Theoretical and Experimental Investigation on Nanoparticles within 14YWT: Nanostructured Ferritic Alloys

Yingye Gan, Clemson University


Nuclear industry has been developing to achieve higher efficiency and safety over the past decades. The major challenge is to design advanced structural materials with enhanced material properties under high temperature, high pressure and high irradiation conditions. Nanostructured ferritic alloy (NFA) is a class of oxide dispersed strengthened (ODS) iron-based alloys that have attracted increasing attention with exceptional mechanical properties and radiation tolerance at high temperatures. The remarkable properties of NFAs are attributed to the nanoparticles such as the oxide/carbide/nitride based nanoprecipitates (NPs) and the O-enriched nano-clusters (NCs) with ultrahigh density. Typically, the oxide/carbide/nitride based nanoprecipitates tend to coarsen with elevated temperature due to the relatively low surface energy compared to the bulk energy inside the particle. However, the O-enriched amorphous like NCs are ultra stable without coarsening under high temperature and high pressure conditions. The NCs not only serve as pins to the dislocation migrations and grain boundary sliding, but also serve as the traps to gas bubbles generated during the nuclear reactions. The bubbles are limited to uniform small size without coarsening to form large voids that induce failure. Therefore, the mechanical strength as well as the irradiation resistance is largely enhanced in NFAs both at room and elevated temperatures. For instance, 14YWT has been observed to contain an ultrahigh density of Y-Ti-O enriched NCs with the size of 2-4 nm. Researchers have identified the role of pre-existing vacancies to the formation of O-enriched NCs within NFA and the role of internal strain energy introduced by Ti and Y to the growth of O-enriched NCs. However, it is still unclear about the interaction mechanism between the gas bubble and nanoparticles within 14YWT, as well as the contribution of nanoparticles to the mechanical properties enhancement at elevated temperatures. In this work we first perform the first principles theory calculation to investigate the underlying mechanism of helium bubble formation and bubble behavior near O-enriched NCs within 14YWT. The helium displays a strong affinity to the oxygen:vacancy pair. By investigating various local environments to the vacancy, we find that the energy cost for helium growth increases with the appearance of solutes in the reference units. A growth criterion is proposed based on the elastic instability strain of the perfect iron lattice in order to determine the maximum number of He atoms at the vacancy site. Due to the fact that nuclear structural materials operate under various pressured states, the helium behavior in 14YWT under pre-existing strain are investigated with density functional theory (DFT) method. The energetic study on helium interaction with micro-structures in 14YWT show that pre-existing strain has pre-dominant effect on the elasticity of 14YWT. The energy cost is lower where tensile strain is presented. However, if compressive strain is presented, the energy cost to trap helium is higher. Similar to strain free 14YWT, the helium atom displays strong affinity to O:Vac pair. Simulating various local atomic environments in 14YWT, it is found that typical nano-cluster structures limit the absorption of helium atom, regardless of pre-existing strain. The elastic properties of 14YWT under pre-existing strain is also investigated by performing first principles calculation. With strain-energy method, the elastic constants of 14YWT with helium impurity under various pre-existing strain conditions are determined. The pre-existing tensile strain causes softening in the elastic constants while compressive strain induces hardening. The introduction of solute atoms and helium atom can induce hardening of its surrounding matrix due to the production of compressive strain under fixed-volume constraints. But on the other hand, helium atom can degrade overall mechanical performance due to local interaction. Experimentally, the mechanical performances of three types of NFAs, including 9YWTV, 14YWT-sm13, and 14YWT-sm170, are studied via high-energy synchrotron X-ray diffraction technique. The samples are stretched under uniaxial tensile loading until failure at room temperature (RT), 300°C, 500°C and 600°C. The lattice strain variation on ferritic grains is extracted from the diffraction patterns for various orientations. Based on the Kroner's model, the elastic constants, Young's modulus and Poisson's ratio are determined. The changes in these parameters denote the softening of the materials and indicate an increasing elastic anisotropy with rising temperature. The 14YWT samples show smallest change in ultimate tensile strength (UTS) and elastic anisotropy when temperature increases from RT to 600°C, implying a strong strengthening effect of nanoparticles. Mean internal stress is calculated to elucidate the nanoparticles strengthening effect in the three NFAs. The difference between the mean internal stress and the actual applied stress is a reflection of the strengthening effect.