Role of Deformable Fine Spinel Particles in High-Strain-Rate Superplastic Flow of Tetragonal ZrO 2
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Role of Deformable Fine Spinel Particles in High-Strain-Rate Superplastic Flow of Tetragonal ZrO2 Koji Morita, Keijiro Hiraga, Byung-Nam Kim and Yoshio Sakka National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan ABSTRACT The role of MgAl2O4 spinel particle dispersion in high-strain-rate superplasticity (HSRS) of tetragonal ZrO2 was examined by characterizing microstructural changes during deformation. The dispersed spinel particles elongate with strain along tensile direction and the elongation tends to be pronounced with increasing strain rate. In the elongated spinel particles, intragranular dislocations lying along the elongated direction were observed, suggesting that the elongation relates to the dislocation motion. The flow behavior characterized by a stress exponent of ≈2.0 suggests that grain boundary sliding (GBS) is the predominant flow mechanism. The dislocation-induced plasticity in the spinel particles may assist the relaxation of stress concentrations exerted by GBS, leading to HSRS in tetragonal ZrO2. INTRODUCTION Recently, we attained high-strain-rate superplasticity (HSRS) in fine particle dispersed ceramic composites; Al2O3-spinel-ZrO2 [1,2] and ZrO2-spinel [3-5]. For tetragonal ZrO2 dispersed with 30vol% spinel, the fine spinel particle dispersion can improve strain rate by 102-103 times in attaining similar tensile elongation in monolithic tetragonal ZrO2 [6-8]. The tensile elongation, ef, ． exceeds ≈250% at 1823 K and at an initial strain rate of ε0 ≈ 0.7 s-1 [3-5]. A common microstructural feature in the high-strain-rate superplastic deformed ceramic composites is that the dispersed particles appear to be plastically deformable to some extent during deformation. Particularly in the ZrO2-spinel composite, the dispersed spinel particles elongate along tensile direction after deformation. This aspect indicates that the dispersed particles play an important role in the attainment of HSRS in ceramics. The present study was therefore performed to examine the role of deformable particle dispersion in HSRS of ZrO2-spinel composite. In this study, we lay an emphasis on characterizing quantitatively microstructural changes during high-strain-rate superplastic flow. EXPERIMENTAL PROCEDURES The specimens used in this study was prepared by a method described elsewhere [3]. Briefly, 3mol%-Y2O3-stabilized tetragonal ZrO2 (TZ-3Y, Tosoh Co., Ltd.,) mixed with 30vol% spinel
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powders (SP-12, Iwatani Co., Ltd.,) were cold-isostatically pressed at about 400 MPa and sintered at 1673 K for 2 h in air. Constant displacement-rate tensile tests were conducted using an ． Instron-type tensile machine at 1773 K and at ε0 ≈ 2.0×10-3-0.3 s-1 under vacuum. For scanning electron microscopy (SEM) observation, the surface of the specimens were mechanically polished and thermally etched at 1573 K for 10 min. The average grain size, d, was determined as 1.56 times of the average intercept lengths of grains [9]. The grain aspect ratio (GAR) was determined from the intercept leng
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