of aluminium titanate (Fig. 2a) were always well higher than those corresponding to the stoichiometric, 0.68 and 0.60, respectively. The K, radiations emitted by light elements have lower energies and are preferentially absorbed by carbon con￾tamination formed during the spot analyses. This induces an underestimation of oxygen concentration. Also, the ratio Ti/Al (0.85–0.87) was slightly lower than the stoichiometric (0.89). Nevertheless, results of these semi quantitative analyses are valid for comparative purposes. No Ti was detected inside the alumina grains of the composites (Fig. 2a), whose analyses were simi￾Fig. 1. Characteristic microstructures of the studied A10 composites. Scanning electron micrographs of polished and thermally etched surfaces. Alumina grains appear with dark grey colour whereas micrometer sized aluminium titanate grains have lighter gray shade. (a) Composite A10 sintered at 1450 ◦C. Submi￾crometric second phase grains inside the alumina matrix are pointed by arrows. (b) Composite A10 sintered at 1550 ◦C. Detail of nanosized (arrows) aluminium titanate grains located at the boundaries between the alumina grains.