Synthesis of Ti 4 AlN 3 and phase equilibria in the Ti-Al-N system
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INTRODUCTION
JEITSCHKO et al. discovered the Ti2AlN and Ti3AlN[3] ternary compounds in 1963 and 1964, respectively. In 1984, Schuster and Bauer[4] reported an additional ternary compound in that system: Ti3Al2N2. Based on crystallographic and diffraction data, they proposed the Ti3Al2N2 crystal to be a hexagonal unit cell of the space group P31c, with lattice parameters of a 5 0.299 nm and c 5 2.33854 nm. The space group was later revised[5] to P63mc, with a 5 0.29875 nm and c 5 2.3350 nm. Lee and Petuskey[6] re-examined the work of Schuster and Bauer over the 900 to 1873 K temperature range and, based on chemical analysis and an erroneous interpretation of the X-ray diffraction (XRD) results, concluded that Ti3Al2N2 was Ti3Al12xN2 (0 , x , 0.3) and was isostructural with Ti3SiC2. Barsoum and Schuster, however, noted that a c-lattice parameter of 2.3350 nm is incompatible with the Ti3SiC2 structure.[7] Most recently, Barsoum and co-workers have unambiguously shown by high-resolution transmission electron microscopy (TEM) chemical analysis, and Rietveld analysis of neutron diffraction data[1,8] that the correct structure and stoichiometry were neither Ti3Al2N2 nor Ti3Al12xN2, but a new phase, Ti4AlN32x, related to the family of layered machinable hexagonal ternary carbides with the general formula Mn11AXn , where M is an early transition metal, A is group-A element (mostly III-A and IV-A), and X is either C or N.[9–20] Concomitant electron microprobe analysis (EMPA) determined the chemical composition to be 50.8 at. pct Ti, 12.4 at. pct Al, and 36.8 at. pct N, or 4:1:3, which is almost identical to the one claimed by Lee and Petuskey.[6] The rate at which equilibrium is reached in the Ti-Al-N system, around the Ti4AlN32x composition, is slow. Lee and Petuskey annealed their samples for 20 days at 1523 K,[6] whereas Schuster and Bauer used 50-hour anneals at 1573 K.[4] Another complication is the weight loss, shown in this work to be Al loss, reported during these extended anneals. [2]
M.W. BARSOUM, Professor and T. EL-RAGHY, Research Assistant Professor, are with the Department of Materials Engineering, Drexel University, Philadelphia, PA. A. PROCOPIO graduated from the Department in June of 1999 with an MS thesis. Manuscript submitted March 22, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
To avoid the latter problem, synthesis of Ti4AlN32x, in this work, involved a relatively short reactive hot isostatic pressing (“hipping”) process that resulted in closed pores. The extended anneals could then be carried in Ar without loss of Al from the bulk. As noted previously, Ti4AlN32x is a member of a larger family of layered hexagonal ternary carbides and nitrides that have recently been shown to possess an unusual set of properties.[9–20] They are machinable, good thermal and electrical conductors, anomalously soft (Vickers hardness of 2 to 5 GPa), thermal shock resistant, damage tolerant, stiff, have relatively low thermal expansion coefficients (,10 3 1026 8C21), and combine mechanical anisotropy with thermal isotr
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