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

Abstract

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.

Edge dislocation slows down oxide ion diffusion in doped CeO2 by segregation of charged defects

“…The mechanism responsible for the slow diffusion along an edge dislocation in ceria is quite different from those observed in uranium dioxide 43 , alumina 64 and magnesium oxide 65 , in which dislocations were found to be fast pipe diffusion paths. For uranium and alumina, the fast ion diffusion at dislocations is mediated by interstitial type oxygen defects.…”

The stress dependence of olivine creep rate: Implications for extrapolation of lab data and interpretation of recrystallized grain size

“…D Si pipe can be estimated by equating the ratio of the observed and calculated strain rates shown in Fig. 4b (Tang et al, 2003). For comparison, at the same homologous temperature (T /T melting = 0.7), D O vol is 10 −20 m 2 /s for olivine, 10 −21 for Al 2 O 3 and 10 −17 for MgO, illustrating that the diffusion kinetics of these oxides are similar to olivine.…”

“…Following Twiss (1977), we analyze the energy balance between strain energy per unit volume from free dislo- Blue symbols from van der Wal et al (1993) for experiments conducted at 1100 • C to 1400 • C; Red symbols from Karato et al (1980) for experiments conducted at 1500 • C to 1650 • C. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) cations (αρμb 2 ) and surface energy per unit volume from grain boundaries (2γ /d):…”

Oxygen and aluminum diffusion in α-Al2O3: How much do we really understand?

“…On the other hand, Tang et al 50 used an indirect method based on annihilation of dislocation dipoles to determine D pipe . Undoped and 30 ppm Mg-doped sapphire which had been plastically deformed by basal slip ((0 0 0 1) 1/3 [1 12 0]) to a strain of ∼5% at 1375 • C were studied.…”