The Effect of Crystal Orientation and Temperature on Fatigue Crack Growth of Ni‐Based Single Crystal Superalloy

Abstract

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.

Computational Methods for Lifetime Prediction of Metallic Components under High-Temperature Fatigue

“…A straightforward usage of the isotropic incremental lifetime rules (9)- (14) to the single crystal superalloys is rather questionable. According to many investigations on small fatigue cracks [40][41][42], the Ni-base single crystals may exhibit crack propagation on the octahedral slip planes with the fracture surface manifesting a cleavage-like appearance. This tendency is stronger at low or moderate temperatures and higher frequencies, while cracks usually propagate normal to loading direction independent of crystal orientation at high temperature [42,43].…”

“…According to many investigations on small fatigue cracks [40][41][42], the Ni-base single crystals may exhibit crack propagation on the octahedral slip planes with the fracture surface manifesting a cleavage-like appearance. This tendency is stronger at low or moderate temperatures and higher frequencies, while cracks usually propagate normal to loading direction independent of crystal orientation at high temperature [42,43]. This is opposite to the observations in polycrystalline materials, which macroscopic cyclic crack growth is dominated by the normal tensile load in the whole temperature and frequency ranges.…”

Crack growth in single crystal nickel base superalloys under isothermal and thermomechanical fatigue

“…The crack growth behaviour of single crystal superalloys under isothermal conditions has been previously studied [65][66][67][68][69][70][71][72][73][74][75][76][77][78][79][80] , although uncertainties still remain regarding the phenomenon of crystallographic crack growth. The localization of inelastic deformation to {111} planes and crystallographic crack growth along these planes provides a major challenge in terms of test data evaluation and crack growth modelling for single crystal superalloys.…”

Modelling of Crack Growth in Single-Crystal Nickel-Base Superalloys

“…This is especially difficult in single-crystal nickel-base superalloys as a crack can grow in one of two distinct cracking modes, i.e. in the direction perpendicular to the maximum tensile stress in a Mode I fashion or on a crystallographic plane [14][15][16][17][18][19]. As crystallographic crack growth is associated with an increased Fatigue Crack Growth Rate (FCGR) it is of major importance to be able to predict when the cracking mode transition occurs, on which crystallographic plane the crack grows and how fast it then propagates.…”