Materials Science and Engineering A 507 (2009) 58–60 Contents lists available at ScienceDirect Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea Part I: Anisotropy of cracking from oxygen-induced dynamic embrittlement in bicrystals of IN718 William M. Kane a,b , U. Krupp a,c , C.J. McMahon Jr. a,∗ a Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104, United States b Exponent Failure Analysis Associates, 3401 Market Street, Philadelphia, PA 19104, United States c Fachhochschule Osnabrück, P.O. Box 19 40, D-49009 Osnabrück, Germany article info Article history: Received 20 June 2008 Received in revised form 24 November 2008 Accepted 2 December 2008 Keywords: Dynamic embrittlement IN718 Bicrystals Intergranular fracture High-temperature brittle fracture Superalloys abstract It is shown that cracking from oxygen-induced dynamic embrittlement in bicrystals of the nickel-base superalloy IN718 having symmetrical ˙ =5 (013) tilt boundaries is considerably more rapid in the fast- diffusion direction parallel to the tilt axis than in the direction perpendicular to this axis. This lends support to the model that envisions the process as caused by diffusive penetration of grain boundaries ahead of a sharp crack by a surface-adsorbed embrittling element. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Dynamic embrittlement is the diffusion-controlled intergran- ular cracking that can occur in a high-strength alloy that has a cohesion-lowering element adsorbed on a free surface. It was first characterized in studies of oxygen-induced cracking of a Ni-Al inter- metallic [1] and of sulfur-induced cracking of a pressure-vessel steel [2]. One characteristic that is generally noted in polycrystals is the disparity in the susceptibility to cracking among various grain boundaries. This was noted particularly in a study involving the nickel-base alloy IN718 [3]. This behavior was also noted in the highly susceptible Cu–8%Sn alloy [4]. A later study of cracking of bicrystals of this alloy having symmetrical ˙ =5 (013) tilt boundaries showed that it occurred much more rapidly along the tilt axis (i.e., the fast-diffusion direc- tion) than perpendicular to the tilt axis [5]. This supported the proposition that the phenomenon involves diffusive penetration by the surface element into the core of a sharp crack and that the atomic configuration along the grain boundary would play a decisive role. ∗ Corresponding author. E-mail addresses: wkane@exponent.com (W.M. Kane), cmcmahon@lrsm.upenn.edu (C.J. McMahon Jr.). The Cu–Sn alloy study involved the matrix element, tin, which is known to be surface active [4] and is an embrittling element in steels [6], for example. It has also been shown to segregate to grain boundaries in the Cu–Sn alloy during elevated-temperature aging [7]. In contrast, the oxygen-induced cracking of nickel-base alloys involves an element from the vapor phase extraneous to the alloy that is not to be found ab initio in the grain boundaries. The present study was carried out to determine what kind of cracking anisotropy would be found in the commonly used alloy IN718 in experiments similar to the previous ones on the Cu–Sn alloy. This first part of a two-part paper serves as the introduction to both and as the basis for understanding the anisotropy of the cracking process in polycrystalline material, which is the subject of Part II. The same specimen preparation for the bicrystals is used in Part II, and thus Part I also serves to explain that. The ˙ =5 (013) tilt boundary was again selected for this pur- pose because of the great disparity in self-diffusivity in the parallel and perpendicular directions found previously in this boundary in another FCC material, silver. The calculated [8] atomic struc- ture of this boundary in an FCC lattice is shown in Fig. 1(a), and the diffusion results of Mishin and Herzig [9] are shown in Fig. 1(b). The presence of “diffusion pipes” along the tilt axis is readily apparent, and the diffusivity along these “pipes” is about double that in the perpendicular direction for self-diffusion in silver. 0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.12.002