Dong Liu

CrCoNi-based medium- and high-entropy alloys display outstanding damage tolerance, especially at cryogenic temperatures. In this study, we examined the fracture toughness values of the equiatomic CrCoNi and CrMnFeCoNi alloys at 20 kelvin (K). We found exceptionally high crack-initiation fracture toughnesses of 262 and 459 megapascal-meters ^½ (MPa·m ^½ ) for CrMnFeCoNi and CrCoNi, respectively; CrCoNi displayed a crack-growth toughness exceeding 540 MPa·m ^½ after 2.25 millimeters of stable cracking. Crack-tip deformation structures at 20 K are quite distinct from those at higher temperatures. They involve nucleation and restricted growth of stacking faults, fine nanotwins, and transformed epsilon martensite, with coherent interfaces that can promote both arrest and transmission of dislocations to generate strength and ductility. We believe that these alloys develop fracture resistance through a progressive synergy of deformation mechanisms, dislocation glide, stacking-fault formation, nanotwinning, and phase transformation, which act in concert to prolong strain hardening that simultaneously elevates strength and ductility, leading to exceptional toughness.

Polycrystalline graphite has a unique combination of high-temperature properties that has made it the material of choice for many industrial applications. Several nuclear reactor designs that operate between 500°C and 1,000°C include graphite components. These components must maintain their integrity even at the 1,800°C they could be exposed to during an accident. The operational behavior of these graphites during both proof testing of as-manufactured material and postirradiation examination must be determined by measuring physical, mechanical, and thermal properties. For reasons of expense and practicality the properties are measured in (or near to) ambient conditions. It is essential that the measured properties may be extrapolated reliably to high temperatures. Laboratory testing at elevated temperatures therefore provides data for (1) defining temperature-dependent extrapolation curves, (2) informing conceptual models that help to establish confidence in ambient-temperature test methods, and (3) inputs into numerical simulations of operating conditions. The properties of interest for this paper are selected on the basis of current ASTM standards to include those most relevant to current and future fission reactor operation. The effects of fast neutron irradiation on the high-temperature behavior are presented in general terms, and the conventional understanding of the mechanisms behind both the inert and irradiated behavior are outlined. Areas for further research are then highlighted, the findings of which would support design, qualification, operation, and safety monitoring of graphite-moderated nuclear reactors.

β-Ga2O3 was suggested to have excellent irradiation hardness which makes β-Ga2O3-based devices extremely attractive for nuclear and space applications. To discern the fundamental nano-scale structural changes with irradiation, an in situ irradiation experiment in a transmission electron microscope was carried out using 400 keV Ar ions of fluences up to 8 × 1015 cm−2 (equivalent to four displacements per atom). Contrary to previous works, which indicate a phase transition of β-Ga2O3 into the κ polymorph, the β-Ga2O3 structure was found to remain intact throughout except for (i) anisotropic lattice distortions, which are most significant at low levels of irradiation, and (ii) the appearance of additional weak reflections above 2 dpa irradiation. The origin of the extra reflections is discussed.