Phase stability and microstructure of Al-Ti-Fe near Al 3 Ti
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I.
INTRODUCTION
THE intermetallic alloy A13Ti is considered as a potential high-temperature structural material because of its high melting point, low density, superior strength-toweight ratio and high oxidation resistance. However, this alloy is extremely brittle at room temperature since it has the low symmetry tetragonal D022 structure, which is based on the L12 structure. The D022 unit cell can be described as two L 12 unit cells stacked together, with an antiphase vector [a/2 + b/2] in between. One of the remarkable achievements in improving the ductility of A13Ti is to transform the tetragonal D022 into the cubic L 12 structure by adding a few atomic percents of elements such as Cr,11.21 Mn,12.3.41 Fe,15-81 C0,121 Ni,16-9l Cu,[6-111 Zn,tt21 Rh,121 Pal, 12331 Ag, t2'~41 Pt, t21 and Au. 12~
Not only the compound A13Ti but also the other aluminum-rich A1-Ti intermediate phases, including the long-period one-dimensional antiphase domain structures (LPS), AI2Ti, and AIsTi3, are all based on the L12 structure. The LPS, which were reported earlier as AI23Ti9 and AII~Ti5 by Raman and Schubert j~51or as AI24Ti8 and AIsTi2 by van Loo and Rieck, 1~6,~v]are a series of antiphase domain structures based on the LI2 with average domain size between 1 and 2. IJSA91Two types of AI2Ti have been reported in the AI-Ti binary system: AI2Ti~ (Ga2Zr-type) and AI2Ti" (Ga2Hf-type).119.20.211Both of them can be constructed from L12 by displacing the structure by a nonconservative antiphase vector 1/2[---1 0 1] or 1/210 + 1 1] every (0 0 1) plane. A12TiI and AI2Ti" are distinguished by different stacking sequences, and the latter has twice the period as the former along the c-axis. The third type of AI2Ti, AI2Ti m, similar to the other two, but with different stacking sequence, was observed in Fe-modified AI3Ti and reported in a previous article. [221 However, the AI2Ti~u has never been observed in the A1-Ti binary system. Therefore, the added Fe not only can convert A13Ti (or AI2Ti) into L12 but also can change the structure of A12Ti. AIsTi3 is a superlattice first reported by Miida e t al. 1231 It consists of the domains based on the LI2, and the domain boundaries have the Llo-type atomic arrangement, i23] The primary interest in this investigation is to study how the added iron can change the structures of AI3Ti T.Y. YANG, Graduate Research Assistant, and EDWARD GOO, Associate Professor, are with the Department of Materials Science and Engineering, University of Southern California, Los Angeles, CA 90089-024 I. Manuscript submitted July 1, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
or AI2Ti. Also, among reported phase diagrams, Is'6'71 the phase analysis in the region between the binary compounds A1Ti and AI3Ti in the AI-Ti-Fe system is not complete yet: some intermediate phases, such as LPS, AI2Ti, and AIsTi3 have not been specified. Phase stability in this region is also studied.
II.
EXPERIMENTAL
Twelve alloy ingots of compositions listed in Table I were used in this study. These ingots (about 20 g) were prepared by a
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