16 Aug 2016

Next generation's nuclear reactors require materials that can resist higher temperature and radiations. The ODS steels are used as nuclear fuel cladding and structural materials. This is because the ODS steels are enforced with clusters of 5 nm size of Ti. Yttrium phase that can undergo second order transition to the superionic states.

Oxide¬ dispersion ¬strengthened steels (ODS) are the most promising structural materials for next generation nuclear energy systems because they exhibit good radiations resistance to hardening, swelling and embrittlement. These protective properties are partly due to ( originate from) a clusters of nanometric and dense dispersion of complex oxides [1]. Hirata et al [2] recently confirmed that small Y- or Y-Ti non-stoichiometric oxide nano precipitate that are uniformly dispersed in the steel matrix ( so called nano-feature or NF) are responsible for reducing the creep rate by six order of magnitude at 650-900 C and contribute to the excellent tensile ductility and strength of the ODS steel.

Furthermore, Hirata and co-workers [2] believe that these nano precipitate present extremely high stability at temperature close to 90% of the melting temperature (1400 C) and intense neutron radiation field. This unusual stability which are due to the presence of the nano precipitates is not in accordance with the thermodynamics and traditional materials theories. Hirata and co-workers [2] think that the structure and chemical feature of small nano precipitate of size less than 5 nm are very important for understanding the effect of these nano precipitates on ODS steels. They used atom probe tomography (APT) which can give the chemical composition of the oxide nanoclusters with size less than 5 nm.

Hirata and co-workers believe that because the nanoclusters are very small (roughly around 2 -5 nm), embedded in the magnetic bcc-Fe matrix and may have coherent relations with the matrix. Bringa and his co-workers [3] attributed such beneficial effect of these nano scale ( large surface to volume ratio) on ODS steels to be due to the addition of large amount of interfaces between the matrix and the nano scale precipitates. The mechanism proposed here is based on the fact that the fluorite structure[2] of these dense cluster of re-¬enforcing non¬-stoichiometric nanoparticles (Y(x)Ti(y)O(z) of size less than 5 nm) are able to undergo a phase transition to the super ionic state at high temperature and radiations.The super ionic state is characterized by maximum number density of defects ( almost like liquid but in solid state).