Nitrogen activity determination in plasmas
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INTRODUCTION
IN traditional
thermochemical processing of metals and alloys, the feasibility of reactions, the activities of various species, and the solubility of gases in metals and alloys can be determined, in most cases, from the established principles and the available data base in metallurgical thermodynamics. However, such estimations are not possible for many important established and emerging materials processing operations where a solid or a liquid metal is exposed to a plasma. Some familiar examples include various types of welding and the laser, electron beam, and plasma processing of metals and alloys. Although plasmas are electrically neutral, the presence of excited neutral atoms, ions, and electrons precludes any simple extension of the well-established formal treatment of gas-metal systems to plasmacondensed matter systems. Results of recent research at Penn State and elsewhere indicate that the physicochemical behavior of plasmametal systems u-5] is significantly different from that in the corresponding gas-metal systems. For example, when a pure metal, such as copper or iron, is exposed to a glow discharge argon plasma, its interfacial tension is significantly reduced. [3] The rates of vaporization of pure metal drops in a plasma environment are significantly lower f4,sj than those in the corresponding gas environment. Furthermore, the transformation of ordinary molecular species to excited neutral atoms and ions in the plasma leads to enhanced solution of species in metals. Uda et al.[6] and Ohno and Uda tT] have demonstrated that the concentrations of nitrogen in liquid iron and nickel during arc melting are significantly higher than the corresponding equilibrium solubilities when the metals are exposed to diatomic nitrogen molecules under non-arc melting conditions. Katz and King tS1 also observed that, in the presence of an arc discharge, the concentration of A. BANDOPADHYAY, Postdoctoral Research Associate, A. BANERJEE, Graduate Student, and T. DEBROY, Professor, are with the Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802. Manuscript submitted July 29, 1991. METALLURGICAL TRANSACTIONS B
nitrogen in liquid iron was significantly higher than the value predicted by Sievert's Law. All of the previous w o r k s [6'7'8] o n the solubility of a species in metals exposed to a plasma were conducted using electric arcs and relatively large quantities of metals. In these investigations, the metal in contact with the plasma jet was exposed to a strong radial temperature gradient, with the maximum temperature established at the jet axis. It is known from recent theoretical t9'~~ and experimental II~l research that when a liquid metal is heated by an electric arc, the propagation of strong convection currents in the liquid pool due to Marangoni, densimetric, and electromagnetic effects is insufficient to eliminate the commonly present strong temperature gradient within the liquid. Inside the plasma jet, the highest absorption rate corres
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