Integrated Computational Study of Radiation Damage Effects in Grade 92 Steel and Alloy 709 [report]

Haixuan Xu, Lizhen Tan, Li He, Kumar Sridharan
2018 unpublished
EXECUTIVE SUMMARY Radiation resistance of austenitic alloy 709 (named as A709) and ferritic-martensitic steel Grade 92 (G92) were investigated in this project using 316H and Grade 91 (G91) as references, respectively. In the meantime, a model 709 (M709) alloy (Fe-20Cr-24Ni) was prepared to study the matrix response to irradiation. Samples machined from the tab section of three creep-ruptured A709 samples, i.e., 17,106 h at 600 °C, 12,014 h at 650 °C, and 16,448 h at 700 °C, were included in
more » ... work to evaluate radiation effect on the long-term "thermally aged" samples. Primarily Fe 2+ ion irradiation was conducted at 350, 600 and 670 °C for up to 75 displacements per atom (dpa), together with a few proton irradiation experiments at 380-670 °C for up to 1.5 dpa. Additionally, four neutron-irradiated NF709 samples (3.22-8 dpa at ~430-469 °C) were examined. The samples were examined by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS) for microstructures and nanoindentation for radiation-hardening. Unlike the negligible hardening after irradiation at 670 °C, irradiation to nominal 2.5-50 dpa at 350 °C resulted in radiation-hardening of 13.5±5.4 % for A709, which was comparable to that of 13.4±6.2 % for M709 and moderately lower than that of 20.1±5.6 % for 316H. The radiation-hardening induced by the same irradiation condition to the "thermally aged" samples was comparable to A709, which were hardened by 15.0±8.2 %, 13.5±6.0 %, and 10.1±6.1 % for the 600, 650, and 700 °C aged samples, respectively. The radiation-hardening of A709 is significantly lower than that (~94±9 %) of neutron-irradiated NF709 to 8 dpa at ~430 °C. Microstructural characterization indicated dislocation loops, type 1/3<111>, in 316H and A709 after irradiation at 350 °C. The loop size and density in A709 were slightly smaller than that in 316H, e.g., 14.2±0.6 nm with 5.2×10 22 m -3 in A709 vs. 15±1 nm with 8.3×10 22 m -3 in 316H after 50 dpa. However, only line dislocations were developed in the samples irradiated at 600-670 °C. A small amount (~2.8×10 12 m -3 ) of Nb(CN) precipitates in a size of ~72 nm were observed in the as-received A709, which were reduced to ~1.6×10 12 m -3 with a larger size of ~90 nm after irradiation to 50 dpa at 350 °C. A new type of precipitates CrNbN (Z-phase) in ~10 nm was introduced by the rastered beam irradiation to 50 dpa at 670 °C but not by the defocused beam irradiation to 75 dpa at 600 °C. Additionally, some (CrMo)xCy type precipitates were observed in both the high-temperature irradiated A709 samples, which is believed to be the precursor of M23C6. In contrast, a significant amount of multiple types precipitates was identified in 600-700°C aged A709 samples, e.g., (CrMo)23C6, (CrMo)3Ni2Si, Nb(CN), CrNbN, and Ni/Si-rich particles. Large (CrMo)23C6 and (CrMo)3Ni2Si particles, in ~1 µm, grew along grain boundaries, which covered ~53 %
doi:10.2172/1528716 fatcat:uuu6kgmlxnh5tgd3cnul7wrdo4