Solidification kinetics and metastable phase formation in binary Ti-Al

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I.

INTRODUCTION

RECENT investigations indicate that several solidification phases' are thermodynamically accessible with relatively small bulk undercoolings in near-equiatomic alloys of binary Ti-A1.t1"21Thus, the Ti-AI system is excellent for studying the effects of bulk undercooling on the primary solidification phase selection process. Since the competing phases have structures ranging from nonclose-packed (/3, bcc) to close-packed (a, hcp) to ordered (3', Llo; o~2, D019), the Ti-A1 system is also ideal for studying the effects of crystal structure on solidification kinetics as a function of bulk undercooling. In the current study, the above features are explored in detail in the near-equiatomic portion of the binary system. II.

BACKGROUND

Figure 1 contains the present authors' recent revision to the high-temperature, near-equiatomic portion of the Ti-A1 phase diagram, t3] This amendment is based on revised liquidus temperatures (Tliq) determined by optical pyrometry and the concurrent measurement of emissivity with rotating analyzer ellipsometry. The liquidus temperatures have an uncertainty of about + 10 K. They are in agreement with the earlier measurements of Ogden et al. t41 and Kornilov et al. tSj and are used throughout the present article. In near-equiatomic Ti-AI, it is often difficult to distinguish primary solidification of/3 from that of a. Solid-state transformations are unavoidable, and roomtemperature microstructures resulting from/3 solidification are generally quite similar to those resulting from a C.D. ANDERSON, formerly Doctoral Student, Vanderbilt University, is Research Scientist, Intersonics, Incorporated, Northbrook, IL 60062-1818. W.H. HOFMEISTER, Research Associate Professor, and R.J. BAYUZICK, Professor of Materials Science and Engineering, are with Vanderbilt University, Nashville, TN 37235. Manuscript submitted November 19, 1991. METALLURGICAL TRANSACTIONS A

solidification,t1'6-9] Consequently, other information is necessary to eliminate this ambiguity. High-speed thermal data obtained during solidification provide one possible solution. A.

Previous Undercooling Study

Valencia et al. correlated recalescence events in thermal data from electromagnetic levitation experiments with phase transformations in near-equiatomic binary Ti-AI samples that were bulk undercooled at the onset of solidification,tSl Beta was observed to be the primary solidification structure for all undercoolings in Ti-45 at. pct A1 and for undercoolings greater than 85 K in Ti-50 at. pct A1. Alpha was found to be the primary solidification structure in Ti-50 at. pct AI samples undercooled less than 85 K and in Ti-55 at. pet AI samples solidified at all undercoolings. The maximum undercoolings observed were 286 K in Ti-50 at. pet AI and 348 K in Ti-55 at. pct A1. This work provided useful information, but there were some limitations inherent in the temperature measurement instrumentation. The pyrometry used in the Valencia study provided only a 25-ms time resolution. This is insufficient for resolving adjoini

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