Microstructure, Mechanical Properties, and Crack Propagation Behavior in High-Nb TiAl Alloys by Directional Solidificati
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DUCTION
TITANIUM aluminide alloys based on the intermetallic gamma phase are widely considered to be a replacement for Ni-based superalloys above 600 °C due to their low density and high specific strength.[1–3] Compared with conventional TiAl alloys, high Nb-containing TiAl alloys exhibit superior high-temperature strength and oxidation resistance.[4,5] However, Nb as a b-stabilizer usually induces the formation of the b(B2) phase in the matrix at room temperature, which is harmful to the room-temperature tensile properties and high-temperature creep properties of the material.[5,6] In addition, the B2 phase in the matrix can result in stress concentration, which induces the formation of cracks in the bulky B2 phase that propagate along its boundaries.[7,8] Therefore, it is essential to restrict the formation of the B2 phase in the as-cast high Nb TiAl alloy. Chen et al.[9] reported that three a2 + c + B2 phases can form in lamellar structures and that two B2 + c phases
QI WANG, RUIRUN CHEN, XUE GONG, JINGJIE GUO, YANQING SU, HONGSHENG DING, and HENGZHI FU are with the School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China. Contact e-mail: ruirunchen@ hit.edu.cn Manuscript submitted December 17, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
can form at the colony boundary during solidification when the b-stabilizing elemental content in the local segregation region exceeds a threshold value. According to the ternary phase diagrams of Ti-Al-Nb,[9,10] the region of the three a2 + c + B2 phases can form in the low-Al region. This finding indicates that both high Nb and high Al contents in the TiAl alloy will restrict the formation of the B2 phase. In addition, the high Al content can improve the room-temperature elongation in fully lamellar TiAl alloys.[10] Therefore, the combination of high Al and high Nb contents in the as-cast TiAl alloys restricts the formation of the B2 phase and maintains a balance between the room-temperature tensile properties and the high-temperature creep properties. The microstructure of TiAl alloys has a significant influence on the room-temperature tensile properties and high-temperature creep properties of the material, and the microstructure strongly depends on the solidification technique. Yamaguchi and Inui have reported the directional solidification (DS) technique with TiAl alloys in detail.[11–13] A homogeneous and stable microstructure showing significant anisotropy in the mechanical properties can be obtained using the DS technique.[14] However, the use of ceramic crucibles usually leads to the formation of contamination due to ceramic inclusion in the molten TiAl alloys during the DS process due to the high melting and high activity of
TiAl alloys. This effect significantly reduces the room-temperature tensile properties and high-temperature creep properties of the material.[15,16] The electromagnetic cold crucible directional solidification (CCDS) technique can prevent the formation of contaminants and can be used to prepare industrial siz
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