The Effect of Crystal Orientation and Temperature on Fatigue Crack Growth of Ni-based Single Crystal Superalloy

Crack propagation tests were performed by using compact specimens made of Ni-based single crystal superalloy CMSX-4.Three types of specimens with different orientation were machined.The combinations of loading directions and machined notch directions wererespectively.Tests were performed at room temperature, 500°C, 700°C and 900°C for each orientation specimen.In all specimens tested at room temperature, cracks propagated along slip planes (stage-I), resulting fracture surfaces composed of slip planes.In specimens tested at elevated temperature, cracks propagated along the machined notch direction (stage-II) at the beginning of the tests.FEM calculations were conducted for evaluating the relations between mode I-III SIFs and crack length for specimen with inclined cracks.Methods for evaluating stage-I and stage-II crack propagation rates were proposed.Stage-I crack propagation rates were correlated within the range of factor of 4 using resolved shear stress intensity factor range which was calculated from shear stress on a slip plane parallel to the stage-I crack plane.Stage-II crack propagation rates of all specimens were correlated in the range of factor of 3 with energy release rate calculated using anisotropic elastic moduli.It was suggested that the difference in the elastic moduli caused by the difference between specimen orientation and tested temperature influenced primarily stage-II crack propagation rate.While stage-II cracking was predominant, stage-I fracture surfaces were initiated near side faces in some specimens, and the areas of the stage-I fracture surface tended to expand to the inner regions of the specimens with crack propagation.The transition from stage-II to stage-I was evaluated by using resolved shear stress intensity factor range under plane stress condition.