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N

HIGH-NITROGEN stainless steels (HNSS) have been extensively investigated lately due to their excellent combination of mechanical properties and corrosion resistance.[1,2,3] In particular, nitrogen as an alloying element is a powerful solid-solution strengthener and an austenite stabilizer. Other potential benefits include grain size strengthening and an increase in pitting corrosion resistance.[1,2] In the biomedical field, Ni-free stainless steels[4] are highly attractive, since Ni has been linked to severe allergic reactions in the human body. Commercial versions of HNSS are commonly produced by high-pressure melting including electroslag remelting processes. Under these conditions, the nitrogen solubility is strongly dependent on the thermodynamic equilibrium achieved between the melt and the nitrogen pressure. From Sievert’s law, high-nitrogen contents can only be attained under high-N2 pressures. However, the need for expensive equipment for high-pressure steel melting is a major constraint. Alternatively, nitrogen incorporation in the solid state by powder-metallurgy techniques is a viable alternative in processing high-nitrogen steels.[5–9] Successful efforts in obtaining high-nitrogen steels have been reported, such as hot-isostatic pressure nitriding,[5] steel nitriding under mechanical fluidized vacuum,[6] and reactive mechanical alloying.[7,8,9] The main advantages of conventional powdermetallurgy methods are related to the use of simple equipment in incorporating significant nitrogen contents in steels. M.M. CISNEROS and E. VALDE´S, Professors and Researchers, and ´ ZQUEZ, Postgraduate Student, are with the Depto. Metal Meca´nica, D. VA Instituto Tecnolo´gico de Saltillo, C.P. 25280, Saltillo Coah., Me´xico. Con´ PEZ, Professor and Chair, is tact e-mail: [email protected] H.F. LO with the Materials Department, University of Wisconsin-Milwaukee, WI 53201. H. MANCHA, G. MENDOZA, and M. ME´NDEZ, Professors and Researchers, are with CINVESTAV-IPN Unidad Saltillo, C.P. 25000, Saltillo Coah., Me´xico. Manuscript submitted July 6, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A

In addition, mechanical alloying provides an alternate route in producing high-nitrogen stainless steels with novel properties. In this method, powder mixtures corresponding to the alloy of interest are mixed and ball milled together in an attritor under a nitrogen atmosphere. Under these conditions, nitrogen gets absorbed and forms a solid solution with the metallic powder mixture. Enhanced nitrogen solubility can be achieved through these means, concomitant with the development of a nanometric grain structure. However, studies concerning complex iron mixtures processed by mechanical alloying are rather scarce. Among the published works, Miura et al.[8] reported the formation of nanostructures in high-nitrogen Cr-Ni and Cr-Mn stainless-steel powders via mechanical alloying. Also, Rawers and Maurice[7] have shown that over 1 wt pct N can be incorporated in iron powder consisting of nanometric ferritic grains. In general, nitrogen