Effect of Cascade Size and Damage Rate on ?? Precipitate Stability on Fe-15Cr

High Cr ferritic-martensitic (F-M) steels are candidates for nuclear reactor structural components due to their high resistance to swelling and adequate corrosion resistance. However, these steels are susceptible to the formation of Cr-rich α’ precipitates at low to intermediate temperatures (below ~500°C) in both thermal and irradiation environments leading to hardening and embrittlement, commonly known as “475°C embrittlement”. α’ precipitates are not consistently observed under heavy ion irradiation and have dissimilar properties (radius, number density, concentration, volume fraction) than those formed under neutron, proton, or electron irradiation or by thermal aging. The dissimilar properties are a result of ballistic dissolution of the α’ precipitate, which is dependent on both damage rate and cascade size. The objective of this thesis is to understand the roles of damage rate and cascade size on the stability of α’ precipitates in Fe-15Cr under irradiation. A systemic study using heavy ion, proton, and electron irradiation was conducted with variations in damage rate at 400°C to doses of 1 and 10 dpa to examine the stability of α’ precipitates under irradiation. A steady state α’ precipitate distribution was first established in Fe-15Cr samples using 2 MeV proton irradiation at a damage rate of 1x10-5 dpa/s to 1 dpa at 400°C. These samples were then subjected to irradiation with Fe ions, proton, or electrons also at 400°C, to 1 and 10 dpa, over a range of damage rates. The α’ precipitate microstructure evolution was characterized for each irradiation experiment using atom probe tomography (APT). Under heavy ion irradiation, the α’ precipitates were observed to decrease to a steady state size and Cr concentration or completely dissolve. At doses of 1 dpa, the α’ precipitate was the same size and was shown to be independent of damage rate. For higher doses of 10 dpa, the α’ radius is stable for low damage rates but the precipitate dissolves completely at higher damage rates. However, under electron and proton irradiation, the α’ precipitates increased in size and Cr concentration. The ballistic dissolution parameter (BDP), a constant describing the atom flux ballistically ejected from a precipitate surface per dpa, was calculated for both proton and heavy ion irradiation using two methods to describe the effects of cascade size and damage rate on ballistic dissolution of α’ precipitates. A critical temperature analysis was further used to explain the balance in the roles of ballistic dissolution and radiation enhanced diffusion between heavy ion, proton, and electron irradiation. This work provided substantial insight into the roles of cascade size and damage rate on α’ precipitate stability under irradiation.PHDNuclear Engineering & Radiological SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/174310/1/knthomas_1.pd