13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- A Local Approach to Creep-Fatigue -Oxidation Interactions in Inco 718 Alloy André Pineau1, Raul De Moura Pinho2, Stéphane Pierret2, Caroline Mary2, 1 Mines ParisTech, Centre des Matériaux, UMR CNRS 7633, BP 97, 91003 Evry Cedex, France 2 Snecma-SAFRAN, Villaroche, 77550 Moissy-Cramayel, France * Corresponding author: stephane.pierret@snecma.fr Abstract . Understanding and modeling hold time effects on fatigue crack growth rate (FCGR) above 500 °C in Inco 718DA is a great challenge. Fatigue tests with a trapezoidal wave shape signal including hold times from 90 s to 3600 s were carried out on Inco 718DA with a small grain size (5-15 µm) over a wide range of temperatures (500 °C-650 °C). FCGRs were measured using potential drop technique. SEM observations were carried out to correlate the measured FCGRs with the trans- or inter-granular aspect of the fatigue fracture surfaces. Two regimes must be distinguished. The first regime associated with relatively short hold times can be represented by a power law, where da/dN is proportional to t α, α ~ 0.25. This first regime corresponds to the situation where the fatigue crack does not propagate during the hold time but its propagation takes place during the cyclic part of the loading over a damaged zone ahead of the crack tip. The second regime associated with longer hold times corresponds to the case where crack propagation during hold time (creep crack growth) is predominant. This regime is purely time dependent. Simple models are introduced to describe both regimes. Keywords: Inco 718, High temperature fatigue, hold time effect, fatigue crack growth 1. Introduction Inco 718 alloy is a Ni- base superalloy which is widely used in the fabrication of a number of gas turbine components, especially turbine disks, operating at high temperatures (up to 650 °C). Both the low cycle fatigue properties and the fatigue crack growth rate (FCGR) behavior of this material have been investigated in some detail by a number of investigators (see e.g. [1-12]). The creep crack growth resistance of this alloy has also been examined (see e.g. [13-15]). It is also well established that this alloy, like other Ni-base superalloys, is sensitive to oxidation. This oxidation effect depends on the microstructure, in particular the grain size [5]. These studies constitute the basis for the application of a defect tolerance approach. However most of these studies have been limited to relatively short hold times (< 300 s) which are clearly too small compared to in-service conditions. The recent work by Gustafsson et al. [10-12] constitutes one exception. The first aim of the present study is an extension of the effect of hold time, tm, on FCGR over longer times (up to 1 hour). It is also well established that this alloy, like other Ni-base superalloys, is sensitive to oxidation. This oxidation effect depends on microstructure, in particular the grain size [5]. In gas turbine design, the main load cycle is typically defined by the start-up and shut-down of the engine. The loading varies from component to component but often is a combination of both temperature variations and mechanical loads. These two events defining the main load cycle are separated by few hours in the case of an aero engine, and weeks or months in turbines for power generation. When searching for a model for predicting the interaction between pure cyclic damage (PF), creep (C), oxidation (Ox) the following requirements will appear [10] : (i) the interactions between the time dependent and cyclic load need to be separated, (ii) a damage accumulation between the time dependent damage and the cyclic damage must be considered. This is the second aim of the present study.
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