Determination of Pipe Diffusion Coefficients in Undoped and Magnesia-Doped Sapphire (α-Al2O3): A Study Based on Annihilation of Dislocation Dipoles

• Published in: Journal of the American Ceramic Society 2003-04, Vol.86 (4), p.560-65
• Main Authors: Tang, Xiaohong, Lagerlöf, K. Peter D., Heuer, Arthur H.
• Format: Article
• Language: English
• Subjects: alumina; Condensed matter: structure, mechanical and thermal properties; Defects and impurities in crystals; microstructure; deformation; Deformation and plasticity (including yield, ductility, and superplasticity); Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.); dislocations; Exact sciences and technology; Mechanical and acoustical properties of condensed matter; Mechanical properties of solids; Physics; sapphire; Structure of solids and liquids; crystallography
• ISSN: 0002-7820; 1551-2916
• DOI: 10.1111/j.1151-2916.2003.tb03341.x

The breakup of dislocation dipoles in plastically deformed samples of undoped and 30‐ppm‐MgO‐doped sapphire (α‐Al2O3) was monitored using conventional TEM techniques. Dislocation dipoles break up into prismatic dislocation loops in a sequential process during annealing; i.e., dislocation loops are pinched off at the end of a dislocation dipole. This pinch‐off process is primarily controlled by pipe diffusion, and pipe diffusion coefficients at temperatures between 1300° and 1500°C were estimated by monitoring the kinetics of the dipole breakup process. We determined DPU= 8.1(–4.3+9.1) × 10–3 exp [–(4.5 ± 1.3eV)/kT)] m2/s for the undoped material. The pipe diffusion kinetics for the MgO‐doped crystal was determined at 1250° and 1300°C and was about 6 times higher than for undoped sapphire. Finally, climb dissociation of the dislocations constituting the perfect dipoles in sapphire is common; annihilation of one set of partials can result in the formation of faulted dipoles, which can pinch off to form faulted dislocation loops. DPU for faulted dipoles in the undoped material was determined at 1300° and 1350°C, and was about 4–10 times higher than for perfect dipoles.