Doctoral Dissertations

Date of Award

12-1984

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Metallurgical Engineering

Major Professor

Carl J. McHargue

Committee Members

J. E. Spruiell, B. F. Oliver, C. R. Brooks, E. E. Stansbury

Abstract

This work investigated the sensitivity of microstructural evolution, particularly precipitate development, to increased helium content during thermal aging and during neutron irradiation. Helium (110 at. ppm) was cold preinjected into solution annealed (SA) DO-heat type 316 stainless steel (316) via cyclotron irradiation. These specimens were then exposed side by side with uninjected samples. Continuous helium generation was increased considerably relative to EBR—II irradiation by irradiation in HFIR. Data were obtained from quantitative analytical electron microscopy (AEM) in thin foils and on extraction replicas.

Normal aging produced small amounts of M23C6 (τ) precipitation at 560 to 600°C, and t plus large amounts of Laves phase at 650 to 700°C. Preinjected helium eliminated τ and replaced it with Laves at 600 and 700°C. Interstitial "black-dot" Frank loops were introduced via preinjection. These grew and coarsened upon long-term aging at 400 to 600°C, and then unfaulted at 700°C, apparently to provide vacancies for growing helium-vacancy clusters and bubbles.

Normal EBR-II irradiation produced dislocation loops and network, precipitate-associated and matrix voids, and precipitation of mainly MgC (n) plus a minor assortment of irradiation-produced phases [G, γ (Ni3Si) plus A and B phosphides] at 500 to 630°C and 8.4 to 36 dpa (except at 630°C and 36 dpa, where Laves plus η formed). Helium preinjection eliminated void formation and eliminated RIS-induced or -modified precipitation at 500 to 625°C and 8.4 dpa. Early fine bubble nucleation apparently caused an overwhelmingly bubble-dominated sink structure that also diluted RIS to eliminate precipitation at SOO'C and yet allow enhanced thermal precipitation at 625°C.

The results of HFIR irradiation at 500 to 650°C were intermediate to those described above due to intermediate levels of bubble nucleation and evolution. Void formation was initially suppressed but then, at higher fluences, enhanced; maximum RIS was observed at 425 to 450°C, but RIS intensity was reduced at 515 to 555°C and virtually eliminated at higher temperature. Enhanced thermal precipitation dominated above 550°C, and void swelling appeared cut off above 650°C.

This thesis includes an extensive literature survey of data on SA 316 and of relevant theoretical work which provides the perspective necessary to discuss and understand the data.

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