The fundamental elastic properties of metals are determined by the behavior of dislocations - which are systematic distortions introduced into the crystalline lattice. When the material is under stress, these dislocations can move. The ease with which they move, or more generally, by whether and if they can move, determines the elasticity (or plasticity) of the metal. The dislocations can be impeded in their movement by irregularities in the crystal - in the form of grain boundaries, or in the form of impurities - which can act as 'pinning centers' or 'sinks' for the dislocations.
The size and orientation of the grains within the metal relative to the dislocations, determines whether, and how easily, the dislocations can be pinned at the grain boundaries. As the grain size decreases, the dislocations are more easily pinned, and this increases the yield strength of the metal (that is, it delays the onset of plasticity). However, below a grain size of about 100 nm, a reversal of this rule has been observed - with the decline in yield strength flattening out, or the yield strength even declining with decreasing grain size.
The idea that impurities within a crystal can act as pinning centers for dislocations has been intuitively grasped for centuries, as well as the idea that deliberate introduction of misaligned dislocations and defects in a metal through heat treatment or cold rolling can lead to an overall strengthening of the metal. For example, the strength of steel itself arises from the incorporation of carbon atoms in interstices within the iron lattice, which act as pinning centers for dislocations. A similar idea underlies the concept of alloying for increased strength.
Advances in metallurgy led to heat treatments of alloys, followed by differential quenching. Deeper investigations revealed that such treatments led to the formation of a precipitate of the alloyed species, with both the alloyed species and the matrix existing in a metastable supersaturated solution. When the quenching is followed by moderate heating, the precipitate finely diffuses into the matrix, forming a dense network of pinning centers that provides very high tensile strength for the alloy. This technique became known as precipitation hardening.
The idea of deliberately introducing as fine a powder of the pinning species as possible into the steel matrix was then taken up, and eventually led to the development of steels with nanoscale powders of transition metal oxides (such as those of Yttrium or Chromium) dispersed through the matrix. These are known as Oxide Dispersion Strengthened Steels (ODSS) which, in addition to displaying high yield strength, have also displayed low irradiation and thermal creep, as well as high corrosion resistance. This is why they are being intensively investigated as possible structural and cladding materials for next generation nuclear reactors.
In my presentation Multi-scale Modeling of Radiation-Induced-Dislocation Pinning in Oxide-Dispersion Strengthened Steels (ODSS)- Part 2 , I discuss these desirable properties in detail, as well as the scientific questions that arise given the existence of these properties. For example, the precise effect that the specific alloyed species has on the properties is not known. Similarly, the role played by the metal oxide dispersion in strengthening corrosion resistance is not known, nor the details of how the dislocation pinning actually occurs, or whether there are any other novel effects, such as spin interactions, which might conceivably impact the properties of these steels. Such understanding is not only scientifically motivated, but is also necessary to develop a predictive understanding of the properties of the steels that may then be rolled into probabilistic safety analyses of the reactors themselves.
Multiscale Materials Modeling, which I have discussed in another post, naturally lends itself to addressing these issues, and is the focus of my current work. At present, the behaviour of reactor materials is modeled by phenomenological methods involving approximate relationships and observed correlations, which may not extrapolate into the operating conditions (and desired design lifetimes) of the new generation (Gen IV) reactors. The research I am undertaking is intended to fill an important and critical knowledge gap in the path toward these reactors.
It is important to realize the compelling criticality of Materials Issues in the path toward the Nuclear Renaissance - no matter which path is taken - whether a new coolant is introduced or whether new fuel cycles are introduced - the stress on the structural and cladding materials for the new generation of reactors changes both qualitatively and quantitatively. New materials will be needed that can reliably withstand this stress, and their behaviour needs to be understood and modeled, before such materials can be qualified for use in these reactors.
The Materials Research Society Fall Meeting in Boston next month is devoting an entire session solely to a Discussion of Materials Science Solutions to Impediments to the Nuclear Renaissance, in addition to a Symposium on Materials for Future Fission and Fusion Reactors, which includes several sessions both on Multiscale Modeling and on Oxide Dispersion Strengthened Steels.
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