The effect of orientation, applied stress state, environment, and temperature on the crack growth and crack-path behavior of single-crystal specimens of UDIMET 720 has been examined. Stage I cracking has been promoted by mixed-mode loading, plane stress, and vacuum conditions; increasing the test temperature to 600 ЊC does not suppress stage I crack growth in vacuum. Consideration of the local resolved shear-stress intensity and local resolved normal-stress intensity for each slip system as it intersects the nominal crack-growth plane allows the prediction of stage I crack paths, clarifying the importance of secondary single-crystal testing orientation (i.e., nominal crack-growth direction as well as effective tensile axis). A combination of both opening and shearing are found to promote stage I crack growth, and boundary conditions have been established within which stage I cracking is promoted. Highly deflected stage I cracking gives rise to significant shielding effects, but under suitable mixed-mode loading, highly oriented, coplanar stage I crack growth can be produced. Intrinsic stage I cracking under mixed-mode loading appears to be greatly accelerated compared with mode I-dominated stage II crack growth for comparable stress-intensity levels.

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