Pore Formation Upon Nitriding Iron and Iron-Based Alloys: The Role of Alloying Elements and Grain Boundaries

• Published in: Metallurgical and materials transactions. A, Physical metallurgy and materials science Physical metallurgy and materials science, 2014-12, Vol.45 (13), p.6173-6186
• Main Authors: Schwarz, B., Göhring, H., Meka, S. R., Schacherl, R. E., Mittemeijer, E. J.
• Format: Article
• Language: English
• Subjects: Alloy development; Alloying elements; Applied sciences; Characterization and Evaluation of Materials; Chemistry and Materials Science; Exact sciences and technology; Ferrous alloys; Grain boundaries; Heat treatment; Iron; Materials Science; Metallic Materials; Metals. Metallurgy; Nanotechnology; Nitriding; Pore formation; Porosity; Production techniques; Structural Materials; Surfaces and Interfaces; Thermochemical treatment and diffusion treatment; Thin Films
• ISSN: 1073-5623; 1543-1940
• DOI: 10.1007/s11661-014-2581-x

Pure iron and a series of iron-based Fe-Me alloys (with Me = Al, Si, Cr, Co, Ni, and Ge) were nitrided in a NH 3 /H 2 gas mixture at 923 K (650 °C). Different nitriding potentials were applied to investigate the development of pores under ferrite and austenite stabilizing conditions. In all cases, pores developed in the nitrided microstructure, i.e. , also and strikingly pure ferritic iron exhibited pore development. The pore development is shown to be caused by the decomposition of (homogeneous) nitrogen-rich Fe(-Me)-N phase into nitrogen-depleted Fe(-Me)-N phase and molecular N 2 gas. The latter, gas phase can be associated with such high pressure that the surrounding iron-based matrix can yield. Thermodynamic assessments indicate that continued decomposition, i.e. , beyond the state where yielding is initiated, is possible. Precipitating alloying-element nitrides, i.e. , AlN, CrN, or Si 3 N 4 , in the diffusion zone below the surface, hinder the formation of pores due to the competition of alloying-element nitride (Me x N y ) precipitation and pore (N 2 ) development; alloying elements reducing the solubility of nitrogen enhance pore formation. No pore formation was observed upon nitriding a single crystalline pure iron specimen, nitrided under ferrite stabilizing conditions, thereby exhibiting the essential function of grain boundaries for nucleation of pores.