Investigation of B2 and Related Phases in the Ti-Al-Nb Ternary System

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INVESTIGATION OF B2 AND RELATED PHASES IN THE Ti-Al-Nb TERNARY SYSTEM LEONID A. BENDERSKY AND WILLIAM J. BOETTINGER Metallurgy Division, National Institute of Gaithersburg, MD 20899, USA

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ABSTRACT Alloy compositions around Ti 2 AINb were studied to establish phase equilibria and transformations during cooling from 1100°C and 1400°C. In addition to general results obtained on a wide range of compositions, which include evidence for a broad B2 phase field, transformation of BCC Ti 4 Al 3 Nb to a phase with an omega-type structure is reported. Detailed analysis indicates that this phase has the B8 2 structure after annealing at 700°C. INTRODUCTION Titanium aluminides (Ti 3 AI and TiAl) with Nb additions have received considerable attention as potential low density, high strength and creepresistant materials for high temperature application. Because the presence of BCC and BCC-related phases in these alloys seems to have an important role in their mechanical properties, in-depth studies of the BCC phase field are necessary. Surprisingly, the phase equilibria in this ternary system are poorly understood. The only detailed studies are for alloys of Ti 3 AI with Nb substitution for Ti up to 20 at% by Strychor et al.[1] and Banerjee et al. [2,3]. Both show the existence of a high temperature BCC or B2 phase which generally transforms during cooling to an ordered hexagonal DO1 9 phase. The variety of observed microstructures reported shows extreme sensitivity of the alloys to the details of heat treatment. Probably because of this, the two studies have reported quite different results. Banerjee et al.[3] found that upon slow cooling both B2 and D0 1 9 phases transform partially to an orthorhombic phase with symmetxy Cmcm. They suggest a structural relationship between this phase and the hexagonal DO19 phase. No such phase was observed by Strychor et al. [1]. Instead, an ordered omega phase was observed. The phase precipitates from the retained B2 phase during low temperature annealing. Arguments about further transformation of the omega phase to a B8 2 (Zr 2 Al) phase were also presented. Recently we have begun an experimental study on phase equilibria and phase transformations in the ternary Ti-Al-Nb system. In order to cover a wide range of compositions, five different alloys including and surrounding Ti 2 AINb were chosen for study. In this paper we present preliminary results on possible equilibrium phases at 1400°C and 1100°C for these alloys. The focus of the paper is on the alloy with the Ti 4 AI 3 Nb composition for which a complex transformation of BCC to an omega-type structure was observed.

EXPERIMENTAL PROCEDURE Alloys with compositions TiAl 2 Nb(alloy #3), Ti7AINb(#30), TiAAI 3 Nb(#4), Ti 3 AI 2 Nb3 (#2) and Ti 4 AINb 3 (#l) were prepared by arc melting. In excess of ten remelts were necessary to ensure mixing of the components. Cast buttons were homogenized at 1400°C for three hours in a furnace under 2/3 atm gettered Ar. It was found necessary to rest the samples on a Y2 0 3 coat