A general model for partitioning of gases between a metal and its plasma environment
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INTRODUCTION
IN many important metal processing operations, such as welding and laser, electron beam, and plasma processing, a metal or an alloy is exposed to a plasma. It has become clear in recent years that the physicochemical behavior of a plasma-metal system is significantly different from that of the corresponding gas-metal system. For example, rates of vaporization of metals are significantly lower in the presence of a plasma than in an environment without a plasma, m2j Similarly, the interfacial tensions of pure metal drops are significantly reduced in the presence of plasma, t3j Results of recent research at the Pennsylvania State University and elsewhere t4-Sj indicate that the gas solubilities in the metals exposed to plasma are significantly higher than the values predicted by Sieverts law 191 for corresponding gas-metal systems. In the presence of a plasma, the transformation of ordinary molecular species to excited neutral atoms and ions in the gas phase leads to enhanced solubility of gaseous species in the metal. Oxygen and nitrogen contents as high as 0.7 and 0.2 wt pct, respectively, have been obtained in low-carbon steel welds, tl~ These concentration levels are significantly higher than the values predicted by Sieverts law. Ohno and Uda t41 and Uda and Ohno tSl have demonstrated that the concentrations of nitrogen in liquid nickel and iron during arc melting are significantly higher than the corresponding equilibrium solubilities when the metals are exposed to diatomic nitrogen under nonarc-melting conditions. Katz and Kingt61 also observed that in the presence of an arc discharge, the concentration of nitrogen in liquid iron was significantly higher than the value predicted by Sieverts law. Bandopadhyay et al. I71 studied plasma-enhanced nitrogen solubility in pure tantalum and niobium at K. MUNDRA, Postdoctoral Fellow, and T. DEBROY, Professor, are with the Department of Materials Science and Engineering, Pennsylvania State University, University Park, PA 16802. Manuscript submitted March 25, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS B
2243 K. Their data showed a much greater nitrogen solubility in each metal in the presence of plasma than without the plasma. Ouden and GrieblingtSJ found that the solubility of nitrogen in pure iron in the presence of an arc was much greater than that calculated by Sieverts law. The plasma-enhanced solubility of gases in metals has been attributed to the presence of one or more species present in the gas phase. Bandopadhyay e t a / . 171 attributed the enhanced nitrogen solubility in tantalum and niobium to the existence of high-energy electrons that produce neutral atoms (N) and ions (N+). They identified excited neutral atoms N*, molecules N* and ions, N + and N~, in the helium-nitrogen plasma by optical emission spectroscopy. Lakomsky and Torkhov t~l~ attributed plasma-enhanced solubility or nitrogen in metals to excited nitrogen molecules, N*. Katz and Kingt6~ argued that once an excited diatomic molecule is adsorbed on a surface, its vibration i
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