High-temperature nitridation of Ni-Cr alloys
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I.
INTRODUCTION
W H E N a homogeneous binary alloy A-B is heated in a nitriding atmosphere (e.g., a mixture o f N H 3 + H2, N2, etc.), selective nitridation may take place if the external activity of nitrogen is sufficient to nitridize B (solute) but not A. Nitrogen can react with the solute to form a surface nitride layer. However, if the permeation of nitrogen into the metal A is appreciable, the precipitation of the solute B (in the form of nitrides) may also occur inside the alloy. Whether exclusive surface nitridation or formation of internal precipitates (or a combination of both) takes place depends on the activity of nitrogen in the surrounding atmosphere, the activity of the solute B in the matrix of solid solution A(B), and the relative diffusivities of nitrogen and the solute element in the solid solution A(B). Most studies on nitriding behavior have been performed on ferrous alloys. Nitriding of iron-based alloys is a wellknown and widely used commercial process. Internal nitridation of nickel-based alloys, however, was first reported by Barnes and Lai only a few years ago. t~l Later, Rubly and Douglass [2] studied the nitriding behavior of nickel-chromium alloys in an ammonia-hydrogen mixture over the range of 973 to 1173 K. Internal nitridation rates for different Ni-Cr alloys were found to obey the parabolic timedependency law and decreased with increasing chromium content. The aims of the present work were as follows: (1) to investigate the interaction of Ni-Cr alloys with nitrogen at different external pressures (1 to 6000 bar of N2) at 1398 K; and (2) to evaluate the internal nitriding of Ni-Cr alloys using a thermodynamic assessment of the Ni-Cr-N system.
A.A. KODENTSOV, Researcher, J.H. GOLPEN, Postdoctoral Student, and F.J.J. VAN LOO, Professor, are with the Laboratory of Solid State Chemistry and Materials Science, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands. C. CSERHATI, Researcher, is with the Department of Solid State Physics, Kossuth Lajos University, H4010 Debrecen, Hungary. J.K. KIVILAHTI, Professor, is with the Department of Materials Science and Engineering, Helsinki University of Technology, 02150 Espoo, Finland. Manuscript submitted March 10, 1994.
METALLURGICAL AND MATERIALS TRANSACTIONS A
II.
EXPERIMENTAL PROCEDURE
High-purity nickel (99.98) and chromium (99.95) were used as initial materials. Ni-Cr alloys (5.0 to 32.0 at. pct of Cr) were melted in an arc furnace under argon atmosphere using a nonconsumable tungsten electrode. The ingots were cold-rolled to a thickness of 1.5 mm. Slices of 8 X 8 mm were cut from the sheets and homogenized for 100 hours at 1 bar of gas mixture Ar + 10 vol pct H2 (H20 < 5 ppm) and 1373 K. The grain size in the final materials was in the range of 100 to 150 tzm. Nitriding of the alloys at 1 bar of nitrogen was performed in a vacuum fumace equipped with a gas inlet system which enabled heat treatment in isostatic gas conditions. If quenching of the sample was necessary, a horizontal tube furnace with flowing nitrogen w
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