Nitriding of Fe-Cr Powdered Alloys by Ball Milling in Nitrogen Gas

Elemental iron and Fe100-XCrX (X=30, 50 and 70) powder mixtures were milled in a vibrational ball mill under nitrogen atmosphere. Structural changes during milling, and thermal stabilities and magnetic properties of the milled powders were examined by means of X-ray analysis, HRTEM, DSC and Mössbauer spectroscopy. It was found that Fe-Cr-N powdered alloys with very high concentrations of nitrogen were able to be prepared by this technique. The structure of the powder particles was composed of nanometer-sized b.c.c grains supersaturated with nitrogen. With the nitrided Fe50Cr50 alloy, the grain size decreased to about 2 nm after alloying with about 12 ar% of nitrogen, and then X-ray diffraction took on an amorphous-like pattern. On annealing of the nitrided powders, a heat evolution due to the precipitation of chromium nitride was observed at temperatures around 850 K. The Mössbauer analysis revealed that the nitrided Fe70Cr30 alloy had paramagnetic and ferromagnetic components. The nitrided Fe50Cr50 alloy became entirely paramagnetic after the reduction in grain size below about 5 nm.