Thermochemistry and diffusion of nitrogen in solid molybdenum
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I.
INTRODUCTION
M O L Y B D E N U M is used as an important alloying element in many alloy and stainless steels, both for imparting hardenability and as a substitute for chromium and vanadium. Furthermore, molybdenum and its alloys are used in power generation, propulsion devices, and in many other high-temperature applications, t~,2`3j There have been several investigations to determine the role of the environment in order to understand the purification, alloying sequence, and high-temperature applications of molybdenum and its alloys. Significant property changes can occur during fabrication or use of these materials due to the uptake of small amounts of interstitials. Therefore, the solubilities and diffusivities of these elements, especially nitrogen, have been of considerable interest. [4-91 The difficulty in the determination of a formal relation between the purity of molybdenum and a gas atmosphere such as nitrogen originates, to a large extent, due to (1) contamination of the highly reactive molybdenum in contact with commonly used containers at elevated temperatures and (2) problems involved in the accurate experimental determination of the interstitials dissolved in the high melting point metal. In view of these impediments, it is not surprising that conflicting data have been reported about the solubility of nitrogen in molybdenum and the terminal solubility of nitrogen in molybdenum with respect to its principal nitride, Mo2N. The discrepancy in the experimental data due to various sources can be appreciated from Figure 1, which is taken mainly from the compilation of Evans and Eyre. 17j Although some additional works tS,9j have been reported since the publication of their paper, the discrepancies have not been adequately resolved, and the need for reliable data in this field cannot be overemphasized. The solubility of nitrogen in molybdenum was determined at temperatures close to the melting point by designing a special holder to avoid sample contamination and to facilitate rapid cooling to preserve the actual ni-
GANG QI, Visiting Scientist, W.M. SMALL, Associate Professor, and T. DEBROY, Professor, are with the Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802. Manuscript submitted August 14, 1990. METALLURGICAL TRANSACTIONS B
trogen concentration of the sample at high temperature for subsequent analysis. Apart from providing important information about thermochemical and kinetic aspects related to nitrogen solubility in molybdenum, it is hoped that the present data will be useful in resolving some of the discrepancies in results reported in the literature. II.
EXPERIMENTAL
The high-purity molybdenum used in this study was obtained from Amax Specialty Metal Corporation, Ann Arbor, MI. A detailed chemical analysis of this metal is given in Table I. Nitrogen and argon were of ultra high purity (99.999 pct). Any trace amounts of oxygen in nitrogen and argon were removed by separate passage of each gas through tubes packed with copper turn
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