Mechanical and structural properties of radiation-damaged allanite-(Ce) and the effects of thermal annealing

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.