Management of nitrogen in chromium

Work by Scruggs in the 1960's demonstrated that tensile ductility could be achieved at room temperature in sintered and extruded powder-metallurgical Cr alloyed with MgO. During sintering, some of the MgO converted to the MgCr₂O₄ spinel phase, which was hypothesized to getter N from the Cr, rendering ductile. Recent efforts at Oak Ridge National Laboratory and the University of Alabama have succeeded in duplicating this original effect. Room-temperature tensile elongations of 4% were obtained for hot-pressed Cr-6MgO-(0-1)Ti wt.% and 10% for hot-pressed/extruded Cr-6MgO-0.75Ti wt.%. Direct incorporation of N into the MgCr₂O₄ phase was not detected; however, impurities (particularly N and S) were observed to segregate to and/or precipitate at interfaces between the MgO/MgCr₂O₄ phases and the Cr matrix. Elevated temperature exposure of these alloys in N-maintaining environments resulted in Cr₂N subscale formation. Possible mechanisms by which the MgO additions may important room-temperature tensile ductility in Cr and potential applications for this unique class of materials are discussed.