Effect of oxygen on vacancy cluster morphology in metals
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I.
INTRODUCTION
D I S P L A C I V E irradiation can be used as a tool to investigate materials parameters and to modify the properties of metals and ceramics. The study of vacancy clusters provides useful information about the physics of metals, and vacancy clusters created by irradiation can exert a large effect on physical properties such as strength and density. The most common vacancy cluster morphologies in face-centered cubic (fcc) metals are the stacking fault tetrahedra (SFT), the void, and perfect and faulted dislocation loops. There have been numerous theoretical studies of the relative stability of these cluster geometries using continuum mechanics 1~,2] and atomistic [3-91 models. These studies have found that the cluster geometry of lowest energy at small cluster sizes is typically the SFT or faulted loop, particularly in metals with a low stacking fault energy and/or high surface energy. Void formation is generally predicted to be unstable for pure metals. Some investigations have attempted to incorporate temperaturedependent effects by explicitly including entropy contributions.[l~ Experimental studies of vacancy cluster formation have produced a confusing set of results. Various irradiation and quenching studies have observed that the dominant vacancy cluster geometry for a given metal can be the SFT, the void, or the faulted or perfect loop, depending on minor changes in the experimental procedure. [2A2-141 Void formation is commonly observed at homologous irradiation temperatures of 0.35 to 0.6 TM, where T~ is
S.J. ZINKLE and E.H. LEE, Research Staff Members, are with the Metals and Ceramics Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6376. This paper is based on a presentation made in the symposium "Irradiation-Enhanced Materials Science and Engineering" presented as part of the ASM INTERNATIONAL 75th Anniversary celebration at the 1988 World Materials Congress in Chicago, IL, September 25-29, 1988, under the auspices of the Nuclear Materials Committee of TMS-AIME and ASM-MSD. METALLURGICAL TRANSACTIONS A
the melting temperature. On the other hand, SFT and dislocation loops are the most prevalent geometry at temperatures below 0.35 TM. It was recognized at an early stage t14-19] that impurities (particularly gaseous) played an important role in determining the preferred vacancy cluster geometry. However, a comprehensive quantitative assessment of the physical mechanisms associated with the general phenomenon of impurity-aided cluster formation has been lacking. The role of oxygen in promoting void formation in metals has been recently addressed in a simple quantitative model that is based on continuum mechanics and thermodynamic principles. 12~ Specific calculations have been performed for copper, [2~ nickel, 121[ and type 316 stainless steel.j22] The physical basis for this model lies in the reduction in surface energy upon chemisorption of oxygen on a free surface. A brief review of the thermodynamic effects of oxygen interaction with metals is given in the f
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