Effect of heat treatment on microstructure and mechanical properties of cast and directionally solidified high-Nb contai
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Two kinds of heavy-alloying b-type TiAl-based alloys Ti44Al6Nb1.0Cr2.0V (A1) and Ti44Al6Nb1.0Cr2.0V0.15Y0.1B (A2) are newly designed. They are prepared by vacuum consumable melting (VCM) and cold crucible directional solidification (CCDS). Via the theoretical analysis and tentative experiment, five alternative heat treatment (HT) schedules are proposed and studied that the corresponding microstructure and room temperature (RT) tensile property are investigated, and finally the optimized HT schedules are acquired. After HT5 (heat preservation in b phase region and at 1290 °C, and then ladder cooling), A2 alloy cast by VCM exhibits a better tensile property with average elongation of 1.20%. For the two CCDS ingots, after HT3 (mainly annealing at 1280 °C), B2 phase and (B2 1 c) blocky morphology are reduced, the columnar grains and small angle lamellas are reserved, and the tensile property also has a moderate improvement.
I. INTRODUCTION
TiAl-based alloys are the promising candidate to replace the widely used Ni-based superalloys partly at 600–800 °C,1–3 the service temperature can be much higher by proper alloying or post treatment.4,5 However, due to high melting point and high activity, its preparation is generally very difficult. After years of efforts, different kinds of processing methods are applied successfully.6,7 For metal materials, casting is always one of the most direct and effective processing methods. Compared with thermo–mechanical treatment,8–10 no deformation structure is brought by casting, which is benefit for creep resistance and endurance property at high temperature. Furthermore, contamination and defect in the cast can be well-controlled nowadays and the cost is lower. Cold crucible directional solidification (CCDS)11,12 is an effective technique to prepare TiAlbased alloy ingots with industry size, directionally solidified (DS) macrostructure and better axial mechanical properties, but without contamination. According to the service condition, alloying is an effective method to improve a certain property by adding specific elements, and nowadays “lower Al content and higher b-stabilizers” is the main development tendency. Chen13,14 has proposed “High-Nb TiAl-based alloys,” because of their higher service temperature, better mechanical properties and excellent deformation processing capacity, more and more attention has been attracted. Contributing Editor: Yang-T. Cheng a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.298 J. Mater. Res., Vol. 30, No. 21, Nov 13, 2015
However, a tough problem that lots of B2 phase in this kind of alloy induced by more addition of Nb has always not been well-treated. As well known, B2 phase is a kind of hard phase at room temperature (RT), but the soft phase at high temperature, and too much B2 phase will be harmful for the mechanical properties.15 Heat treatment (HT) is an effective method to solve this problem. Appropriate HT can eliminate, reduce or disperse B2 phase, and then improve the properties. Certainly, HT ca
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