Crystallography of Bcc/T 1 /T 2 Three-Phase Microstructure in the Directionally Solidified Mo-Nb-Si-B Alloy
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Crystallography of Bcc/T1/T2 Three-Phase Microstructure in the Directionally Solidified Mo-Nb-Si-B Alloy Naoki Takata 1, Nobuaki Sekido 2, Masao Takeyama 1 and John H. Perepezko 3 1
Dept. Metallurgy and Ceramics Science, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo, 152-8552, Japan 2 Research Center for Strategic Materials, National Institute for Materials Science, 1-2-1 Sengen Tsukuba, Ibaraki, 305-0047, Japan 3 Dept. Materials Science and Engineering, University of Wisconsin-Madison, 1509 University Avenue, Madison, WI 53706, U.S.A. ABSTRACT In the present study, the crystallographic features of bcc/T1/T2 three-phase microstructure in a directionally solidified Mo–32.2Nb–19.5Si–4.7B (at.%) alloy have been examined by electron back-scattering diffraction (EBSD) analysis. The alloy was directionally solidified using an optical floating zone (OFZ) furnace in a flowing Ar gas atmosphere at a constant growth rate of 10 mm/hour. The microstructure of the directionally solidified alloy is characterized by an elongated T2 phase surrounded by inclusions of bcc and T1 phases with an interwoven morphology. The T2 grains are faceted on the (001) planes and elongated along the [110] ― direction. The T2 phase has an orientation relationship of (001)T2 // (011)bcc and [130]T2 // [21 1]bcc with the bcc phase, whereas any particular orientation relationships of T1 phase with bcc and T2 phases have not been found. These crystallographic features of bcc/T1/T2 three-phase microstructure suggest that the primary T2 phase crystallizes and grows along the [110] direction in liquid phase, followed by nucleation of the bcc phase on the interface between T2 and liquid phases, resulting in bcc/T1 two-phase eutectic reaction surrounding the elongated T2 phase. INTRODUCTION The commonly used Ni-based alloys applied in jet engines and stationary gas turbines are exposed at extremely high temperatures. To further increase their performance and efficiency, the gas inlet temperature needs to be increased. The highly refined cooling and coating schemes to prevent the Ni-based alloys from melting in the current high-pressure turbine can reduce the surface temperature below 1100 oC, but they incur a penalty as a loss of efficiency [1,2]. In order to advance beyond Ni-based systems, alternative materials such as refractory metals are needed to significantly improve the performance of jet engines and gas turbines. The candidate materials are Mo and Nb based alloys. In particular, the Mo–Si–B ternary alloys consisting of α-Mo, Mo3Si (A15), and the Mo5SiB2 (T2) phases have been widely studied [3-11]. The addition of Nb into the Mo–Si–B alloys can destabilize the A15 phase, resulting in the formation of a three phase region of bcc + T2 + Mo5Si3 (T1) [7]. The solidification microstructure exhibits the organized interwoven morphology [2, 7], which suggests a three-phase eutectic decomposition of L → bcc + T1 + T2 in the Mo–Nb–Si–B quaternary system. It is essential to understand the threephase eutectic reaction in order to control the
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