Investigating the Microstructural Stability of Nanocrystalline Oxide Dispersion-Strengthened Alloys under Proton Irradiation by X-ray Line Profile Analysis
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Authors
Stenstrom, Zachary
Date
2025-01-13
Type
thesis
Language
eng
Keyword
Nanomaterials , Materials Science , Nuclear Energy , Radiation Damage , Crystallography , X-ray Diffraction , ODS Alloys
Alternative Title
Abstract
Nanocrystalline (NC) alloys contain unique defect structures compared to conventional Coarse-Grained (CG) alloys. NC alloys have smaller grains, and therefore higher Grain Boundary (GB) volume fraction, higher dislocation density and more ordered dislocation structures. Experimental and computational studies of NC alloys show improved resistance to degradation of material properties under irradiation due to increased point-defect (PD) sink densities. As a result, there is a reduced concentration of PDs available for clustering into higher-dimensional defects, which are chiefly responsible for the degradation of properties at the engineering scale. This work aims to experimentally validate the approach of limiting radiation damage by introducing pre-existing defect structures, as well as to gain a better understanding of the defect interactions that govern the thermodynamics and kinetics of irradiation-induced microstructural evolution. A Ni-based superalloy (Inconel 617, solution-annealed and aged) and two Oxide Dispersion-Strengthened (ODS) alloys, Ni and Fe-based (MA754 and MA957), are processed by High-Pressure Torsion (HPT), a form of Severe Plastic Deformation (SPD). Convolutional Multiple-Whole Profile (CMWP) analysis of high-resolution Xray diffraction (XRD) patterns reveals these sample to have dislocation densities on the order of 1016 m-2 and crystallite sizes of approximately 15 nm. Following low temperature (T < 0.35Tm) proton irradiation up to two displacements per atom (dpa), XRD experiments are repeated. Irradiation is found to cause partial annihilation of the dislocations created by HPT, with the mechanism of dislocation annihilation seemingly depending on crystal structure. Irradiation-induced grain growth, thought to be due to cascade-induced thermal spikes overlapping with grain boundaries, is observed in all samples. However, Inconel 617 exhibits “NC stability”, exemplified by a stable grain size with increasing dose – the greater NC stability of Inconel 617 agrees with earlier differential scanning calorimetry recrystallization experiments. XRD phase analysis suggests that the nanoscale oxide dispersions in MA754 and MA957 undergo ballistic dissolution due to low-temperature irradiation. Thus, they are unable to exert a Zener pinning force on mobile GBs to improve NC stability, nor can they serve as effective recombination sites for PDs, indicating that NC-ODS alloys do not show significant promise in the pursuit of material stability in extreme environments. The results suggest that NC stability is better approached by finetuning the alloy system to reduce GB energy. Finally, the irradiation-induced precipitation of α-Cr is found to occur in all Ni-based samples, with the effect being far more prevalent in MA754. Experimental evidence for radiation-induced segregation occurring concurrently with grain growth (previously investigated via phase-field models) is presented, with XRD revealing a grain size dependence on Cr
concentration.
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This publication is made available by the authority of the copyright owner solely for the purpose of private study and research and may not be copied or reproduced except as permitted by the copyright laws without written authority from the copyright owner.
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ProQuest PhD and Master's Theses International Dissemination Agreement
Intellectual Property Guidelines at Queen's University
Copying and Preserving Your Thesis
This publication is made available by the authority of the copyright owner solely for the purpose of private study and research and may not be copied or reproduced except as permitted by the copyright laws without written authority from the copyright owner.
Attribution 4.0 International