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14 декабря, 2021
Refractory metals and alloys offer attractive and promising high-temperature properties, including high-temperature strength, good thermal conductivity, and compatibility with most liquid metal coolants, many of which are suitable for applications in nuclear environments. Though many of the refractory alloys have been known for more than 60 years, there are significant gaps in the materials property database for both unirradiated and irradiated conditions. In addition, significant issues related to low-temperature irradiated mechanical property degradation at even low neutron fluences restrict the use of refractory metals. Protection from oxidizing environments also restricts their use, unless suitable protection or a liquid metal coolants is used.
Much of the early research on refractory metal alloys was centered on applications in aerospace as well as cladding and structural materials for fission reactors, with particular emphasis on space reactor applications. Reviews concerning the history of these programs and the development of many of the alloys whose irradiated properties are discussed in this chapter can be found elsewhere.1-5 Due to cancellations and reintroduction of new mission criteria for these space reactor programs, the materials database shows similar waves in the gains of intellectual knowledge regarding refractory alloy and irradiated property behavior. Unfortunately, as seen in the subsequent sections of this chapter, much of the irradiated property database for refractory metals consists of scoping examinations that show little overlap in either material type, metallurgical conditions (i. e., grain size, impurity concentrations, thermomechanical treatments), radiation conditions (i. e., spectra, dose and temperature), or postirradiation test conditions or methods.
The irradiation behavior of body-centered cubic (bcc) materials is known. Irradiation-induced swelling because of void formation in the material lattice is typical for temperatures between 0.3 and 0.6 Tm, where Tm is the melting temperature. Maximum swelling in refractory metals is <10% for displacement damage levels up to 50 dpa (displacements per atom), but typical values for fission reactor applications are <4%. Alloy additions can further reduce the sensitivity to swelling, for example, rhenium additions to molybdenum or tungsten. These levels of swelling are manageable through the appropriate engineering design of components.
The generation of dislocation loops and point defects provide significant irradiation-induced strengthening or hardening of refractory metals and alloys. This in turn creates reductions in the ductility and fracture toughness of the material. This is most pronounced at temperatures <0.3 Tm, where defect mobility is reduced. The increase in the yield strength of the material because of the irradiation — induced defects can exceed the fracture strength of the material, leading to brittle behavior. These degradations in material property can begin to occur at neutron fluences as low as 1 x 102°ncm~2, or ^0.03 dpa3 and increase in severity with dose. As irradiation temperatures increase, dislocation loop and void sizes increase, whereas their densities are reduced, providing improvements in ductility, though at a reduced strength of the material. At high enough temperatures, recovery of properties to levels close to that of the unirradiated values is possible, though changes in material properties may be further influenced by microstructural changes such as segregation or precipitate formation of solute and transmuted species or recrystallization, which can lead to further deterioration of properties. Detailed information on the effects of radiation on materials is presented in Chapter 1.03, Radiation-Induced Effects on Microstructure, and in Chapter 1.04, Effect of Radiation on Strength and Ductility of Metals and Alloys. In general, the use of refractory alloys in radiation environments is not recommended at temperatures <0.3 Tm. However, new research work, particularly on molybdenum and its alloys, has shown that control over interstitial element contamination levels, grain size, and morphology, as well as the introduction of oxide dispersion strengthening, can lead to improvements in the low — temperature irradiation behavior. This is discussed in detail in this chapter.
The following sections of this chapter deal with the irradiated properties database of niobium, tantalum, molybdenum, and tungsten, as well as their alloys. While vanadium may sometimes be considered a refractory metal, its melting temperature is considerably lower than that of the other materials mentioned. However, its radiation effects database is considerable and well advanced relative to some refractory metals and it is therefore discussed separately in Chapter 4.12, Vanadium for Nuclear Systems. The irradiated properties database for refractory alloys is particularly thin, especially involving fracture toughness properties, irradiation creep effects, and combined radiation effects with high thermomechanical loads such as those experienced in plasma facing components or spallation target materials. Where needed, a comparison of the unirradiated and irradiated properties of a material is given.