Issue 35

E. Fessler et alii, Frattura ed Integrità Strutturale, 35 (2016) 223-231; DOI: 10.3221/IGF-ESIS.35.26 224 K EYWORDS . DA Inconel 718; Fatigue crack growth; Hold time effect; Crack growth threshold; Intergranular to transgranular transition. I NTRODUCTION eroengine manufacturers have to demonstrate that critical components such as turbine disks meet the certification requirements in term of fatigue crack propagation life, using damage tolerance approaches. Crack propagation laws are usually identified from sinusoidal wave shape fatigue tests. Trapezoidal wave shape signal tests, with a hold time at maximum load, are also studied as they are more representative of the in service loading conditions (i.e. the take off – cruise – landing cycle). This study aims at investigating the deleterious effect of hold time on the crack propagation behaviour of DA Inconel 718 [1-3], a polycrystalline nickel based superalloy widely used for aeroengines turbine disks manufacturing. Modelling the hold time effect is challenging as models have to take into account the coupled effects of fatigue, creep and environment (see e.g. [4-6]). Established models are often conservative. Introducing the crack growth threshold under hold time conditions in models is a way to reduce the degree of conservatism. This paper focuses on the characterization of the crack growth threshold under hold time conditions and crack growth mechanisms involved in the low ΔK regime. Crack growth tests were carried out under hold time conditions using a K-decreasing procedure. This procedure, described in [7], is generally used to determine the fatigue crack growth threshold. Different hold times (300 s and 1200 s) have been investigated at temperatures ranging from 550 °C up to 650 °C. Studies regarding the crack growth threshold under hold time conditions are scarce, but the reader can refer to the work of Lynch [8] and the more recent work of Li [9]. In the present paper, the emphasis is laid on the characterization of the fracture mode involved in the low ΔK regime. Scanning electron microscopy (SEM) was used to characterize the transgranular or intergranular aspect of the fracture surfaces. Electron backscattered diffraction (EBSD) analyses were also carried out to observe the crack path at a microscopic scale in the low ΔK regime. M ATERIAL AND EXPERIMENTAL PROCEDURE Material data he material used in this study is the direct aged version (DA) of Inconel 718, a wrought polycrystalline nickel based superalloy widely used for aeroengines turbine disks manufacturing. The chemical composition of the material is presented in Tab. 1. The material was delivered in the form of a forged disk which was given the DA heat-treatment (720 °C for 8 h and 620 °C for 8 h) directly after forging. The DA Inconel 718 exhibits a γ matrix with a face-centered cubic structure and a small grain size of 5 to 15 µm. Two strengthening particles, the γ’ (Ni 3 (Ti, Al)) phase and the metastable γ’’ (Ni 3 Nb) phase, are precipitated inside the γ matrix. A large amount of stable δ (Ni 3 Nb) phase particles are formed mainly along the grain boundaries. The δ phase is the stable version of the γ’’ precipitates, thus a γ’’ precipitates depleted zone is observed around δ particles giving rise to a local softening. Element Ni Cr Fe Nb+Ta Mo Ti Al Co Mn Cu C B Weight % Balance 17.97 17.31 5.4 2.97 1 0.56 0.14 0.08 0.03 0.023 0.0041 Table 1 : Chemical composition of DA Inconel 718. Experimental procedures Tests were carried out on KBr specimens with a rectangular cross-section of 8.3 x 3.5 mm². A semi-circular starter notch of depth 0.3 mm is introduced by EDM. Specimens were fatigue-precracked at 450 °C at a frequency of 10 Hz, using a K- decreasing method according to ASTM E-647 [7]. This was done to obtain a sharp semi-circular crack of 1.3 mm length with a reduced plastic zone ahead of the crack tip. Specimens were then heated up to the test temperature. Temperatures ranging from 550 °C to 650 °C have been investigated. A T