Doctoral Dissertations

Abstract

"Void formation in high purity aluminum resulting from irradiation to fluences between 1.5 x 10¹⁹ and 1.6 x 10²² neutrons/cm² (E > 0 .1 MeV) at a temperature of 55 ± 5ºC was studied, primarily by means of transmission electron microscopy. In particular the effects of neutron fluence and flux were examined, as well as the possible influences of both preexisting and transmutation-produced impurities. A very low concentration of voids (about 10¹²/cm³) resulted from irradiation to 1.5 x 10¹⁹ neutrons/cm²; they averaged about 150 Å in diameter. Void concentrations and sizes increased with fluence, reaching values of 6 x 10¹⁴ voids/cm³ averaging 500 Å in diameter at 1.6 x 10²² neutrons/cm². Void formation initially was nonuniform in distribution, but tended to be homogeneous at higher fluences. The void shapes were always consistent with those of octahedra bounded by {111} planes, sometimes with the vertices truncated by {100} planes. Void size distribution curves were obtained for all fluences, and from these the mean void size was found to increase in proportion to the fluence raised to the one-sixth power. The void concentration displayed a fluence dependence best described by a power law, N ~ (Φt)ᵃ, in which the exponent decreased from 2 at 1.5 x 10¹⁹ neutrons/cm² (E > 0.1 MeV) down to only 0.1 at 1.6 x 10²² neutrons/cm². Treating the swelling with a similar power law, ΔV/V ~ (Φt)ᵇ, a similar saturation effect was observed, with the fluence exponent b̲ decreasing roughly from 2.5 to 0.5 over the range of fluence studied. At 1.6 x 10²² neutrons/cm² the total void volume or swelling amounted to 7.4%. Microhardness increases in the irradiated specimens can be explained on the basis of void contributions to the impeding of dislocation motions. Irradiation at a factor-of-ten lower flux produced effects upon the void morphology (e.g., half as many but larger and more elongated voids) much like those resulting from irradiation at a higher temperature. In each case, the lowering of the vacancy supersaturation during irradiation is a consequence. Irradiation, annealing to remove damage, and reirradiation of a specimen, each exposure to a fluence of 5 x 10²⁰ neutrons/cm² (E > 0.1 MeV), yielded twice as many voids of smaller maximum size, having the same total void volume as those produced by the first irradiation alone. The effect is attributed to submicroscopic remnants of the original voids surviving the anneal and serving as preexisting nuclei in the second irradiation. In postirradiation void annealing experiments performed inside a high vacuum electron microscope, the rate at which void radius changed, dr/dt, varied considerably from void to void. A transient effect in dr/dt was observed upon abrupt changes in the annealing temperature. Both effects are ascribed to the actions of local vacancy sinks near the voids, such as a suggested population of submicroscopic gas bubbles. The experimental observations are compared with the current models for void formation. The two models most consistent with the experimental evidence both involve transmutation-produced helium playing a crucial role in void nucleation--one involving helium-stabilized spikes and the other small helium bubbles as the nuclei of voids. Throughout this work effects were found which demonstrate a strong influence of impurities upon void formation in high purity aluminum"--Abstract, pages ii-iv.

Advisor(s)

Bolon, Albert E., 1939-2006

Committee Member(s)

Leighly, Hollis P.
Frad, William A.
Gerson, Robert, 1923-2013
Moore, Robert E., 1930-2003

Department(s)

Materials Science and Engineering

Degree Name

Ph. D. in Metallurgical Engineering

Sponsor(s)

National Science Foundation (U.S.)

Comments

The author held a National Science Foundation Graduate Traineeship; The financial assistance provided by the Traineeship; is gratefully acknowledged.

Publisher

University of Missouri--Rolla

Publication Date

1970

Pagination

xii, 147 pages

Note about bibliography

Includes bibliographical references (pages 140-146).

Rights

© 1970 Nicolas Hayes Packan, All rights reserved.

Document Type

Dissertation - Open Access

File Type

text

Language

English

Subject Headings

Aluminum -- Effect of radiation onNeutron fluxNeutron irradiationIonizing radiation

Thesis Number

T 2415

Print OCLC #

6024144

Electronic OCLC #

858940331

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