Next generation nuclear technologies will require materials to sustain extreme irradiated conditions; hence studying the behavior of nuclear materials under extreme environments is essential in improving existing materials or designing radiation tolerant materials for next generation nuclear reactors.
Irradiated metals are metals that have undergone microstructural changes due to radiation impact from energetic particles (such as neutron or fission fragments); these energetic particles collide with the atoms of a particular metal material, transferring to the atoms some energy and knocking the atoms out of the natural lattice positions. As the primary atom (pka) gets knocked away, this knocked-on atom then causes additional collisions with other atoms, thereby generating a cascade of displaced atoms. Radiation dosage is usually measured by displacement per atom (dpa), which is the average number of displacements per lattice atom.
In the typical nuclear environment, the average energy of a neutron is about 2MeV while the threshold energy to displace an atom from its lattice position in metals is just 20-40 eV; this means that about 50,000 atoms are displaced in a typical collision. In most cases, 90% of the displaced atoms recombine with vacated lattice positions and move back to its original position. However, the remaining non-correct but stable defects and rearrangements from radiation damage will affect material's mechanical properties.
Two common defect clusters are formed from vacancies or self-interstitial atoms (SIAs). Voids, bubbles are both three-dimensional vacancy clusters. However, a void refers to a vacancy not dependent on internal pressure while bubbles are defined as pressurized vacancies such as those formed from helium bubbles. Cavities refer to voids or bubbles formed from vacancies.
There are four common geometric configurations that arise from these vacancy and SIAs defect clusters: two planar dislocation loops either for vacancies or SIAs and two 3-D configurations, which are stacking fault tetrahedral (SFT) and cavities, for vacancy clusters. The dislocation loops are faulted or perfect and generally form on closed packed planes in FCC materials. SFTs are usually observed in FCC materials.
Irradiation of a metal can cause strengthening by two widely proposed mechanisms: source hardening and friction hardening.
Source hardening applies to the increase in stress of initial plastic deformation by which the dislocation is unlocked and could be multiplied. When defect clusters are introduced into the metal, they form in the vicinity of Frank-Read dislocation sources, thereby pinning or locking dislocation lines. Before this source can operate, multiply, and expand under an applied stress, the dislocations must be unpinned from the defect clusters. This requires an increase in stress more than that of the intrinsic metals. Once the stress level is sufficient to release the source, the moving dislocations can destroy the small defect clusters and reduce the stress needed to continue the deformation.
Friction hardening applies to the increase in stress required to sustain plastic deformation, which is often referred to as the flow stress. The resistance to dislocation movement arises from dislocation network and obstacles such as defect clusters, precipitates, and other impurities. The resistance can come from long range stresses caused by dislocation-dislocation interaction by dislocations' stress fields or from short range stresses that originate between the dislocation and discrete obstacles in the slip plane. The total applied stress required sustaining plastic deformation combines short and long stress ranges.
Radiation damage causes a few common observable physical threats to materials at different operating temperatures and damage levels. The common threats are embrittlement, volumetric swelling from void formation, creep, phase transitions, and swelling due to gas bubbles. In many metal samples, the point defects harden the material. However, too much hardness can introduce material embrittlement. Irradiation creep, a permanent deformation, could occur due to the change in the metal's damaged structure in which the material is more prone to deform in a particular direction. Moreover, vacancies formed from knocked on displaced atoms could congregate together and induce volumetric swelling inside the material such as bubbles. Lastly, phase transitions can be instigated by irradiation which may trigger the redistribution of alloy metal elements through a local transformation by changing to a more energetically favored shape.
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