Seed Preparation and Orientation Control of PST Crystals of Ti-47Al Alloy
JIN Hao1,2,JIA Qing1,LIU Ronghua1,XIAN Quangang1,CUI Yuyou1,XU Dongsheng1,YANG Rui1()
1. Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 2. University of Chinese Academy of Sciences, Beijing 100049, China
Cite this article:
JIN Hao, JIA Qing, LIU Ronghua, XIAN Quangang, CUI Yuyou, XU Dongsheng, YANG Rui. Seed Preparation and Orientation Control of PST Crystals of Ti-47Al Alloy. Acta Metall Sin, 2019, 55(12): 1519-1526.
Button ingots were prepared by arc melting and feed bars for directional solidification were prepared by optimized drop casting technique. The Ti-43Al-3Si directionally solidified bars with lamellar boundaries perpendicular to the growth direction were prepared at the growth rate of 180 mm/h in an optical floating zone furnace. Cylindrical sections were cut from these perpendicular lamellae with appropriate direction and then fixed on polycrystalline TiAl bars by mechanical setting method to serve as the initial seeds. The ultimate Ti-43Al-3Si seeds with parallel lamellar microstructure were successfully prepared from these initial seeds at the growth rate of 5 mm/h. To avoid the nucleation of stray grains, drop-cast bars with the shape of frustum of a cone were used for the preparation of ultimate seeds. At the growth rates of 5 and 180 mm/h, the primary phase of Ti-43Al-3Si alloy was always the α phase. Polysynthetically twinned (PST) crystals of Ti-47Al alloy were obtained from the Ti-43Al-3Si ultimate seeds and the seeding process was studied by microscopic analysis. Lamellar microstructure of the seed kept stable and recrystallization of the seed was not found. Lamellar orientation of Ti-47Al PST crystals was successfully controlled by the ultimate Ti-43Al-3Si seed.
Fig.1 Schematics of drop casting (a) and crystal growth in an optical floating zone furnace (b), and dimensions of the drop-cast bar with the shape of frustum (unit: mm) (c) (P1, P2—pressure; Tmax—temperature of melting zone)
Fig.2 Macrostructures of the longitudinal section of drop-cast bar processed at the pressure differential of 30 kPa (a) and 60 kPa (b)
Fig.3 Microstructures of the Ti-43Al-3Si DS bar solidified at the growth rate of 180 mm/h(a) the perpendicular lamellae (b) dendrites morphology in the quench zone
Fig.4 Cylindrical seed section from the Ti-43Al-3Si DS bar solidified at the growth rate of 180 mm/h (a), and connections of the seed section with the holding bar by argon arc welding (b) and mechanical setting (c)
Fig.5 Microstructures (a~c) and macrograph (d) of the ultimate seed solidified at the growth rate of 5 mm/h(a) bottom section of the seed (b) end part of the seed(c) dendrites morphology in the quench zone (d) photo of the ultimate seed bar
Fig.6 Ti-47Al PST crystals grown from the ultimate seed(a) macrostructure of the longitudinal section(b~g) microstructures of the seed after seeding process (b), the transition region (c), the stray grains in the transition region (d, e), the main body (f) and γ phase near the quench zone (g) corresponding to the boxes in Fig.6a
Fig.7 BSE image of the precipitate in the quench zone (a) and the EDS analyses (b)
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