Effects of lamellar spacing on microstructural stability and creep properties in ß-solidifying ?-TiAl alloy by directional solidification

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Vol. 711; p. 508
• Main Authors: Wang, Qi, Chen, Ruirun, Yang, Yaohua, Wu, Shiping, Guo, Jingjie, Ding, Hongsheng, Su, Yanqing, Fu, Hengzhi
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier BV 10.01.2018
• Subjects: Alloys; Colonies; Creep (materials); Creep rate; Creep tests; Crucibles; Directional solidification; Dislocations; Heat treatment; Intermetallic compounds; Lamellar structure; Microstructure; Slip resistance; Stability; Stress concentration; Titanium aluminides; Titanium base alloys
• ISSN: 0921-5093; 1873-4936

Ti44Al6Nb1Cr (at%) alloys with different lamellar spacing were prepared by cold crucible directional solidification. Creep tests were conducted at 750 °C under 260 and 300 MPa, and the microstructure before and after creep testing were observed and analyzed. The results show that the prepared TiAl alloys have similar macro/microstructure except for lamellar spacing, which are different from the heat-treated TiAl alloys with obviously changed macro/microstructure. The refinement of lamellar spacing can improve creep properties, especially the steady-state of fine lamellar alloy lasted for more than 600 h with creep rate at 7.3 x 10-9 S-1. The improvement of creep properties by refined lamellar spacing are revealed as following two reasons. (1) Fine lamellar spacing improves the stability of γ lamellae and increases the resistance for dislocation slip in γ lamellae. (2) It disperses stress concentration and delays the formation of globular structure at colony boundary. Moreover, the alternating β and γ laths in a-segregation zone can improve microstructural stability during creep. In β-solidifying γ-TiAl alloy, the local stress concentration on β-segregation at colony boundary promotes colony boundary sliding and the formation of void with globular structure, which further accelerates the creep failure.