The aging of the NC 19 Fe Nb alloy (Inconel 718), previously quenched from 990 °C, is characterized by a hardness peak at 650 °C, then a maximum in hardness at about 750 °C. Over this temperature, the hardness progressively decreases. In the 550–650 °C temperature range, TEM observations have revealed that β (Ni3Nb) precipitates are formed as long platelets parallel between them within the same grain, as well as extremely fine γ′[Ni3(Ti, Al)] particles responsible for the observed improvement in hardness. For a tempering temperature higher than 650 °C, a first hardening occurs after a 4 h treatment, which has been associated with the γ′ phase precipitation, with a more or less spherical shape. Beyond this time, a second hardening takes place linked to the γ″ phase precipitation (Ni3Nb, bct D022 structure), as thin platelet shaped, perfectly coherent with the matrix. The misfit between the γ and γ″ phases is about 3% in the 〈001〉γ″ direction and lower than 1% in the 〈100〉γ″ and 〈010〉γ″ directions. During a longer aging at 750 °C, the γ″ platelets progressively dissolve while β precipitates grow.
Taking into account some attractive properties of the thorium phosphate−diphosphate (Th4(PO4)4P2O7), called TPD, we focused our research on its potential use as a ceramic for the long term storage of some radionuclides including trivalent and tetravalent actinides. In this objective, we already investigated with success several ways of preparation of the pure TPD and solid solutions with tetravalent actinides either by wet or dry chemical methods. In the field of nuclear applications and considering handling operations on the radioactive materials, the preparation of this compound in the ceramic form was developed. Therefore, a precursor of TPD was especially synthesized in order to get sintered samples after performing an uniaxial pressing at room temperature followed by a heat treatment at high temperature. The synthesis, the characterization, and the sintering procedure of the precursor are described in this paper. The influence of the heating temperature on the specific area of the powder is examined and then correlated with the granulometry and the densification of the solid. For all of the samples, the apparent and effective relative densities obtained are in the range of 87−99% of the value calculated from XRD data. Moreover, some physical properties such as thermal diffusivity, thermal conductivity, and specific heat were measured from room temperature up to 1000 °C. The main results are given in this paper. Finally, the durability of the sintered TPD samples during leaching tests was studied. The apparent dissolution rates remain low even in very corrosive media (1.25(2) × 10-3 g d-1 in 5 M HNO3 at 25 °C) which shows that the TPD can be considered as a promising candidate for the immobilization of actinides and some fission products for a long-term storage.
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