Crack growth threshold under hold time conditions in DA Inconel 718 – A transition in the crack growth mechanism

E. Fessler et alii, Frattura ed Integrità Strutturale, 35 (2016) 223-231; DOI: 10.3221/IGF-ESIS.35.26 Focussed on Crack Paths Crack growth threshold under hold time conditions in DA Inconel 718 – A transition in the crack growth mechanism E. Fessler SAFRAN Snecma, Villaroche, 77550 Moissy-Cramayel, France ONERA, The French Aerospace Lab, F-92322 Châtillon, France S. Pierret SAFRAN Snecma, Villaroche, 77550 Moissy-Cramayel, France E. Andrieu CIRIMAT, INP-ENSIACET, 31010 Toulouse, France V. Bonnand ONERA, The French Aerospace Lab, F-92322 Châtillon, France ABSTRACT. Aeroengine manufacturers have to demonstrate that critical components such as turbine disks, made of DA Inconel 718, meet the certification requirements in term of fatigue crack growth. In order to be more representative of the in service loading conditions, crack growth under hold time conditions is studied. Modelling crack growth under these conditions is challenging due to the combined effect of fatigue, creep and environment. Under these conditions, established models are often conservative but the degree of conservatism can be reduced by introducing the crack growth threshold in models. Here, the emphasis is laid on the characterization of crack growth rates in the low ΔK regime under hold time conditions and in particular, on the involved crack growth mechanism. Crack growth tests were carried out at high temperature (550 °C to 650 °C) under hold time conditions (up to 1200 s) in the low ΔK regime using a K-decreasing procedure. Scanning electron microscopy was used to identify the fracture mode involved in the low ΔK regime. EBSD analyses and BSE imaging were also carried out along the crack path for a more accurate identification of the fracture mode. A transition from intergranular to transgranular fracture was evidenced in the low ΔK regime and slip bands have also been observed at the tip of an arrested crack at low ΔK. Transgranular fracture and slip bands are usually observed under pure fatigue loading conditions. At low ΔK, hold time cycles are believed to act as equivalent pure fatigue cycles. This change in the crack growth mechanism under hold time conditions at low ΔK is discussed regarding results related to intergranular crack tip oxidation and its effect on the crack growth behaviour of Inconel 718 alloy. A concept based on an “effective oxygen partial pressure” at the crack tip is proposed to explain the transition from transgranular to intergranular fracture in the low ΔK regime. 223 E. Fessler et alii, Frattura ed Integrità Strutturale, 35 (2016) 223-231; DOI: 10.3221/IGF-ESIS.35.26 KEYWORDS. DA Inconel 718; Fatigue crack growth; Hold time effect; Crack growth threshold; Intergranular to transgranular transition. INTRODUCTION A 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. MATERIAL AND EXPERIMENTAL PROCEDURE Material data T 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’ (Ni3 (Ti, Al)) phase and the metastable γ’’ (Ni3Nb) phase, are precipitated inside the γ matrix. A large amount of stable δ (Ni3Nb) 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 Kdecreasing 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. 224 E. Fessler et alii, Frattura ed Integrità Strutturale, 35 (2016) 223-231; DOI: 10.3221/IGF-ESIS.35.26 The first part of the test consisted in a fatigue crack growth test under load control conditions. The tests were performed using 10-X-10 trapezoidal load cycles, where 10 is the time in seconds used to load and to unload the sample, and X denotes the hold time (in seconds) at maximum load. Hold times of 300 s and 1200 s hav (...truncated)