Intergranular cracking under creep-fatigue deformation in lamellar TiAl alloy Young Sam Park a , Soo Woo Nam a, * ,1 , Sun Keun Hwang a,b,1 a Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 373-1 Kusong-dong, Yusong-gu, Taejon, 305-701, South Korea b Department of Metallurgical Engineering, Inha University, Yonghyun-dong, Nam-gu, Inchon, 402-751, South Korea Received 28 March 2001; accepted 19 May 2001 Abstract Total strain range controlled low-cycle fatigue tests (R=1, strain rate=410 3 /s) indicate that lamellar structured Ti– 46.6Al – 1.4Mn – 2Mo (at.%) alloy shows cyclic stability behavior regardless of test conditions. Fatigue life is drastically reduced with the application of tensile hold time and this reduction of fatigue life is understood to be due to the additional creep damage occurring during tensile hold time. Microstructural analysis for continuous fatigue tests indicates that surface crack is observed to be the main defect and the transgranular fracture mode is predominant. However, in creep-fatigue tests, the initiation of internal grain boundary cracks is remarkable and the intergranular fracture mode is predominant. This implies that the introduction of creep damage during tensile hold time is responsible for grain boundary weakness and intergranular cracking. Furthermore, grain boundary morphology observation and compositional analysis indicate that the grain boundary weakness and the intergranular cracking in the creep-fatigue test are induced by a 2 !g phase transformation at the grain boundary, which controls the creep-fatigue fracture behavior. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Titanium aluminide; Lamellar structure; Low-cycle fatigue; Tensile hold time; Fatigue crack; Phase transformation 1. Introduction The lamellar TiAl alloy has attracted a lot of research interest because of its high specific strength at elevated temperatures. Recent research trends indi- cate that studies of the alloy have been mainly concentrated on the mechanical properties, especially on tensile or creep properties [1]. For creep proper- ties, it has been reported that the rate-controlling process of creep deformation is a 2 !g phase trans- formation accompanying dislocation generation at the lamellar interface [2 – 4]. Furthermore, an intermedi- ate Ti 2 Al phase has been newly discovered during the phase transformation with a new atomic config- uration model for a 2 !Ti 2 Al!g phase transforma- tion process [5]. The lamellar TiAl alloy designed for light and high-temperature materials is generally exposed to low-cycle fatigue conditions. For the purposes of safety and performance improvement, it is necessary to investigate fatigue crack initiation and growth behavior under practical application conditions. How- ever, in the case of fatigue considerations, fatigue crack growth resistance and fatigue fracture mode 0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII:S0167-577X(01)00513-4 * Corresponding author. Tel.: +82-42-869-3318; fax: +82-42- 869-3310. E-mail address: namsw@cais.kaist.ac.kr (S.W. Nam). 1 Jointly Appointed at the Center for the Advanced Aerospace Materials. www.elsevier.com/locate/matlet April 2002 Materials Letters 53 (2002) 392 – 399