Slip transfer and dislocation nucleation processes in multiphase ordered Ni-Fe-Al alloys
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I. INTRODUCTION
LOW ductility, fracture toughness, and impact resistance of NiAl at room temperature continue to be the major limitations of NiAl-based alloys for high-temperature structural applications.[1] Multiphase alloys containing an ordered b (B2) NiAl-based phase and a ductile g (fcc)-based or g 8(L12)-based second phase are known to exhibit good room-temperature ductility, independent of the processing conditions.[2–6] Although some investigators have cited processing-related[2] or extrinsic mechanisms[3] for the high ductility of these alloys, detailed study of the deformation behavior[4,7,8] has indicated that slip transfer from the ductile g or g 8 phase to the harder b phase is the primary mechanism responsible for ductility of these alloys. However, a complete understanding of the slip transfer process across interphase boundaries in these ordered alloys is needed to allow tailoring of interfaces which facilitate slip transfer and thereby enhance the ductility of intermetallic alloys. For slip transfer across grain boundaries, the detailed mechanisms and the criteria for predicting the slip system activated in a grain as a result of slip transfer from the adjoining grain are well documented.[9,10] The most common slip transfer mechanism is the one involving the bounday reaction: b1 5 b2 1 br , where b1 is the Burgers vector of the dislocation in grain 1, b2 is the Burgers vector of the dislocation in grain 2, and br is the Burgers vector of the residual dislocation left behind in the boundary.[9,10] The slip system activated in grain 2 is the one which satisfies the following three criteria:[10] (1) Geometric condition: The angle between the lines of intersection at the grain boundary of the incoming and outgoing slip planes should be minimized. In most cases, the active slip plane is determined by this condition.
A. MISRA, formerly Graduate Student, Department of Materials Science and Engineering, University of Michigan, is Limited-Term Technical Staff Member, Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545. R. GIBALA, Frances E. Van Vlack Professor, is with the Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109-2136. Manuscript submitted January 9, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
(2) Resolved shear stress (RSS) condition: The resolved shear stress acting on the outgoing slip system from the piled-up dislocations should be maximized. (3) Residual Grain Boundary Dislocation Condition: The magnitude of the Burgers vector of the residual dislocation left at the grain boundary should be minimized. This and the resolved shear stress condition usually determine the active slip direction.[9,10] Little literature exists on the details of slip transfer across interphase boundaries. Early work on the deformation behavior of a(fcc)/b(bcc) brass bicrystals has shown that slip in the harder b phase can nucleate at stresses up to 40 pct lower when the two phases have a favorable orientation relationship, i
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