Mass transport of carbon in one and two phase iron-nickel alloys in a temperature gradient
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INTRODUCTION
W H E N a temperature gradient is imposed on a homogeneous material the components of the system, interstitial solutes for an example, move toward either the hot or colder portion of the specimen. This phenomenon is referred to as thermotransport or thermomigration. Conversely, a heat flux and an accompanying temperature gradient occur in a material in which a concentration gradient exists. This is referred to as the Dufour effect. The similarities between thermotransport and heat conduction are evident in the theoretical work of Fiks ~ which shows that thermotransport results in part from the forces exerted on the migrating atoms by two types of heat-carrying agents in a metal: namely, phonons and electrons. Thermotransport is of technological importance in high temperature alloys or compounds (particularly those containing interstitial atoms) that are subjected to a large temperature gradient for a prolonged period of time. Indeed, the net force of an atom due to a temperature gradient may be greater than that arising from a concentration gradient in isothermal diffusion. 2 Iron-nickel alloys containing additions of chromium and carbon form the basis for a group of alloys identified as Alloy 800 that are characterized by their good high temperature creep strength and oxidation resistance. This has led to the use of these alloys in high temperature applications such as coal gasification containment vessels, breeder reactor fuel element claddings, and superheaters or reheater tubes in helium-cooled reactors. Many high-temperature components, such as heat exchanger tubes, nozzle guide vanes, and turbine blades are exposed to large temperature gradients of the order of 102 K mm -~. Accordingly, much work has been done on determining the thermodynamic properties 3"4and diffusion and electrotransport5~6behavior of carbon in the Fe-Ni-C system for various carbon concentrations. Hitherto, these studies have been restricted to I. C. I. OKAFOR, formerly with Ames Laboratory, DOE, is now with Anambra State University of Technology, P . M . B . 1660, Enugu, Nigeria. O . N . CARLSON is with Ames Laboratory-DOE and the Department of Materials Science and Engineering, Iowa State University. D.M. MARTIN is with the Department of Materials Science and Engineering and Engineering Research Institute, Iowa State University, Ames, IA 50011. Manuscript submitted October 30, 1981.
METALLURGICALTRANSACTIONS A
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isothermal measurements. This work was undertaken, therefore, as part of a study of the mass transport of interstitial solutes in high temperature metals and alloys by which we are attempting to gain a better understanding of the migration of interstitial solutes under the influence of a temperature gradient. Thermotransport studies are usually carried out on single phase alloys. In many service situations, however, the solute concentration is so high or the solubility limit so low that an alloy part operating in a temperature gradient is either partially or completely in a two-phase region of the system. The
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