Loading...
Impact of cellular dislocation structures produced by additive manufacturing on neutron irradiation damage evolution in stainless steel 316L
Collette, Ryan A.
Collette, Ryan A.
Citations
Altmetric:
Advisor
Editor
Date
Date Issued
2021
Date Submitted
Collections
Research Projects
Organizational Units
Journal Issue
Embargo Expires
2022-06-25
Abstract
Additive manufacturing (AM) has emerged as a practical fabrication technique to producegeometrically complex components for challenging industrial applications. Laser powder bed
fusion (L-PBF) and laser powder forming (LPF) are both well researched additive manufacturing
processes that build three-dimensional structures layer-by-layer from computer-aided-design
models. The nuclear industry is exploring additive manufacturing technologies for reactor internal
applications in order to enhance performance, reduce economic costs, and shorten production
cycles. However, engineering use of AM materials requires that they compare favorably to
conventional materials in their ability to resist degradation of mechanical properties.
Microstructure plays a critical role in establishing a component’s mechanical reliability; thus, prior
to being qualified for use in a nuclear setting, the role of AM process-specific microstructure
features and their potential impact on irradiation-induced damage mechanisms must be studied. Stainless steel 316L (SS-316L) samples were irradiated in the Advanced Test Reactor overa range of fast neutron fluences as part of a grant received by the Colorado School of Mines.
Characterization of the samples prior to irradiation revealed a network of sub-grain cellular
dislocation structures decorated with oxide nano-inclusions and enrichment of heavy alloying
elements at the cell boundaries. These features are thought to be responsible for AM SS-316L’s
improved material performance in comparison to traditionally manufactured SS-316L, suggesting
an ability to overcome the strength-ductility tradeoff that is common to traditional strengthening
approaches in poly-crystalline materials. It is hypothesized that Lomer-Cotrell (L-C) locks
observed in high resolution imaging of the cellular walls contribute significantly to the
preservation of the strength-providing dislocation cell network and promote plasticity by
unlocking when local stress thresholds are exceeded. This study also conducted molecular dynamics (MD) simulations to develop a predictivecapability for the effect of pre-existing defects on radiation damage accumulation. The evolution
of synthetically generated dislocations in a Fe-Cr-Ni system in response to multiple radiation
collision cascades suggest that dislocation dense regions reduce the probability of surviving point
defects forming clusters. This finding results from the dislocation cores’ ability to absorb point
defects, thereby limiting the formation of clusters during the recombination phase. However, after
successive cascades, the ability of the dislocation entanglement to prevent defect formation is
diminished and defect cluster formation trends to an equilibrium. Accordingly, it is hypothesized
that in an experimental setting the dislocation cells will initially act as neutral sinks for point
defects, potentially delaying the onset of radiation damage effects; but, these structures will be
degraded due to radiation-enhanced diffusion and lose their effectiveness at doses above 1.5-2
displacements per atom. Post-irradiation examination of the L-PBF and LPF samples showed the damage to bedominated by small defect clusters. The solidification-induced cellular dislocation structures were
partially recovered and more diffuse, but remained intact. Accordingly, the MD model
satisfactorily predicted that the primary radiation-induced damage mechanism would be a high
density of defect clusters but underestimated the dose necessary to fully recover the dislocation
density within the cell boundaries. This observation, coupled with evidence of defect denuded
zones adjacent to cell boundaries, suggests that the cellular structures in AM SS-316L function as
effective sinks for point defects at low dose and temperature. At a minimum, they increase the
dose necessary to reach a steady state defect population. Therefore, ensuring stability of the AM
cell structures at high dose and temperature will be key to maximizing their radiation resistant properties. In conclusion, AM’s ability to produce complex and feature-specific microstructures suggests promise for the design of radiation tolerant materials.
Associated Publications
Rights
Copyright of the original work is retained by the author.