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Title:The effects of irradiation on the creep and hardening behavior of crystalline and amorphous materials
Author(s):Jawaharram, Gowtham Sriram
Director of Research:Dillon, Shen J
Doctoral Committee Chair(s):Dillon, Shen J
Doctoral Committee Member(s):Averback, Robert S; Heuser, Brent J; Maass, Robert E
Department / Program:Materials Science & Engineerng
Discipline:Materials Science & Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
in-situ TEM
Abstract:Understanding the mechanical behavior of nanostructured and amorphous materials under harsh environments such as high irradiation dose and temperature is one of the major constraints in the development of novel materials for nuclear applications. This thesis explores the effects of irradiation on the mechanical properties of several model alloys in the context of irradiation induced creep and irradiation induced hardening. Three different systems were studied to elucidate the creep behavior under energetic particle irradiation; single crystal Ag, nanocrystalline high entropy alloys, and amorphous alloys. 1) In the Ag nanopillars, the ion irradiation induced creep rate increases linearly at lower stresses, i.e. below ≈ 2/3 the high temperature yield stress and parabolically with pillar diameter for diameters less than ≈ 300 nm. The size dependence results from a competition between the relative flux of point defects to surfaces versus internal sinks; i.e. dislocations. 2) In nanocrystalline high entropy alloys, the measured ion beam induced creep compliance varies directly with the inverse grain size. The activation energy for vacancy migration in the recombination regime is measured to be ~ 0.7 eV. The sluggish diffusion behavior often cited as a novel feature of these alloys has very limited effect on the observed creep behavior. 3) A low (200 keV or 80 keV) energy electron beam is used to induce creep in SiO2, Fe79B16Si5, CuTi and CuTa metallic glasses. In SiO2 and Fe79B16Si5 irradiation induced creep rates are sufficiently high to suggest that recoils as low as ≈ 1 eV should contribute to creep. The distinct microstructure resulting from irradiation induced self-organization of highly immiscible Cu-W alloys system is used to understand the relative importance of composition, solubility, precipitate distribution, morphology on hardening mechanisms. 1) During room temperature irradiation, a nanograin solid-solution strengthening mechanism with a linear compositional dependence is observed for the as-grown alloys and for the alloy samples irradiated to 0.5 dpa. Solid solution strengthening is the major strengthening mechanism for Cu99.5W0.5 at all irradiation doses. Irradiation induces precipitation in samples with W concentrations greater than or equal to 1 at% W at doses above > 0.5 dpa. The growth of 1-4 nm precipitates enhances the hardness of these alloys, and the degree of strengthening is determined by the interparticle spacing. While the alloys exhibit steady-state properties after relatively low dose (≈1 dpa), the different time scales associated with detwinning and damage accumulation in pure Cu lead to transients at higher doses (>5 dpa). 2) High temperature irradiation and annealing following room temperature irradiation results in the formation of precipitate denuded zones, distinct areas around grain boundaries that are devoid of W precipitates The PDZs in the irradiated samples are a result of self-organization and the hardness scales linearly with the width of the PDZs.
Issue Date:2019-05-28
Rights Information:Copyright 2019 Gowtham Jawaharram
Date Available in IDEALS:2019-11-26
Date Deposited:2019-08

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