Effect of orientation on crystallographic cracking in notched nickel-base superalloy single crystal subjected to far-fie
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I.
INTRODUCTION
S I N G L E crystals of nickel-based superalloys are being widely used in various commercial applications like gas turbine engines, etc., where the elimination of grain boundaries has substantially improved their creep lives. The study of mechanical properties of these alloys has to a large extent been limited to stress rupture, creep, tt,2] cyclic fatigue, 13-6] and tensile fatigue crack growth, t71 Crystallographic stage I tensile fatigue has been commonly observed in nickel-base superalloys at room temperature, t6,8] In addition, low-cycle fatigue studies on single-crystal MAR-M20019't~ have shown that the initial fatigue crack terminates in cleavage-type failure along {11 1} planes. The crack path is along planes which are subjected to large shear stresses and shear strains. There are several models proposed to explain the mechanism of fatigue crack growth. A slip-band decohesion m o d e P j deals with the reversed motion of dislocations on a series of parallel planes forming an array of dipoles in the plastic zone. The tensile stress at the crack tip due to the dipoles acts across the slip plane, and this stress, combined with the applied stress, is believed to cause localized separation of the atomic bonds across the slip plane, leading to the formation of a crack. This model is characterized by a cleavage like fracture surface. However, evidence of ductile tear ridges on the fracture surface 15}indicates that the fracture process is not an atomistically brittle process but involves deformation by shear stresses. These observations suggest the importance of both shear and normal stresses on the fracture mechanism. In their model, Koss and Chan [81 have explained the tensile stage I crack growth and the associated cleavage P.B. ASWATH, Assistant Professor, is with the Mechanical and Aerospace Engineering Department and the Materials Science and Engineering Program, the University of Texas at Arlington, Arlington, TX 76019. Manuscript submitted November 18, 1992. METALLURGICAL AND MATERIALS TRANSACTIONS A
like fracture surface as a consequence of crack-tip plasticity, in particular on the basis of the inability of coplanar slip to relax the normal stress components of the crack-tip field. This confines the crack to the primary slip plane. Tensile fatigue crack propagation along slip bands also was examined by Tanaka and Mura. t~] They showed that crack propagation was a repetition of microcrack nucleation within slip bands of the cracks. Studies 171 of tensile crack growth in MAR-M200 single crystals under either unidirectional or multiaxial cyclic tensile loads suggest that cracks can grow by simultaneous cracking on two {111}-type planes leading to the formation of ridges on the fracture surface. Despite the wealth of information on tensile fatigue crack growth in single crystals of nickel-based superalloys, little is known about their behavior in cyclic compression. The presence of a defect or a notch serves as a stress concentration leading to a localized increase in stresses and strains.
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