X-RAY-DIFFRACTION AND MOSSBAUER CHARACTERIZATION OF ATTRITION-MILLED NANOSTRUCTURED IRON AND IRON-NITROGEN POWDERS

To date there has been limited information on the microstructure of me chanically infused interstitial elements in metal powders, and, althou gh interstitials have a limited solubility in iron alloys, they are an important means of increasing tensile strength in large-grained struc tures. In this study, iron powder was mechanically processed in argon and mechanically alloyed in nitrogen to produce micron-sized particles with nanosized grains. Although the total mechanically infused nitrog en concentration in the milled particles greatly exceeded the thermody namic equilibrium interstitial solubility limit of iron, no nitrides f ormed. The nitrogen distribution was determined by a combination of X- ray diffraction (for nanostructure characterization) and Mossbauer spe ctroscopy (for atomic site characterization). Analysis revealed that t he majority of the nitrogen was not intragranular but intergranular, a ssociated with the nanograin boundary as either surface impurities or incorporated into the outer lattice cell of the grain boundary where i t distorted the bcc cells to form bet Fe(N) clusters.