Mechanical and structural properties of radiation-damaged allanite-(Ce) and the effects of thermal annealing
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The onset of thermally induced, heterogeneous structural reorganization of highly radiation-damaged allanite-(Ce) begins at temperatures below 700 K. Three strongly disordered allanite samples (S74 20414: ~ 0.55 wt% ThO2, 22.1 wt% REE oxides, and maximum radiation dose 3.5 × 1018 α-decay/g; LB-1: ~1.18 wt% ThO2, 19.4 wt% REE oxides, and maximum radiation dose 2.0 × 1019 α-decay/g; R1: ~ 1.6 wt% ThO2, 19.7 wt% REE oxides, and maximum radiation dose 2.6 × 1018 α-decay/g) were step-wise annealed to 1000 K in air. Using orientation-dependent nanoindentation, synchrotron single-crystal X-ray diffraction (synchrotron XRD), X-ray powder diffraction (powder XRD), differential scanning calorimetry and thermogravimetric analysis (DSC/TG), mass spectrometry (MS), 57Fe Mössbauer spectroscopy and high-resolution transmission electron microscopy (HRTEM), a comprehensive understanding of the structural processes involved in the annealing was obtained. As a result of the overall increasing structural order, a general increase of hardness (pristine samples: 8.2–9.3 GPa, after annealing at 1000 K: 10.2–12 GPa) and elastic modulus (pristine samples: 115–127 GPa, after annealing at 1000 K: 126–137 GPa) occurred. The initially heterogeneous recrystallization process is accompanied by oxidation of iron, the related loss of hydrogen and induced stress fields in the bulk material, which cause internal and surface cracking after step-wise annealing from 800 to 1000 K. HRTEM imaging of the pristine material shows preserved nanometer-sized crystalline domains embedded in the amorphous matrix, despite the high degree of structural damage. The results show that hardness and elastic modulus are sensitive indicators for the structural reorganization process.
KeywordsAllanite Radiation damage Mechanical properties Nanoindentation Thermal annealing Oxidation
This research was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)—BE 5456/2-1 (T.B. and C.E.R.). R.C.E. was supported by the U.S. Department of Energy through the Energy Frontier Research Center “Materials Science of Actinides” under Award Number DE-SC0001089. A.N. was supported by the U.S. Department of Energy Grant DE-FG02-97ER14749. L.A.G. acknowledges the support of the Natural Sciences and Engineering Research Council of Canada (NSERC), funding reference 06434. We thank Thomas Malcherek for sample orientation, Gregor Hofer and Warren Oliver for fruitful discussions, the Geological Survey of Norway (NGU) for a quick and helpful response concerning the age database of the NGU, Peter Stutz for sample preparation and Jan Cempírek, the Pacific Museum of Earth (UBC), and the Fersman Mineralogy Museum for samples (LB-1 and R1). The constructive comments and helpful suggestions of two anonymous reviewers are gratefully acknowledged.
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