Modeling of grain-boundary effects and intergranular and transgranular failure in polycrystalline intermetallics
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UMINIDE- and silicide-based ordered intermetallics generally have low densities, high melting points, good thermal conductivities, and superb high-temperature strengths. However, material failure in these ordered intermetallics (for example, Su et al.,[1] Pope and Liu,[2] and George et al.[3]) can result from intrinsic factors, such as poor grainboundary (GB) cohesion arising from the large valency and electronegativity between the different intermetallic phases, difficulty of slip transmission across GBs, dislocation mobility in the GB region, GB migration, or extrinsic factors, such as moisture or the presence of GB additives (George et al.[3]). The intrinsic characteristics of GB structure, orientation, and distribution are essential microstructural factors in characterizing ductile and brittle failure initiation and transgranular and intergranular failure paths and modes in ordered intermetallics. Most polycrystalline analyses do not account for the effects of GBs on the evolution of dislocation densities between collective grains. However, as experimental investigations by Lynch et al.,[4] for disordered intermetallics, and Kimura and Pope,[5] for ordered intermetallics, have clearly shown for polycrystalline intermetallics, it is essential to consider the microstructurally altering effects of GBs, and to understand how dislocation-density transmission and blockage can affect the nucleation and evolution of large strain deformation and failure modes. Dislocation-density pileups at GBs and dislocation-density emissions in crack-tip regions can clearly affect crack-growth directions and failure paths. Therefore, it is crucial if intermetallics are to T. KAMEDA, Assistant Professor, is with the Institute of Engineering Mechanics and Systems, University of Tsukuba, Tsukuba, Ibaraki, 3058573, Japan. M.A. ZIKRY, Professor, is with the Department of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, NC 27695. Contact e-mail: [email protected] A.M. RAJENDRAN, Adjunct Full Professor, is with North Carolina State University, and Senior Scientist, with the Army Research Office, RTP, NC. Manuscript submitted May 12, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
be considered for operational service in harsh environmental conditions that failure paths be accurately predicted as a function of microstructural features, such as GB effects and dislocation-density evolution both within grain interiors and across GB interfacial regions. Hence, the major objective of this study is to gain a more detailed understanding and characterization of the interrelated effects of GB orientation, dislocation-density evolution, and stress distribution in the crack-tip region on intergranular and transgranular crack growth and material failure in nickel aluminide polycrystalline aggregates. In an earlier study by Kameda and Zikry[6] and Zikry and Kameda,[7] an inelastic three-dimensional dislocation-densitybased constitutive formulation and a computational scheme were introduced and used for an investigation of coinci
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