Structure of a Fe-Cr-Mn-Mo-N alloy processed by mechanical alloying
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NTRODUCTION
VARIOUS reports on the infusion of nitrogen into iron powders and steels have shown that the maximum N2 content that can be introduced in the ␣ and ␥ phases is strongly influenced by the type and amount of defects present. In particular, mechanical alloying under a N2 atmosphere makes it possible to obtain very high nitrogen levels in Fe powder mixtures. This method is currently used in the development of nanostructured materials, as well as in glassy phases. The synthetic species produced by these means are typically under nonequilibrium thermodynamics. Hence, after powder milling, annealing is commonly employed to achieve crystallization and homogenization of the nanostructured material. In addition, the structure is fully relieved from the internal stresses induced by the milling process. High nitrogen stainless steels are potentially important due to their superior mechanical and corrosion properties as a result of nitrogen in solid solution.[3,4,5] However, mechanical alloying has not been employed in producing these steels, even though this technique has been widely used in investigating binary or ternary systems.[6,7] During the manufacture of alloys by mechanical alloying, two main goals need to be achieved in order to obtain the desired microstructure: (a) an adequate nitrogen content during the milling process; and (b) the stable phases, which are obtained by a suitable annealing process. The lack of phase diagrams with more than three elements for microstructure design purposes makes it difficult to select alloy compositions, which will ensure a desirable alloy performance. However, the basic Fe-Cr-N ternary system is known and has been reviewed several times.[8,9,10] From [1,2]
M. ME´NDEZ, H. MANCHA, G. MENDOZA, and J.I. ESCALANTE, Research Scientists, are with the Center for Research and Advanced Studies of IPN, 25000 Saltillo, Coah., Me´xico. Contact e-mail: hmancha@ saltillo.cinvestav.mx M.M CISNEROS, Research Scientist, is with the Tech´ PEZ, nological Institute of Saltillo, 25280 Saltillo, Coah., Me´xico. H.F. LO Professor and Chair, is with the Materials Department, University of Wisconsin, Milwaukee, WI 53201. Manuscript submitted March 8, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
these reviews, a thermodynamic analysis has been provided that enables the derivation of Gibbs free energy parameters for the ␣, ␥, , CrN, Fe4N, and liquid phases. In addition, the effect of Mn on the stability of austenite in Fe-Cr-Ni alloys has been reported.[11] From this report, it is apparent that the main role of Mn in nitrogenated stainless steels is not so much to stabilize the austenite, but to enhance the effect of Cr on increasing the nitrogen solubility in this phase. The present work examines the exhibited microstructures obtained in an annealed Fe-Cr-Mn-Mo-N powder mixture processed by mechanical alloying under a nitrogen atmosphere.
II. EXPERIMENTAL PROCEDURE High-purity (99.99 pct) elemental Fe, Cr, Mn, and Mo powders with mean particle sizes of 44.7, 50.17, 45.0, and 22.96 m, resp
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