© 2013 Siemens Energy, Inc. All rights reserved. Orientation and temperature dependences on fatigue crack growth (FCG) behavior of a Ni-base directionally solidified superalloy Yangyang Zhang1, Huiji Shi1,*, Jialin Gu2, Changpeng Li3, Kai Kadau4, Oliver Luesebrink5 (1Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China) (2Department of Material Science, Tsinghua University, Beijing 100084, China) (3Corporate Technology, Siemens Ltd. China, Beijing 100102, China) (4Siemens Energy Inc., Charlotte, USA) (5Siemens AG Energy, Mulheim an der Ruhr, Germany) * Corresponding author. Email: shihj@mail.tsinghua.edu.cn Abstract Fatigue crack growth (FCG) behaviors of a widely-used nickel-based directionally solidified (DS) superalloy were investigated. Direct-current potential drop method was employed to capture the crack length according to ASTM E647. Standard compact tension (CT) specimens in longitudinal, transverse and diagonal directions were cast and tested at T0 and T0+250 0C to reveal the orientation and temperature dependence. Moreover, the post-test fractography were observed through SEM and OM to understanding the underlying mechanism responsible for the fracture modes. Results suggested that cracks in all three orientations propagated transgranularly, no obvious differences were found between each other, and thus, the orientation dependence appeared to be very weak, all da/dN-DK curves in three orientations fell into one narrow band. However, temperatures showed significant dependence in diagonal directions, while in transverse direction the dependence became weak and even none in longitudinal orientation. Finally, methods for characterizing the FCG behaviors in different orientations and at both temperatures were proposed, which were able to explain the orientation and temperature dependences. Key words Fatigue crack growth, directionally solidification, nickel-based superalloy 1. Introduction Superalloys, either in the conventionally-cast (CC) or directionally solidified (DS) forms may contain porosity and shrinkage cracks, so that the life of rotating components, such as turbine blades, may be limited by crack propagation from these defects. It is therefore of interest to determine the resistance of the oriented grain structure, produced by DS, to the propagation of fatigue cracks [1]. Although lots of research have been focused on the fatigue crack growth behaviors of superalloys, few have shed light on those behaviors of directionally solidified superalloys[1-6]. In 1976, Scarlin [1] investigated the fatigue crack growth behaviors of nickel-based IN738 DS, and high cycle fatigue crack growth rates had been measured at room and high temperatures and for crack propagation both parallel and perpendicular to the solidification directions. However, comparable crack growth rate seemed to be found in his results. Okazaki et al [2] conducted low cycle fatigue tests to study the stage-I short crack growth behavior RENE80+Hf and CM247LC DS at 6000C. Later, Highsmith et al [4, 6] and Yoon et al [5] studied the FCG behaviors of nickel-base GTD-111 DS, both in longitudinal and transverse directions, where the stress ratio effects and temperature effects were focused on, respectively. For DS material, it is seen that only crack growth in
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