Issue 35

E. Fessler et alii, Frattura ed Integrità Strutturale, 35 (2016) 223-231; DOI: 10.3221/IGF-ESIS.35.26 230 this assumption and the proposed concept of an effective oxygen partial pressure at the crack tip, characterizations of oxides formed on the transgranular fracture surfaces, in the low ΔK regime will be carried out. C ONCLUSION his work aimed at studying crack propagation in the low ΔK regime, under hold time conditions in DA Inconel 718 alloy. Crack growth test were carried at high temperature using a K-decreasing procedure. The results presented in this paper focuses on the fracture mode and the crack growth mechanisms. Based on SEM observations of the fracture surfaces and EBSD analyses along the crack path, a transition in the crack growth mechanism is evidenced in the low ΔK regime. Results and proposed analyses can be summarized as follow: 1. In the low ΔK regime, transgranular fracture was observed on the fracture surfaces and along the crack path on samples tested under hold time conditions at maximum load. Under the same loading conditions, and at higher ΔK, the fracture is intergranular. Transgranular fracture is usually observed on fracture surfaces of sample tested under pure fatigue loading conditions, at higher frequency. 2. Slip bands have been observed at the tip of an arrested crack tested under hold time conditions at low ΔK (ΔK≈13 MPa√m). This is usually observed under pure fatigue loading conditions and indicates a crack propagation mechanism governed by cyclic plastic strain at the crack tip. This, associated with the transgranular fracture, could be explained assuming the hold time cycle acts, at low ΔK, as an equivalent pure fatigue cycle. 3. To explain the transition from intergranular to transgranular fracture as ΔK decreases, the concept of an effective oxygen partial pressure at the crack tip is proposed. This could be sustained by an oxygen trapping phenomenon at the crack tip, related with reduced CTOD in the low ΔK regime. Lower oxygen partial pressure at the crack may favour the build-up of a protective Cr 2 O 3 oxide layer. This would prevent the formation of Ni and Fe rich intergranular oxides at the crack tip. These oxides have a deleterious effect on the grain boundaries fracture toughness, then leading to intergranular fracture. Preventing the formation of these oxides would prevent intergranular fracture and thus explaining the transgranular fracture observed in the low ΔK regime. Furthermore, the presence of slip bands at the crack tip allows the rapid diffusion of Cr toward the crack tip, thus favouring the build- up of a protective Cr 2 O 3 oxide layer. Further experiments will be carried out to confirm the proposed mechanisms to explain the observed transition to a transgranular fracture at low ΔK. Crack growth tests using the K-decreasing procedure will be carried out with a continuous DCPD monitoring of the crack growth. This way, crack growth occurring during the hold time can be separated from the crack growth occurring during the cyclic part of the cycle. The crack growth occurring during the hold time is expected to progressively vanish at low ΔK. This way, the assumption of an equivalent pure fatigue cycle could be confirmed. Regarding the proposed concept of an effective oxygen partial pressure at the crack tip, in depth characterization of oxides formed on the fracture surfaces will be carried out. The proposed concept will be confirmed if different oxides layers are observed on the transgranular domain, corresponding to the low ΔK regime, and on the intergranular domain. A CKNOWLEDGMENTS he authors would like to thank D. Boivin and N. Horezan, from the microscopy Castaing Lab. (Onera), for their technical support and for having performed EBSD analyses. R EFERENCES [1] Pédron, J.P., Pineau, A., The effect of microstructure and environment on the crack growth behaviour of Inconel 718 alloy at 650°C under fatigue, creep and combined loading, Mat. Sci. Eng., 56 (1982) 143-156. DOI: 10.1016/0025- 5416(82)90167-7. [2] Pierret, S., De Moura Pinho, R., Pineau, A., Hold time effect on the crack growth behaviour in Inco 718 alloy, in 8 th International Symposium on Superalloy 718 and Derivates, TMS, (2014). T T