Intergranular fracture by slip/grain boundary interaction
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I.
INTRODUCTION
O N E of the outstanding problems in materials science is to explain why certain intermetallic compounds are inherently strong but notoriously weak at grain boundaries (GBs). For instance, while both aluminum and nickel of face-centered cubic (fcc) structure are ductile, polycrystalline Ni3A1 ordered alloys of the L12 structure (fee-based) are brittle at room temperature, exhibiting intergranular fracture. It is therefore necessary to understand first, as a prerequisite, the physical mechanism responsible for intergranular fracture in Ni3A1 before discussing the ductilizing effect of small additions of boron, tl,2] Recent atomistic simulation studies have indicated that the cohesive energies of GBs are comparable for nickel and Ni3A1.[3,4] These results suggest that the intrinsic brittleness of polycrystalline Ni3A1 must be related not only to GB cohesive strength but also to the localized microplasticity at and near GBs. In order to investigate the role of slip/GB interaction in intergranular fracture, crystallography, dislocation reactions, stress analyses, and microcrack formation pertaining to a E = 9 tilt boundary in the LIE structure have been studied. [5,6] The most important conclusion of the recent w o r k [6] w a s the prediction of cleavage fracture and not intergranular fracture. The purpose of this paper is to extend the previous analysis to a generalized symmetric tilt boundary in both fcc metals and L12 ordered alloys. Geometric descriptions of dislocation absorption and transmission are given in Section II. The crystallographic and coincidence site lattice (CSL) relationships of a symmetric tilt boundary are discussed in Section III. In Section IV, the longrange elastic interaction between a dislocation and a GB is estimated. Microcrack initiation at the head of a singleended dislocation pileup is treated in Section V. Finally, M.H. YOO, Senior Research Staff Member, is with Oak Ridge National Laboratory, Oak Ridge, TN 37831-6115. A.H. KING, Professor of Materials Science, is with the Department of Materials Science and Engineering, State University of New York at Stony Brook, Stony Brook, NY 11794-2275. This paper is based on a presentation made in the symposium "Interface Science and Engineering" presented during the 1988 World Materials Congress and the TMS Fall Meeting, Chicago, IL, September 26-29, 1988, under the auspices of the ASM-MSD Surfaces and Interfaces Committee and the TMS Electronic Device Materials Committee. METALLURGICAL TRANSACTIONS A
in Section VI, a comparison of our results to those experimental data which exist currently is presented. II. GEOMETRY OF DISLOCATION REACTIONS The processes of dislocation absorption and transmission by GBs were discussed earlier by several investigators, t7,8,9] The geometric relationships involving these two processes are illustrated in Figures 1 and 2. The GB plane (HKL) and an active slip plane (hk/) intersect along the common axis, ~, at an angle ~b. The reference vector of [UVW]~ referred to crystal (1) is perpendicular to
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