Hot forging characteristics of transformation-toughened Al 2 O 3 /ZrO 2 composites
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I. INTRODUCTION Recent studies have shown that fine-grained (0.3 /mi), tetragonal ZrO 2 ( + 2.3 or 3.0 mol % Y 2 O 3 ) exhibits extensive plastic deformation in both tension1 and compression23 at temperatures exceeding 1400 °C, and can generally be classified in superplastic. A stress exponent of 2 was obtained in two of these studies. 12 Largegrained (7.5 ^ m ) , cubic ZrO 2 (6.6 mol % Y 2 O 3 ) fails under similar, initial compressive stresses (70 MPa) used to deform the tetragonal material. Two phase Al 2 O 3 /ZrO 2 materials [one containing 20 vol. % cubic ZrO 2 (6.6 mol % Y 2 O 3 ), the other containing 80 vol. % tetragonal ZrO 2 (3 mol % Y 2 O 3 )] have also been shown to exhibit extensive plastic deformation at 1500 °C. 24 In the Al 2 O 3 /20 vol. % ZrO 2 composite, the average size of the A12O3 and ZrO 2 grains was 1.1 and 0.7 fxm, respectively. Single-phase A12O3 with an average grain size of 3 fxm, and with abnormal grains exceeding 10,um, failed at 17% deformation after 4 h at 1500 °C under similar forging conditions. It is known that the ZrO 2 phase inhibits the growth of the A12O3 grains, suggesting that one of the roles of the ZrO 2 phase in these composites is to produce a finer-grained material and thus, a more deformable composite.5 Composites were therefore fabricated containing different amounts of tetragonal ZrO 2 . The deformation behavior and microstructure of these composites were analyzed in an attempt to understand the role of the ZrO 2 phase. II. EXPERIMENTAL PROCEDURES Alumina/zirconia composites containing 0, 5, 10, 15, 20, and 30 vol. % tetragonal ZrO 2 ( + 3 mol % Y 2 O 3 ) were prepared from A12O3 (from Sumitomo, Tokyo, Japan) and ZrO 2 (from TSK, Toya Soda, Tokyo, Japan) powders. Colloidal processing techniques were used to both ensure good homogeneity and to remove J. Mater. Res. 3 (3), May/Jun 1988
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large agglomerates, as described elsewhere.6 Mixed slurries were consolidated by pressure filtration,7 dried overnight in an oven, and sintered in air to relative densities exceeding 0.98 at 1600 °C for \ h. Single-phase, tetragonal ZrO 2 material was prepared by a similar route, but sintered at 1400 °C/1 h. Specimens were diamond cut, ground, and polished, to an orthogonal shape (approximately 4 x 4 x 8 mm). Stress-strain rate behavior was determined using a cantilevered deadweight creep apparatus containing the high-temperature creep furniture and extensometer8 shown in Fig. 1. A constant load was applied to the specimen through polished SiC pads. The specimen's cross-sectional area increases during forging, decreasing the true stress on the specimen. True stress was calculated from the change in the specimen thickness during forging by assuming both constant specimen volume and uniform cross-sectional area. The effect of friction hill stresses were ignored because specimen barreling was minimal. Strain rates were determined from the specimen thickness-time record. Two experiments were carried out for each material. The grain boundaries depicted in scanni
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