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DISLOCATIONS AND TWINNING ACTIVATED BY THE ABRASION OF Al2O3
Abstract
ÐDefects generated at the surface of single crystal Al2O3 during abrasion on diamond have been
analysed by high spatial resolution cross-sectional electron microscopy. Plastic deformation predominantly
occurs by basal twinning and pyramidal slip 1/3h1120i{1101}. Basal twins have platelet morphologies, with
widths d0001=2n1/6[0001] and vd0001v< <100 nm, aspect ratios d1010/d0001 or d1120/d0001 > 10, and ABCBA
stacking of (0006)Al planes across twin interfaces. Basal twins and dislocations occur clustered around
steps in macroscopic (0001) abraded surfaces, whereas 2{1120} and 2{1010} prismatic plane abrasion
results in a more uniform 1±5% deformation twin cover with basal twin penetration> >that for basal
abrasion. Extensive microcracking along {1102} rhombohedral and (0001) twin habit planes occurs, but no
widespread rhombohedral twinning, prism glide or non-basal dislocation Burgers vectors were observed.
Implications for surface preparation of
Emission of partial dislocations from amorphous
intergranular boundaries in deformed nanocrystalline ceramics
A theoretical model is suggested that describes emission of partial lattice dislocations from amorphous intergranular boundaries
into crystalline grains in deformed nanocrystalline ceramics. Within the model, a dipole of immobile lattice dislocations is generated
at an amorphous intergranular boundary through local shear events in this boundary. One of the dislocations then undergoes transformation, resulting in emission of a partial dislocation into a grain. It is shown that the emission process is energetically favorable
in nanoceramic SiC deformed at high stresses.
2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Dislocation mechanism of deformation and strength of Al2O3±YAG
single crystal composites at high temperatures above 1500
Abstract
A new unidirectionally solidi®ed eutectic Al2O3±YAG composite has recently been fabricated by accurately controlling the unidirectional solidi®cation. The eutectic composite has a new microstructure, in which single crystal Al2O3 and single crystal YAG are
three-dimensionally and continuously connected and ®nely entangled without grain boundaries. The dislocation structure is
observed in both single crystal Al2O3 and YAG in the plastically deformed specimens in the tensile and compressive tests at high
temperatures for the Al2O3±YAG single crystal composite, showing that the plastic deformation occurred by dislocation motion.
The Al2O3±YAG single crystal composite fabricated has the following properties: (1) the ¯exural strength at room temperature can
be maintained up to just below melting point (about 1830C), (2) the compressive ¯ow stress at 1600C and a strain rate of 10ÿ4/s is
about 13 times higher than that of sintered composites of the same composition. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Al2O3±YAG; Composites; Dislocations; Microstructure-®nal; Strength
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