Hardening Mechanisms in Neutron- and Ion-Irradiated Fe-9Cr ODS Alloy
October 17, 2014
Janelle P. Wharry, Assistant Professor
Department of Materials Science and Engineering
Boise State University
Oxide dispersion strengthened (ODS) alloys are leading candidates for cladding and structural components in advanced fission nuclear reactors. ODS alloys contain a fine dispersion of Y2Ti2O3 nanoparticles in an Fe-Cr matrix, which enhance the radiation resistance and high-temperature strength of the alloy. But in service, ODS components will be subject to extreme operating conditions of up to 500 displacements per atom (dpa) at up to 700oC, which will alter the designed microstructure and mechanical properties of the material. In particular, the oxide nanoparticles exhibit limited stability under irradiation. As such, the hardening mechanisms of the alloy will change as the nanoparticles evolve. The objective of this talk is to understand the evolution in hardening mechanisms of an Fe-9Cr ODS alloy under irradiation, especially as it relates to the microstructural evolution of the alloy.
In this study, a model Fe-9Cr ODS alloy was irradiated to two conditions: with neutrons to 3 dpa at 500oC, and with 5 MeV Fe++ ions to 100 dpa at 400oC. Specimens were examined using a combination of transmission electron microscopy (TEM) and local electrode atom probe (LEAP) tomography to characterize the irradiated microstructure. Irradiation hardening was measured by nanoindentation. The Orowan dispersed barrier hardening model was used to calculate hardening from microstructure observations, but it significantly underestimated hardening measured by nanoindentation. This disparity was attributed to oxide nanoparticle instability under irradiation. However, when the nanoparticle instability was considered in a solid solution strengthening model, measured and calculated hardness fell into close agreement.