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. 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.

Crack growth in single crystal gas turbine blade alloys under service-like conditions

“…It has been observed, both in the present work and by other researchers [81][82][83][84][85][86][87][88][89][90][91][92][93][94][95][96][97][98] that crystallographic crack growth in single crystal nickel base superalloys is closely related to localized inelastic deformation along the {111} crystal planes. The dislocation motions that give rise to this inelastic deformation are driven by shear stresses on the {111} planes.…”

“…To make this modelling strategy possible, reliable predictions of the transition from Mode I crack propagation to crystallographic crack propagation is of key importance. This transition has been studied previously by Kagawa 91 , Reed 88 and Busse 12 and is further studied and modelled in Paper 4 of this thesis. A cracking mode transition criterion is proposed, which is inspired by the work of Kagawa 91 et al and based on the anticipated crystallographic crack growth rate relative to the mode I crack growth rate, evaluated locally along the crack front.…”

“…This is further discussed in Paper 1, in relation to RQ2. Kagawa and Mukai 91 found that the propensity for crystallographic crack growth in CMSX-4 was indeed higher at room temperature than at 500°C. The unexpected behaviour with regard to crystallographic crack growth at room temperature remains a topic for future investigation.…”

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

“…Thus, the cracks cannot be expected to follow the conventional Mode I Stress Intensity Factor (SIF) dependency (i.e. crack growth perpendicular to the applied load) as commonly observed in conventional materials [13][14][15][16][17][18]. Hence, an appropriate Crack Driving Force (CDF) parameter needs to be developed to be able to describe the observed fracture behaviour in single-crystal nickel-base superalloys.…”

“…The crystallographic orientation has a substantial influence on the stress allocation and thus the crack growth behaviour [16,20,25,26]. In order to model the correct material response, it is important to know the correct crystallographic orientation, as the misalignments from the casting process are present in the specimens.…”