Experimental assessment of crack-tip dislocation emission models for an Al 67 Cr 8 Ti 25 intermetallic alloy
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INTRODUCTION
RICE and Thomson (RT) tu sought to account for cleavage fracture in materials with low or moderate critical resolved shear stress (CRSS) by modeling the energetics of dislocation emission from atomically sharp cracks. The RT dislocation-emission model shows that an energy barrier may exist for dislocation emission from a crack tip, depending on several material parameters and on the slip system geometry relative to the crack, but independent of CRSS. The dislocation-emission model led to the formulation of a fracture-mode criterion for mode I loading, as follows. When the remote stress satisfies the Griffith criterion for fracture~ the model predicts whether dislocation emission from a crack tip is energetically favorable. If dislocation emission is not energetically favorable, sharp cracks are not blunted and the material is predicted to fail by cleavage fracture. If dislocation emission is energetically favorable, the spontaneous emission of dislocations occurs prior to reaching the Griffith criterion, and the crack is blunted. The blunted crack remains trapped at its original position until the external stress is increased sufficiently to promote cracktip damage processes that eventually lead to fracture by other mechanisms. More recent refinements of the model introduce effects associated with intercrystalline fracture, loading mode, and other factors, t2-5~ The RT model W.G. MENG, Graduate Student, and J.A. WERT, Professor, are with the Department of Materials Science and Engineering, University of Virginia, Chalottesville, VA 22903. M.D. VAUDIN, Research Scientist, is with the Ceramics Division, NIST, Gaithersburg, MD 20899. M.F. BARTHOLOMEUSZ, formerly Graduate Student with the Department of Materials Science and Engineering, University of Virginia, is Research Metallurgist with Reynolds Metals Co., Richmond, VA 23219. Manuscript submitted June 3, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
predictions of fracture mode correlate well with experimentally observed fracture modes for a variety of metals, ceramics, and ionic compounds. IL6J Turner, Powers, and Wert (TPW) 171 first applied the RT model to intermetallic alloys. They showed that the RT model is consistent with cleavage fracture of A167NisTi25, a trialuminide intermetallic alloy in which the Ni addition changes the crystal structure of A13Ti from D022 to L12. Room-temperature plastic deformation of A167NisTi25 in compression was shown to occur by the glide of undissociated (150){111} superlattice dislocations, tT,sl which permitted direct application of the RT model. Other intermetallic alloys with the L12 crystal structure deform by the glide of dissociated superlattice dislocations, t9-131complicating application of the original RT model, which considers crack-tip emission of single perfect dislocations. Turner, Powers, and Wert applied the RT model to a variety of intermetallic alloys with the L12 crystal structure by considering two limiting cases. If the antiphase boundary (APB) or superlattice intrinsic stacking fault (
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