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Title:Radiation-induced microstructural and mechanical property modifications in advanced alloys
Author(s):Liu, Xiang
Director of Research:Stubbins, James F.
Doctoral Committee Chair(s):Stubbins, James F.
Doctoral Committee Member(s):Bellon, Pascal; Heuser, Brent; Allain, Jean P.
Department / Program:Nuclear, Plasma, & Rad Engr
Discipline:Nuclear, Plasma, Radiolgc Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Radiation effects
austenitic alloys
ferritic/martensitic alloys
ODS alloys
dislocation loops
radiation-induced segregation
void swelling
irradiation hardening
transmission electron microscopy
atom probe tomography
Abstract:As part of the solutions towards resolving the challenges in energy shortage and climate changes, advanced nuclear reactor systems are being developed as reliable and sustainable clean energy sources with enhanced energy efficiency and safety features. These improved attributes necessitate advanced structural materials that can withstand harsh reactor environments such as high pressure, high temperature, corrosion, and most importantly, high dose fast neutron irradiation. Upon irradiation, the atoms in metals and alloys undergo successive displacements and point defects are created far exceeding the equilibrium concentrations. The material system is driven into thermodynamic nonequilibrium and the microstructure can be modified in many ways: dislocation loops are formed due to the aggregation of self-interstitial atoms (SIA), the accumulation of vacancies leads to the nucleation of voids, preferential coupling between alloying elements and the defect fluxes results in the elemental segregation at defect sinks, the stability of precipitates is strongly impacted by displacement cascades and radiation-enhanced diffusion, etc. A comprehensive understanding of the various microstructural modifications and their impacts on the material properties is therefore critical for the designing and screening of radiation-resistant structural materials. In this study, the radiation responses of several different advanced alloys, including Fe–20Cr–25Ni–Nb austenitic steel Alloy 709, Fe–9Cr ferritic/martensitic steel T91, and Fe–14Cr ferritic ODS steel MA957, were investigated using a combination of microstructure characterizations and nanoindentation measurements. Several different types of irradiation, including ex situ bulk ion irradiation, in situ transmission electron microscopy (TEM) ion irradiation, and neutron irradiation were employed in this study. Radiation-induced dislocations, precipitates, and voids were characterized by TEM. Scanning transmission electron microscopy with energy dispersive X-ray spectroscopy (STEM-EDS) and/or atom probe tomography (APT) were used to study radiation-induced segregation and precipitation. Nanoindentation was used for hardness measurements to study irradiation hardening. Austenitic steel Alloy 709 was bulk-irradiated by 3.5 MeV Fe++ ions to up to 150 peak dpa at 400, 500, and 600C. Compared to neutron-irradiated stainless steel 316, the Frank loop density shows similar dose dependence at 400C, but very different temperature dependence. The radiation-induced segregation (RIS) of Ni and Si was observed in all irradiated conditions and was found at various sinks: line dislocations, dislocation loops, void surfaces, carbide-matrix interfaces, and MX particle-matrix interfaces. Radiation also induced the formation of Ni,Si-rich precipitates. Radiation-induced change in the orientation relationship of preexisting MX precipitates was observed. Intragranular Cr-rich carbides with a core-shell structure, i.e. Cr-rich carbide core and Ni,Si-rich shell was found at 500 and 600C in the highest dose (150 peak dpa) specimens. Coarse voids (~30 nm in diameter) were only commonly found at 500C in the 50 and 150 peak dpa specimens in regions less than 750 nm in depth. The highest swelling for A709 irradiated to 50 and 150 peak dpa at 500C is about 0.44% and 0.37%, respectively. Nanoindentation measurements show that the irradiation hardening decreases with increasing temperature. Microstructure-property correlation shows that the measured hardening at low dose (~5 dpa) was mostly contributed by Frank loops at temperatures above 400C and network dislocations at 300C. T91 is a Fe–9Cr ferritic/martensitic alloy. The chromium content is close to the alpha–alpha prime phase boundary. Irradiation hardening in T91 is largely due to dislocation loops and network dislocations. The dislocation structure evolution was characterized by TEM during in situ 1 MeV Kr++ irradiation at 300, 400, and 500C. At 500C, the loop density at 1–3 dpa is significantly lower and the loops are dominantly <100> type. The loops at 400C consist of both <100> loops and <111> loops. The changes in yield strength were calculated at different doses and temperatures based on the microstructure observations. Nanoindentation measurements on bulk-irradiated specimens show that T91 and G92 have similar hardening behavior. The temperature dependence and dose dependence shown by nanoindentation measurements are consistent with the typical hardening behavior of neutron-irradiated F/M steels. The neutron radiation response of ferritic ODS steel MA957 was studied using TEM and APT. The size distribution of the Y–Ti–O nanoclusters became narrower with increasing dose, with concurrent decrease in the peak Ti and O concentrations. The dislocation loop evolution can be roughly divided into two regimes: loop nucleation under ~1 dpa and loop growth above ~1 dpa. Radiation enhanced alpha prime precipitation was studied and a nonclassical nucleation and growth mechanism is suggested. Radiation-induced Ni segregation was observed at the Y–Ti–O particle-matrix interfaces. Radiation-induced voids were also studied by TEM. Finally, comparative study on the differences between thin foil ion-irradiation and bulk ion-irradiation was carried out. The surface effects for thin foil irradiation was quantitatively studied for Alloy 709. Results show that the surface effects have noticeable impacts on the loop size and density at temperatures above 400C. It was also found that the loop character (Burgers vector and habit plane) is also impacted during thin foil irradiation, as the loop character show strong thickness dependence and many different types of loops were found mainly in the thin region. One needs to be cautious when using thin foil irradiation as a surrogate of bulk irradiation, especially for low sink density materials and/or at high temperatures. In summary, radiation-induced microstructural and mechanical property modifications in advanced alloy systems, including austenitic steel Alloy 709, ferritic/martensitic steel T91, and ferritic ODS steel MA957, were systematically investigated in this study. The results and discussions advance the understanding of the material performance under extreme irradiation environment. The results of accelerated ion beam irradiation also serve as reference for further neutron irradiation studies.
Issue Date:2018-04-17
Rights Information:Copyright 2018 Xiang Liu
Date Available in IDEALS:2018-09-04
Date Deposited:2018-05

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