Radiation Response and Corrosion Behavior of High-Throughput Additively Manufactured 316L Stainless Steel Doped with Hafnium

Abstract

The development cycle for qualifying a nuclear material for reactor use can be an extensive process involving multiple experiments in harsh environments such as high temperatures, irradiation, corrosion, and stress. A method of high-throughput experimentation was proposed, which utilizes additive manufacturing (AM) to reduce time and cost and increase efficiency in evaluating a material. Two 316L stainless steel samples were fabricated via directed energy deposition (DED), an as-built specimen and a recrystallized specimen that was thermo-mechanically treated to simulate a wrought grain structure. Each sample had three regions of increasing Hf dopant and was used to study the effects of Hf on the pristine microstructure, radiation response and corrosion behavior. Hf was observed to refine the grain structure of the as-built sample by reducing grain size and dissolving the cellular sub-grains and delta ferrites, which are typical to AM microstructure.

Samples were irradiated with 2.5 MeV protons at 360 °C to damage level of 2.5 dpa at ~ 10 μm below the surface. Suppression of the radiation damage was observed with the increasing Hf dopants. As an oversized solute, Hf traps radiation-induced vacancies in the atomic lattice, reducing mobility of and enhancing the recombination rates of point defects. Therefore, a reduction in cavities, dislocation loops and radiation-induced segregation was observed. The as-built specimen was determined to be superior in radiation resistivity due to the additional sink strength of delta ferrites in the matrix.

Molten salt corrosion of the as-built AM 316L samples was conducted in eutectic NaCl-MgCl2 at 700 °C. The primary corrosion mechanism was determined to be intergranular grain boundary attack, with preferential attack of high-angle grain boundaries (HAGBs). Hf was found to mitigate grain boundary corrosion by trapping vacancies and slowing Cr diffusion to the grain boundary and surface. Calculation of corrosion rates proved the effectiveness of Hf on corrosion mitigation.

This collection of data for the Hf effects on the radiation response and corrosion behavior of AM 316L proved the efficiency of high-throughput additively manufactured materials as a means to qualify a material for reactor use. Only a total of four gradient samples were used.