Mechanical and Microstructural Differences Between Neutron Irradiated and Thermally Aged SS-316H Produced via Laser Powder Bed Fusion

One of the primary goals of the Advanced Materials and Manufacturing Technologies (AMMT) program is the establishment of a comprehensive framework for rapid qualification of new structural materials, with a specific focus on additively manufactured materials to be deployed in advanced reactors operating at elevated temperatures. This framework relies on a combination of high-throughput optimization of 316H printing parameter envelopes, the down-selection of compositional specifications and post-processing conditions, and the combined use of in-situ build data and modeling/simulation as a combined quality assurance/property validation tool. As such, a key aspect of the program relies on establishing the key microstructural features associated with increased irradiation performance in elevated temperature operating environments relevant to advanced reactor technologies. Recent neutron irradiations were performed in the High Flux Isotope Reactor targeting irradiation temperatures of 400°C and 600°C to displacement damage levels of ~2 displacements per atom (dpa). The irradiated specimens include both wrought and additively manufactured SS-316H using laser powder bed fusion (LPBF). In parallel, samples of LPBF SS-316H have been subjected to thermal aging at two representative temperatures (550°C and 650°C) for up to 5000h as part of separate investigations as to the aging-induced deterioration of these LPBFproduced materials. Of primary interest in these experiments is the deconvolution of irradiation-induced and thermally-induced mechanical property deterioration due to effects such as (1) evolution of dislocations inherent to the printing process , (2) carbide precipitation/sensitization, and (3) irradiation-induced cavity formation and/ or precipitation behavior. In this work, the mechanical properties are presented for solution-annealed and stress-relieved variants of LPBF SS-316H subjected to irradiation for 1 cycle in HFIR (~576h) vs. those subjected to thermal aging to 500h are presented. Tensile and fracture toughness data is presented on neutron irradiated specimens tested at room temperature and at the target irradiation temperature (600°C) and are compared to thermally-aged specimen tests at the target aging temperatures (550°C and 650°C). These temperatures were chosen to provide a window of potential as-measured irradiation temperatures based on historical margins of error for experimental research-reactor irradiation conditions. An example of the tensile property evolution is shown below in Fig. 1. In addition to mechanical property evaluations, scanning transmission electron microscopy (STEM) comparisons are provided between thermally aged and irradiated conditions, with a focus on differences in precipitation state, dislocation structure evolution, and cavity distributions at elevated irradiation temperatures.