Change in direction of carbon thermotransport in Nb-V system with alloying
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I.
INTRODUCTION
THE selection of a material for a particular application is dictated by its properties that meet certain predetermined design criteria. These properties often depend strongly on chemical composition and microstructure, and their deterioration during use limits the useful life of an engineering component. Therefore, any phenomenon that causes changes in the structure or the properties of a material during service is of great concern. One such phenomenon is thermotransport. Thermotransport is a mass transport phenomenon that occurs in the presence of a temperature gradient. The components of a material system undergoing thermotransport may migrate toward either its hot or colder regions. This gives rise to concentration gradients, hence a gradation of material properties with possible harmful effects, t1'2'31 Studies on thermotransport of interstitial solutes (O, N, C, H) in refractory metals and their alloys are of interest from a basic science standpoint since their solid solubility in these materials is considerably larger than in others, t*l Furthermore, most binary refractory metal alloys exhibit a complete series of solid solutions at temperatures of interest. tS] These characteristics allow for the effects of temperature, tr] and substitutional t7'8'91 and interstitial t6,7,1~ solute concentration to be studied over rather wide ranges, leading to a better understanding of the phenomenon. Such studies are important from a practical standpoint also, inasmuch as interstitial impurities have marked effects on the properties of the refractory metals and alloys. These materials are strong candidates for high temperature and power reactor applications where large temperature gradients may exist. [9'12A31 It should be noted that studies concerning thermotransport and a good understanding of the phenomenon will be of increasing technological interest as many engineering materials are subjected to higher temperatures for improved thermal efficiency. MEHMET UZ, formerly a Graduate Research Assistant, Ames Laboratory, Iowa State University, is an Assistant Professor, Department of Metallurgical Engineering, Lafayette College, Easton, PA 18042. O.N. CARLSON is an Emeritus Professor, Department of Materials Science and Engineering and an Associate, Ames Laboratory, USDOE, Iowa State University, Ames, IA 50011. Manuscript submitted August 7, 1987.
METALLURGICALTRANSACTIONS A
Most of the thermotransport work reported has been on dilute binary solid solutions, t141 with very little work on ternary systems, t7'8'91 The latter is needed since multicomponent alloys are often of greater technological importance than pure metals. The purpose of this investigation is to determine the effects of substitutional solute composition on the thermotransport behavior of carbon in Nb-V alloys.
II.
THEORY
A discussion of the theory of thermotransport and related references are given in earlier publications, t9'~2'131 Here, a review of the phenomenological and atomistic aspects of the phenomenon will be presented with em
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