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 × 10 18 α-decay/g; LB-1: 1.18 wt% ThO2, 19.4 wt% REE oxides, and maximum radiation dose 2.0 × 10 19 α-decay/g; R1: ~1.6 wt% ThO2, 19.7 wt% REE oxides, and maximum radiation dose 2.6 × 10 18 α-decay/g) were step-wise annealed to 1000 K in air. Using orientationdependent 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), 57 Fe 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 800-1000 K. HRTEM imaging of the pristine material shows preserved nanometer-sized crystalline domains embedded in the amorphous matrix, despite the high degree of structural 2 damage. The results show that hardness and elastic modulus are sensitive indicators for the structural reorganization process.
Nanoindentation high-resolution mapping has been used to probe the mechanical properties [elastoplastic factor (S2/P, where S is the contact stiffness and P is the load), indentation hardness (H), and elastic modulus (E)] of a natural, highly zoned zircon (ZrSiO4). The zoning, on a scale of ∼ 5 to 400 μm, is due to variations in the U and Th concentrations, resulting in a range of α-decay event doses of ∼ 3.7× to 7.5 × 1018 α-decays/g. Thus, this single, zoned zircon crystal can be used to investigate the effects of α-decay radiation-damage on mechanical properties. The results also illustrate how multilayered ceramics accommodate volume expansion and change in mechanical properties as a function of radiation dose. Further, the detailed investigation of fractures in the lesser damaged, higher crystalline domains provides a better understanding of crack propagation in the initially crystalline material due to the strain induced by heterogeneous damage distribution. This is an important consideration in designing materials for the immobilization of plutonium from dismantled nuclear weapons, as plutonium decays by α-decay events. The directly measurable stiffness2/load (S2/P) provides a useful estimate of the degree of radiation-damage. The evolution of E provides experimental evidence for the predicted second percolation transition that denotes the end of percolation of the crystalline fraction.
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