Effect of ternary additions on the structural stability and electronic structure of intermetallic compounds: Al 3 Ti + C
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The effects of substituting Cu for either Ti or Al in Al3Ti were studied by the first-principles local density self-consistent linear muffin tin orbital method. The Cu atoms are found to strongly favor Al sites and to promote the stability of the Ll 2 phase, in agreement with experiment. Surprisingly, the rigid band model is closely followed when Cu substitutes for Al. However, the structure of the density of states undergoes remarkable changes if Cu substitutes for Ti; a much weaker hybridization occurs in both Ll2- and DO22-like structures, giving rise to weaker binding effects. The modification of bond directionality upon Cu addition is determined by comparing the charge density for the structures calculated with those of pure Al3Ti. The effect of tetragonal distortion is also examined. By comparing with the bonding characteristics of pure Al3Ti in the Ll 2 and DO22 structures, the addition of Cu to Al3Ti is found to be equivalent to the tetragonal distortion in DO22 Al3Ti as far as bonding is concerned, resulting in the stabilized Ll2-like structure for (AlCu)3Ti. The semi-empirical inverse relation between the structural stability and the density of states at Fermi energy is well established. I. INTRODUCTION
Among intermetallic compounds, titanium-aluminides are considered as some of the most promising candidates for aerospace applications and to have superior properties to the Ni-based superalloys.1 Although the compounds Ti3Al and TiAl have been the targets of materials research for more than a decade,2'3 Al3Ti has not been studied extensively until recently.4"7 Despite various efforts, its poor ductility at low temperatures has still to be improved. The success of improving the ductility in another compound, Co3V, by transforming it from the DOi9 structure with relatively low symmetry to the cubic Ll 2 structure with partial replacement of the Co by Fe8 has encouraged the application of similar ideas to other compounds. Since the stable Al3Ti crystallizes in the DO22 structure, i.e., an Ll 2 derivative with tetragonal symmetry, it is natural to try to stabilize the closely related cubic Ll 2 structure. So far, a number of ternary elements (e.g., Cu,9"14 Ni;9,lU2,15-17
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promote the Ll 2 structure in Al3Ti, but no improvement in ductility has been reported. Meanwhile, similar efforts with a wider range of ternary additions on another DO22 compound, Al3Nb, failed to stabilize the Ll2 structure.24 Hence, it appears important to understand the effects of ternary additions on the properties of intermetallic compounds. Previous first-principle calculations5"7 have focused on the binary system. Recently, Fu performed some model calculations on Al3Sc25 based on observations that although it is a natural Ll 2 compound, it shows brittle fracture by transgranular cleavage, just like some of the Ll2-modified 330 http://journals.cambridge.org
J. Mater. Res., Vol. 6, No. 2, Feb 1991 Downloaded: 12 Mar 2015
Al3Ti19 alloys. Nonetheless, no calculations on Al3Tibased t
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