PSI - Issue 57

A. Radi et al. / Procedia Structural Integrity 57 (2024) 642–648 Achraf radi/ Structural Integrity Procedia 00 (2019) 000 – 000

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particle shearing and Orowan looping, while two mechanisms are mediated by mass diffusion, namely dislocation climb around particles, and to a lesser extent particle dragging [3]. The dominant mechanism depends on both microstructural parameters (e.g. alloy composition, lattice misfit, precipitates size, morphology, and volume fraction) and external conditions (e.g. applied stress, operating temperature and environmental conditions). At room temperature mass diffusion is slow, and therefore the athermal mechanisms of particle shearing and looping by dislocations are dominant. Together with the grain size, the precipitate size distribution and volume fraction are essential microstructural parameters influencing both monotonic and cyclic deformation of the superalloys [2,4,5]. Internal hydrogen, which can be introduced into the material through processes such as cathodic protection or, in the case of parts used in sour gas and in rocket engines, powered by hydrogen fuel, can cause a premature failure of the material, potentially leading to catastrophic disasters. Several mechanisms have been proposed to explain the phenomenon of HE (hydrogen embrittlement), such as hydrogen-enhanced localized plasticity (HELP) by interacting with the lattice defects introduced via plasticity, hydrogen-enhanced decohesion (HEDE) by reducing the cohesion between metallic atoms, hydrogen adsorption-induced dislocation emission (AIDE), and hydrogen-enhanced strain-induced vacancy (HESIV) by vacancy generation due to dislocation-dislocation interactions [6 – 9]. In this study, the impact of hydrogen on the fatigue behavior of a Nickel-based superalloy (Waspaloy) was investigated at room temperature, in relation to the precipitation state in the under-aged domain, where plasticity is localized and precipitates are shearable. To achieve this, two precipitation states (HT0 and HT4) were considered for a constant grain size of 100 µm. Both states were hydrogen-precharged by cathodic charging, and their cyclic behavior was compared to that of non-hydrogen charged states. Internal stresses were evaluated using the Cottrell-Dickson partition of hysteresis loops [10,11]. For both microstructural states, it was observed that hydrogen induced mechanical softening in relation to the specific evolution of long-range internal stress (back stress) and effective stress (short range interaction). Correlating the internal stress states with the inter-band spacing of slip bands characterized by transmission electron microscopy (TEM) and atomic force microscopy (AFM) offer an opportunity to examine the The material under investigation is a Nickel-based superalloy known as Waspaloy, manufactured by Aubert et Duval in the form of a round bar with a 60mm diameter and a length of 700mm. The initially received material underwent heat treatment through a solutionizing process at 850°C for four hours, followed by aging at 760°C for sixteen hours, and subsequent cooling in air. The resulting average grain size and precipitate size were 64µm and 42nm, respectively. In order to examine the impact of hydrogen on the precipitation state, two heat treatments were conducted on our samples within the weak coupling domain. The initial heat treatment, referred to as HT0, involved solutionizing at 1080°C for 4 hours, followed by oil quenching, and aging at 550°C for 4 hours in an argon environment, succeeded by air quenching. The second heat treatment, designated as HT4, underwent the same solutionizing process, followed by aging at 750°C for 6 hours in an argon environment, and then air quenching. The controlled heat treatments resulted in a consistent grain size (D = 100µm) and volume fraction (fv = 20%) for both HT0 and HT4. However, TEM observations revealed a variation in precipitate size, with HT0 exhibiting d = 10nm and HT4 showing d = 30nm. The primary objective was to maintain a constant grain size and volume fraction, ensuring a more accurate comparison between hydrogen-charged and non-charged samples while varying the precipitate size. Consequently, the obtained results can be directly correlated to the interaction between hydrogen-precipitate and hydrogen-dislocation interactions. 2.2. Hydrogen charging and fatigue tests Hydrogen-charging of Waspaloy samples was carried out prior the mechanical testing at 25°C using an electrochemical technique (see [12 – 15] for more details on the charging devices). The charging time was 72 hours with a courant density of -10mA.cm -2 . Those conditions are enough to ensure the saturation of hydrogen in the lattice origin of the observed softening. 2. Experimental procedures. 2.1. Material preparation