Atomic mass transport of carbon in two-phase Nb-1.0Zr-0.1C alloy under a temperature gradient
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I.
INTRODUCTION
THE use
of materials under a temperature gradient results in redistribution of their mobile components due to a phenomenon known as thermotransport. It causes the formation of concentration gradients with respect to these components as they migrate toward either the hot or colder regions. Since the development of composition gradients also gives rise to a gradation of microstructure and properties, studies on thermotransport in materials are of significant technological importance. The effects of thermotransport on the microstructure and properties may occur at temperatures much lower than the melting points of metals and alloys under relatively small temperature gradients, especially if they contain one or more of the highly mobile interstitial solutes (oxygen, nitrogen, carbon, and hydrogen) as solid solution or precipitation-strengthening agents. These solutes can also be picked up from any type of an environment during service under a temperature gradient and subsequently undergo redistribution. For example, Peterson and Smith reported significant redistribution of hydrogen and deuterium in vanadium, niobium, and tantalum ~ and in V-Nb, V-Ti, and V-Cr alloys I/J in a rather low-temperature range of 283 to 333 K under a linear gradient as small as 50 deg/cm. Also, Sawatzky t31 described the pickup of hydrogen and subsequent formation of zirconium hydride in ZIRCALOY-2* cladding UO2 *ZIRCALOY-2 is a trademark of Westinghouse Electric Company, Pittsburgh, PA.
fuel elements in pressurized water power reactors in a range of 573 to 773 K. Some other examples of the effects of thermotransport are given in a paper by Hehenkamp. tal Most of the refractory metals form a complete range of solid solutions with one another tSj and have relatively large solubility for interstitial solutes, t61 allowing thermotransport to be studied over rather wide
composition and temperature ranges. Furthermore, refractory metals and their alloys are high-temperature materials and are candidates for use in fast breeder nuclear and fusion reactors ]7's'91 and in space nuclear power applications, t~~ Hence, studies concerning thermotransport of interstitial solutes in these materials are of interest to a basic understanding of the phenomenon, which is far from being complete, and also to their practical applications. The majority of the thermotransport studies have been in dilute binary alloys, as reviewed by Wever. tt3j There has been very little work in t e r n a r y [2Aa-t7] and twophase systems, t3'18-221 which are particularly important inasmuch as the engineering materials are often multicomponent and are sometimes used in a precipitationstrengthened condition. Our interest in the two-phase Nb-IZr-0.1C alloy was derived from the consideration given to it for use in the space nuclear power applications, tl0.1|.12]
II.
THEORETICAL BACKGROUND
The theory of thermotransport in one- and two-phase systems has been discussed in earlier publications, t~5-231 In an alloy containing an interstitial solute, the matrix atoms
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