Thermodynamic Modeling of the Al-Ti-V Ternary System

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I.

INTRODUCTION

THE Ti-based alloys with their high strength-toweight ratio, good toughness, and corrosion resistance have been extensively used in aerospace engineering and processing equipment. The compound TiAl typically has a high-temperature property with relatively low ductility and formability;[1,2] therefore, the addition of a third element, vanadium, is needed to effectively improve the room-temperature ductility of TiAl and Ti3Al intermetallic alloys.[3] In addition, the most widely used titanium alloy in the world is Ti6Al4V. As early as the 1950s, the Al-Ti-V system was investigated,[4–6] and the previous studies were mainly focused on the Ti-rich corner.[5,7] Today, several thermodynamic descriptions of the Al-Ti-V system are available.[8–11] For example, Hayes[8] assessed the phase equilibria of the Al-Ti-V ternary system in 1995, but there was disputation about the phase relationship of the Al-Ti-V ternary system at high temperature, and the thermodynamic database was not published. Further thermodynamic evaluation was also done by Zhang[9] in 1997; afterward, the sub-binary (Al-Ti, Ti-V, and Al-V) parameters were updated, and there are some differences between these[8,9] and the latest ones. Recently, the thermodynamic optimization of the ordered B2 phase in the Al-Ti-V ternary systems was carried out by Wang et al.,[10] and Kostov and Zˇivkovic´[11] calculated the enthalpies of mixing of the liquid phase and the integral excess Gibbs energy of the Al-Ti-V system at 2200 K (1927 C). The purpose of this study is to obtain a thermodynamic description for the Al-Ti-V ternary system. First, XIONGGANG LU and CHONGHE LI, Professors, are with the Shanghai Key Laboratory of Modern Metallurgy & Materials Processing, Shanghai University, Shanghai 200072, P.R. China, and also with the Shanghai Engineering Technology Research Center of Special Casting, Shanghai, P.R. China. Contact e-mail: chli@staff.shu.edu.cn NA GUI and AITAO QIU, Master Candidates, and GUANGXIN WU, Associate Professor, are with the Shanghai Key Laboratory of Modern Metallurgy & Materials Processing, Shanghai University. Manuscript submitted August 12, 2013. Article published online May 14, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A

the available thermodynamic data for three binary systems (Al-Ti, Ti-V, and Al-V)[12–14] are reviewed. Then, the thermodynamic parameters of the Al-Ti-V system are optimized, including the ternary interaction parameters of bcc_A2, hcp, liquid, and some interaction parameters involving the third element (for example, the interaction between the elements Al and V in the TiAl phase). Finally, several phase diagrams from 1073 K to 1473 K (800 C to 1200 C) and the invariant reactions in this ternary system are calculated and compared to the experimental results.

II.

REVIEW OF LITERATURE INFORMATION

A. Binary Systems 1. Al-Ti system The Al-Ti system consists of liquid (L), hcp(a), bcc_A2(b), bcc_B2, fcc, Ti3Al(a2), TiAl(c), Ti3Al5, Ti2Al5, TiAl2, Ti5Al11, the high-temperature phase H_TiAl3 (D022, TiAl3 type),

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