Effect of lamellar orientation on cracking behavior of the α2/γ lamellar single crystals made by directionally solidified has been examined, by means of in-situ three-point bend test in SEM. The single crystals were grown by optical floating zone furnace. As-grown crystal exhibits fully lamellar microstructure, with 20 % of volume fraction of α2 phase. The average thickness of α2 plate is about 200 nm, whereas that of the γ plate shows two peaks: one about 100 nm and the other 1000 nm. Rectangular single notched specimens with a size of 30 mm in length, 4 mm in height, 2 mm in thickness were cut from the crystals. The notch was made by electro discharged machining to a half of the thickness. The lamellar orientation was defined by two-angle combination (θ, λ) with respect to the notch direction, a parallel kink angle (θ) and a twist angle (λ). The load-displacement curve revealed that the specimen with (0, 0) orientation deforms elastically, followed by catastrophic failure by cracking along an α2/γ interface at maximum load of 16 N. The stress required to delaminate the interface made the “crack opening mode”(Mode I) can be calculated as τc(0,0) = 51 MPa. With increasing the θ up to 45°, the cracking behavior of the specimen with (θ, 0) orientation is similar to that of specimen with (0, 0) orientation, although the maximum load increases. In contract, with increasing the λ up to 45°, the specimen with (0, λ) orientation shows work hardening behavior even after initiation of the primary crack along an α2/γ lamellar interface, and many slip lines across the lamellae become visible. After reaching maximum, the load drops gently to failure with secondary crack bridging. In case of a specimen with (0, 30) orientation, τc(0,30) becomes 513 MPa, an order of magnitude higher than that of the (0, 0) specimen. The big increase in τc is caused by plastic deformation due to an introduction of “parallel shear mode”(Mode III). The specimen with (θ, λ) orientation shows the work hardening behavior, but the maximum load decreases with increasing θ due to the activation of “forward shear mode”(Mode II). Thus, within 45°, increase in the twist angle λ with respect to the loading axis is effect in toughening. The mechanism of cracking behavior as well as that of the specimens with angle larger than 45° will be presented. A Part of this study was carried under the grants-in-aid for scientific research (No.23360301) of “Japan Society for the Program of Science”(JSPS).
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