Ordered Structures in Ti Al Alloys in the Composition Range from Ab to Ab 3
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ORDERED STRUCTURES IN Ti Al ALLOYS IN THE COMPOSITION RANGE FROM AB TO AB 3 A. LOISEAU, A. LASALMONIE Office National d'Etudes et de Recherches Atrospatiales (ONE RA) BP 72 92322 CHATILLON CEDEX (France) G. VAN TENDELOO, J. VAN LANDUYT RUCA, Groenenborgerlaan 171, B-2020 Antwerp, Belgium ABS TRACT This paper describes crystal structures of Ti, Al alloys containing from 40 to 75 % at. Aluminum observed by electron microscopy. Two new phases are in particular discussed : Ti 2 Al and a phase denoted X, which is a non stoichiometric Ti3 Al5
INTRODUCTION 3
The Ti Al alloys belong to3the T B4 family alleys, which are characterized by tfe axat that the atom T (Ti, Zr, Hf) and B (Al, Ga, In) have very different electronic effects since "T", a transition metal, tends to induce short range electronic interactions in opposite to "B". Thus the ordered structures on the T rich side are different from those of the B rich side: for example, the T 3 B compounds are of Cu3 Au type (LI 2 for a cubic lattice and Do19 for an hexagonal lattice) while the TB compounds are long period superstructures of the Cu Au lattice (Do22 or Do ). In this family of alloys, the Ti-Al system is special since the Ti and Al atoms have very similar atomic sizes.The phase diagram exhibits two kinds of ordered structures : in the Ti rich side, the phases are substitutional superstructures of the h.c.p. Ti lattice and on the Al rich side substitutional superstructures of the f.c.c. Al lattice. All the phases are thus close-packed and the compact planes have the stoichiometric composition. Figure 1 shows a recent diagram [11, with on the Ti side the only Ti Al compound and on the Al side TiAl. (Do 2 2 ), Ti25A which corresponds to Ti1l.., described by Raman [2] and 2iAlvarious long period structures based on TiAl3 have been found [Ti 5 A iI, Ti8 Al124- • ] 13 T
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