On the Roles of Temperature and Interfaces in Irradiation and Thermally Induced Solid State Amorphization
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Mat. Res. Soc. Symp. Proc. Vol. 398 01996 Materials Research Society
EXPERIMENTAL PROCEDURES Materials irradiated in this study include coesite, zircon (ZrSiO4 ), various compositions of apatite (Ca 2La 8(Si0 4)60 2, Ca1 o(PO 4)6F2), olivine (Mg 2SiQ 4, Fe 2SiO 4) and spinel (MgAI20 4, FeCr 20 4, Zn(Fe,Mn) 20 4, Zn(Mn,Fe) 20 4). The irradiations were performed on ion milled TEM samples with 1.5 MeV Kr', 200 keV and 1 MeV electrons in a wide temperature range (20-700 K). 1.5 MeV Kr* and 1 MeV electron irradiations were carried out using the HVEM-Tandem facility at Argonne National Laboratory and 200 keV electron irradiation was completed using a JEM 2000FX or a JEM 2010 transmission electron microscope. In situ TEM was performed during irradiations in order to follow changes in the microstructure of the material. Critical amorphization dose, Dc, was recorded at the point where diffraction maxima from the crystalline phase had completely vanished in the electron diffraction pattern. Additional detailed TEM analysis HRTEM was performed after ion irradiation to various doses with the JEM 2010 microscope. The coesite samples were synthesized using a piston-cylinder apparatus at 3.5 GPa and 1173 K for 48 hours. The crystals are on the order of 5-10 microns in diameter. For the study of thermally-induced amorphization, the bulk samples were crushed into small pieces before annealing in order to assure free surfaces on the coesite grains. The edges of the crushed pieces were transparent to 200 keV electrons, and no amorphous rim was observed before annealing. Annealing of coesite was performed at 1200 K for 1 to 3.5 hours in order to study the isothermal growth of the amorphous phase. After the heat treatment, the specimens were dispersed with acetone onto holey-carbon films on Cu-grids for TEM examination. RESULTS AND DISCUSSION All the materials were amorphized at low temperatures (20 to 150 K) by the Kr' ion irradiation after fairly low doses (- 10" ions/cm2 or a fraction of a dpa) except the spinel crystals. In most cases, the critical amorphization dose, Dc, then increased with increasing irradiation temperature at various rates until reaching the characteristic critical temperatures above which complete amorphization could not be achieved [4-7]. HRTEM revealed amorphous domains of displacement cascade size in olivine [13], zircon [5] and apatite [14] at doses well below Dc, indicating amorphization occurred directly within the displacement cascades or by cascade overlap. Using a model developed by Weber et al. [5], the activation energy representing the barrier to recovery processes under the irradiation condition can be obtained from the D,temperature curve. The activation energies determined for olivine, zircon and apatite are all only a small fraction of 1 eV [4,5,7], considerably below the activation energy for thermal recrystallization of fully amorphized crystals (e.g., > 5 eV for zircon [5]). The small activation energy is believed to be associated with irradiation-enhanced intracascade defect recombination
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