The ferrite/oxide interface and helium management in nano-structured ferritic alloys from the first principles

Abstract

Nano-size Y-Ti-oxides are largely responsible for the extraordinary mechanical properties and irradiation tolerance of nano-structured ferritic alloys (NFAs). Here we report a theoretical study to assess the characters and possible roles of the ferrite/oxide interface in managing neutron transmutation product helium in NFAs. Using one observed cube-on-cube orientation relationship, various candidate structures of the ferrite/Y2Ti2O7 interfaces were constructed and the associated energies were carefully evaluated. The interface phase diagram is obtained by expressing the energy as a function of temperature and internal oxygen activity (expressed in terms of oxygen partial pressure). The oxide interfaces are predicted to be Y/Ti-rich at thermodynamic equilibrium for the wide temperature range of interest. Vacancy formation energies are lower and helium segregates to the interfaces, in preference to the iron matrix and grain boundaries. Combined with our previous results on bulk-phase Y2Ti2O7, the profound implications of nano-oxides to helium management in NFAs are discussed.

Characterization of polyhedral nano-oxides and helium bubbles in an annealed nanostructured ferritic alloy

“…NO-matrix interface chemistries and energies ( γ hkl ) are highly dependent on the O partial pressure (P O2 ) [37] , which are not well established. The likely thermodynamic bounds are P O2 levels between in equilibrium Cr 2 O 3 and TiO 2 , which result in nonstoichiometric Y-Ti rich interface terminations with semi-coherent energies between ≈ 0.2 -0.8 J/m 2 [37] . However, APT studies measured higher non-equilibrium levels of unreacted excess O (due to internal oxidation kinetic limits and perhaps excess vacancies) with matrix values in the range of ≈ 0.1 at% or more [21] .…”

“…However, APT studies measured higher non-equilibrium levels of unreacted excess O (due to internal oxidation kinetic limits and perhaps excess vacancies) with matrix values in the range of ≈ 0.1 at% or more [21] . In this case the interface will be highly O enriched, with semi-coherent energies < 0.8 J/m 2 [37] . First principles models indicate that Fe atoms usually match with the O atoms for both Fe-YTO [37] and Table 1 Interface names, labels on polyhedral shown in Fig.…”

“…In this case the interface will be highly O enriched, with semi-coherent energies < 0.8 J/m 2 [37] . First principles models indicate that Fe atoms usually match with the O atoms for both Fe-YTO [37] and Table 1 Interface names, labels on polyhedral shown in Fig. 6 , interfacial orientation relationships, facet surface areas on polyhedral, calculated interface misfit dislocation spacing, near coincidence site lattice (NCSL) area, and relative interface energies.…”

“…Being practically insoluble, He atoms migrate to sinks that are dominated by the YTO NOs. As shown by first-principles modeling, He enters the relatively open pyrochlore YTO structure and is deeply trapped at octahedral interstitial positions within the NOs themselves [37] . The He energy is similar at the interface, where diffusional clustering and bubble formation occurs.…”

“…The NO-matrix crystallographic orientation relationships (ORs) are of fundamental interest because they affect critical properties such as interface energies, structures and misfit strains, as well as NO interactions with dislocations, point defects, and He. Interface characterization is also critical to calibrating first principles and atomistic interface models [36][37][38] . The most common bulk ORs for embedded NOs are cube-on-cube [22 , 23 , 25] and cube-on-edge [22 , 25 , 39-41] .…”