The microstructure of electrodeposited titanium-aluminum alloys
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I.
INTRODUCTION
MUCH of the research in the area of low-density intermetallic compounds has focused on A13Ti-based alloys, with applications envisioned for monolithic and, more likely, composite materials. The materials of interest are ternary or higher order alloys with transition metals, for example, Cu, Ni, Pd, Zn, Mn, or Mo, being added to stabilize the L12 crystal structure (simple cubic Bravais lattice) rather than the equilibrium binary D0z2 structure (body-centered tetragonal). This change in crystal structure is considered desirable, since the higher symmetry of the L12 structure would be expected to increase the number of slip systems, therefore, enhancing toughness and/or ductility. Discussions of slip systems, alloying effects, and mechanical properties have been published for various A1-Ti-X alloys fabricated by casting or powder metallurgy methods, t~-81 The present study examines the microstructure of A1-Ti alloys with 3.6 to 24.1 at. pct Ti* electrodeposited from *All compositions will be given in atomic percent.
a molten chloroaluminate electrolyte. Although the technique of electrodeposition is very different from traditional approaches used to fabricate these alloys, a number of analogies exist between it and rapid solidification. For example, Grushko and Stafford have shown that by using electrodeposition, extended solid solutions, stable and metastable crystalline phases, quasicrystalline phases, and amorphous phases can all be formed in the A1-Mn system. tg-Z3J Electrodeposition is an attractive method for fabricating intermetallic compounds since (1) hightemperature consolidation is unnecessary, (2) it is a G.M. JANOWSKI, formerly NIST/NRC Postdoctoral Research Associate, National Institute of Standards and Technology, is Assistant Professor, Department of Materials Science and Engineering, The University of Alabama at Birmingham, Birmingham, AL 35294-4461. G.R. STAFFORD, Research Chemist, is with the Metallurgy Division, National Institute of Standards and Technology, Gaithersburg, MD 20899. Manuscript submitted October 2, 1991. METALLURGICAL TRANSACTIONS A
near-net-shape process, (3) undesirable compositional inhomogeneities are very limited in scale, and (4) grain sizes are typically very small. The purpose of this article is to report the electrodeposition of titanium-aluminide intermetallic compounds. In addition, it is shown that these materials have unique microstructural characteristics, such as extended Ti solid solubility, extensive twinning, and a metastable, binary L12 phase, which are a result of this processing method. II.
EXPERIMENTAL PROCEDURE
A schematic of the electrodeposition process which was used to fabricate the materials of this study is shown in Figure 1. The electrochemical cell was made of jacketed PYREX* ** glass. Its temperature was maintained at 150 ~ by a Cole-Panner Model 1268-44 Circulator which continually pumped silicone fluid through the cell jacket. All electrodes were placed through a TEFLON t cap, and *PYREX is a trademark of Coming Glass Works, Co
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