Effects of Sink Strength and Irradiation Parameters on Defect Evolution in Additively Manufactured HT9

Abstract

A ferritic-martensitic (FM) steel, HT9, has been studied for use in advanced nuclear reactors for its excellent swelling resistance and high-temperature strength. However, the effects of sink strength (SS) on tailoring the radiation responses in HT9 have not yet been fully studied. The advancement of the mechanistic understanding of how SS affects microstructural evolution is of great importance to the development of new radiation-tolerant materials in the future. Additive-manufacturing (AM) has been drawing attention due to the advantage of its ability to control the complex geometry and composition of the structural components. The SS effects on radiation responses are studied within by using the as-built (ASB) and the post-built heat-treated (called ACO3 and FCRD) AM-HT9, with their starting SSs significantly differ from each other. Heat A of AM-HT9 in this study demonstrated a SS of 12.2×1015/m2 in the ASB and a nearly 5-time reduction in SS to 2.4×1015/m2 and 2.7×1015/m2 for the ACO3 and FCRD specimens, respectively. Ion-irradiations focusing on irradiation dose and temperature are conducted to systematically study the radiation responses and defect evolutions in AM-HT9 alloys. Experimental results showed that the high SS in the ASB drastically suppresses all microstructural evolution with damage levels up to 250 dpa. A higher normalization temperature used in the ACO3 results in a reduction of SS compared to the FCRD, leading to a ten-time-higher swelling rate in ACO3 after irradiation to 250 dpa. The complicated microstructural evolution including all features contribute to evolving defect sinks that collectively tailor the swelling behavior in the AM-HT9, which is verified using a simplified rate-theory model that considers the ratio of biased to neutral SS, Q. It was found that the analytical model does highlight both the overall SS and the balance between biased and neutral sinks are strong indicating factors for increased swelling resistance in AM-HT9. In addition, the Ni/Si/Mn-rich precipitate and dislocation loop coarsening processes are captured with increasing damage levels, whereas these processes are either observed to complete at much lower damage levels, or even not observed indicating an early saturation occurring in the ACO3/FCRD that contain about 5-time lower SS in the starting microstructures. Irradiation temperatures also greatly affected the radiation responses of AM-HT9. The density of a⟨100⟩ type dislocation loops dropped from 3.2×1021/m3 to 3.0×1020/m3 in the ASB specimen and dropped from (5.9-6.4)×1021/m3 to (0.3–0.4)×1021/m3 in the ACO3/FCRD heat-treated specimens. In addition, the higher irradiation temperatures stabilize the a⟨100⟩ loops and enables higher coarsening rates than lower irradiation temperatures. The precipitate evolution is greatly accelerated by the available kinetic energy at higher temperatures to overcome the pinning effects imposed by high SS, while cavity swelling exhibits the typical bell-shaped curve in the heat-treated ACO3/FCRD specimens with varying peak swelling temperatures by 30°C tailored by SSs. The overall result of this work is a wide range of microstructural responses under irradiation that can be obtained by AM fabrication with post-build heat-treatments, through the tuning of SS in the starting and the irradiated microstructures. The radiation response then needs to balance with other factors that are tied to the sink strength of AM-HT9 alloys including the mechanical properties such as tensile strength and fracture toughness. These insights obtained will stimulate further the optimization of using AM to fabricate materials that are highly radiation tolerant for advanced nuclear reactor applications.