Communication A Novel X-ray Micro-diffraction Approach for Structural Characterization of Trace Quantities of Secondary Phases in Al 2 O 3 -C/Fe System M. IKRAM-UL-HAQ, R. KHANNA, Y. WANG, and V. SAHAJWALLA Sessile drop investigations were carried out on Al 2 O 3 - 12.9C refractory substrates in contact with molten iron at 1823 K (1550 °C) for 30 minutes. Chemical reactions in this system resulted in the generation of trace quan- tities of ferro-aluminum alloys. An X-ray micro-dif- fraction approach was developed to localize and structurally identify these phases based on epitaxial mapping using X’Pert materials research Diffractome- ter. Using this technique, a small peak in the standard XRD pattern could be enhanced to four diffraction peaks with much higher intensities, leading to a much improved indexing of diffraction peaks and structural characterization of the reaction product. DOI: 10.1007/s11663-014-0201-1 Ó The Minerals, Metals & Materials Society and ASM International 2014 Carbothermic reduction of alumina has been investi- gated extensively for aluminum production as an alternative to the conventional Hall-Heroult process. [1– 3] The overall reaction for carbothermic reduction: Al 2 O 3 (s) + 3C (s) = 2 Al (l) + 3 CO (g), is known to proceed in the region of 2473 K (2200 °C) for a pressure of one atmosphere. [4] Cox and Pidgeon [5] investigated the Al-O-C system in the temperature range 1700 K to 2200 K (1427 °C to 1927 °C) at reduced pressures and reported on the formation of small quantities of aluminum bearing compounds such as Al 4 O 4 C, Al 2 OC, and Al 4 C 3 . In a recent study by our group, in-depth investigations were carried out on Al 2 O 3 -12.9 pct C refractory in contact with molten steel at 1823 K (1550 °C) in argon atmosphere for periods upto 3 hour. [6] Extensive analysis on the system included measurements on carbon pick-up by molten iron, gas generation, the penetration of molten iron in the refractory substrate, video recording of the reaction process, X-ray diffraction, and scanning electron microscopy. One of the first evidences for chemical reactions occurring at such low temperatures was provided by the presence of an additional peak at 44.19 deg in the X-ray diffraction spectrum. A single peak, while pointing to a possible chemical reaction, is however not sufficient for identifying the new phase, which requires a minimum of three diffraction peaks for accurate phase identification. In an attempt to identify various reaction products, the interfacial region between the metal and the refrac- tory was also analyzed by scanning electron microscopy (SEM). A microstructural image of the cross-sectional region just below the metal droplet (Figure 1) indicates some of the iron penetrated regions of the substrate. This region was also analyzed using elemental mapping and EDS. While the position ‘A’ indicates the presence of unreacted alumina in the refractory substrate, the region at position ‘B’ shows the penetration of iron into the substrate. The EDS analysis of position ‘B’ indicates that the oxygen content was very low at this point clearly pointing to a partial reduction of alumina. The oxygen level was found to be quite low in the metal (Fe and Al) rich regions ‘C’ and ‘D’ as well. The situation presented above is fairly common whenever secondary phases are present in trace quan- tities; phase identification becomes very difficult due to insufficient X-ray diffraction data and inherent limita- tions of EDS analysis for determining elemental con- centration. We have developed a novel X-ray micro- diffraction technique using epitaxial mapping software on PANalytical X’Pert materials research diffractometer (MRD) to overcome this difficulty. In this article, we report a detailed experimental procedure that could be used to localize and structurally characterize trace quantities of secondary phases. Sessile drop investigations were carried out on Al 2 O 3 + 12.9 pct C/(Fe + 0.6 C) system at 1823 K (1550 °C); experimental details have been reported elsewhere. [3,4] To examine new phases formed at the interface, the metal droplet was removed from the substrate after the reaction. The exposed interfacial region was set in an epoxy resin mold to prevent substrate degradation due to the hygro- scopic nature of alumina. [5] The surface X-ray diffraction scan of the reacted assembly was carried out by conven- tional MRD using Cu K a radiation. The scan showed an additional peak, whose diffraction pattern did not belong to any of the reacting constituents such as alumina, iron, or carbon (Figure 2a). An analysis with X’pert high score software indicated that this additional peak, located 44.19 deg, may belong either to FeAl or Fe 3 Al (JCPDS No 33-0020 and 45-1203) respectively. As only one additional peak was observed, a general scan of the whole sample using X’pert MRD was not able to identify the new phase. Using thin film X’pert epitaxy technique, commercial MRD equipment could however be used for detecting small concentrations of reaction products and texture through diffraction space maps based on area, texture, and 1 or 2-axes scans. This software has specific tools that can extract information from the near surface region, M. IKRAM-UL-HAQ, Research Associate, R. KHANNA, Associate Professor, and V. SAHAJWALLA, Professor, are with the Centre for Sustainable Materials Research and Technology, School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW, 2052, Australia. Contact e-mail: ritakhanna@ unsw.edu.au Y. WANG Scientific Officer, is with The UNSW Analytical Centre, The University of New South Wales. Manuscript submitted July 31, 2014. Article published online October 11, 2014. 1970—VOLUME 45B, DECEMBER 2014 METALLURGICAL AND MATERIALS TRANSACTIONS B