13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Fatigue life and initiation mechanisms in wrought Inconel 718 DA for different microstructures Meriem Abikchi1,2,*, Thomas Billot3, Jerome Crepin1, Arnaud Longuet2, Caroline Mary2, Thilo F. Morgeneyer1, André Pineau1 1 Mines ParisTech, Centre des Matériaux UMR CNRS 7633, BP 87, 91003 Evry cedex, France 2 Snecma-SAFRAN group ; site de Villaroche, 77550 Moissy-Cramayel, France 3 Snecma-SAFRAN group ; site de Gennevilliers, 92702 Colombes Cedex, France * Corresponding author: meriem.abikchi@mines-paristech.fr Abstract Wrought Inconel 718 DA superalloy disk zones present a wide range of behavior in fatigue life due to the variability of the microstructure. In order to link the effect of the forging conditions and achieved microstructure to the fatigue life, two microstructures have been tested in fatigue. Fatigue tests under strain control were performed at 450°C. Grain size distributions and phase distributions were characterized in the specimens and related to fatigue failure initiation modes. Fatigue crack initiation was seen to occur on large grains in stage I for the larger grain material whereas for the material with slightly smaller grains initiation from internal nitrides caused failure via so-called fish-eye cracks. The different steps of these failure modes are discussed using data from the literature to gather the ingredients for a quantitative assessment of the fatigue lifetime using fracture mechanics. Keywords Inconel 718, Microstructure, Fatigue, Crack initiation, Propagation mechanism 1. Introduction Inconel 718 is widely used in aircraft industry. Because of a good combination of formability fatigue and weldability properties and low price/performance ratio, this superalloy is widely used for aerospace applications like turbine disks [1, 2]. This wrought material is produced by forging followed by different heat treatments in order to obtain the desired microstructure. The fatigue behavior at high temperature of this superalloy was widely studied [3-6], the main objective is to predict the lifetime of aircraft engine disk by calculation according to microstructures parameters [7, 8]. To improve the fatigue durability, it is important to obtain the finest possible microstructure. A specific heat treatment called Direct Aging (DA) was developed to obtain this kind of microstructure [9]. Contrary to a classical heat treatment, here the material is directly quenched after forging then aged without adding an annealing step. Fatigue tests were carried out on 718DA and even if the fatigue life was improved thanks to a finer grain size, a strong scatter in test results was observed [10]. Fractography revealed that this scatter is linked to different microstructures and crack initiation mechanisms [11]. Two mechanisms were identified, one for the fine grain size material (5-10µm) and another one for the large grain size material (150µm). For the fine microstructure, initiation sites were systematically related to the presence of second phase particles (carbides and nitrides) on the specimen surface with a size of 10-20µm, then Stage II crack propagation occurs until the breaking point. On the other hand, in the larger grain size material, conventional Stage I crack initiation was observed along the slip bands. In conclusion of this work [11], crack initiation always occurs in the larger microstructural phase. For the fine grain size material, it corresponds to the particles. When the grain size is larger than the particles, crack initiation occurs within the grains themselves. Also, in the previous study, the fatigue properties
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