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

Ti44Al6Nb1Cr (at%) alloys with different lamellar spacing were prepared by cold crucible directional solidification. Creep tests were conducted at 750 degrees 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 gamma lamellae and increases the resistance for dislocation slip in gamma lamellae. (2) It disperses stress concentration and delays the formation of globular structure at colony boundary. Moreover, the alternating beta and gamma laths in a-segregation zone can improve microstructural stability during creep. In beta-solidifying gamma-TiAI alloy, the local stress concentration on beta-segregation at colony boundary promotes colony boundary sliding and the formation of void with globular structure, which further accelerates the creep failure.