PSI - Issue 23

M.A. Artamonov et al. / Procedia Structural Integrity 23 (2019) 251–256 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction

Powder metallurgy nickel based superalloys are used for the manufacture of turbine disks (TDs) of modern gas turbine engines (GTE) and power plants. These parts work at temperatures of 650 ° C and above. Moreover, the turbine disks are under the influence of the load caused by the rotation of the disk and the centrifugal forces of the turbine blades. During low-cycle fatigue tests in conditions close to real, fatigue cracks from internal defects could appear. The initial stage of growth of fatigue cracks before they reach the surface occurs in a vacuum condition. The study of the development of the fatigue cracks in TDs is interesting from the scientific and practical points of view. Besides, there is a question on the mechanisms of the fatigue cracks development in the presence of the air and without air access. The previous studies demonstrated a significant difference in the crack growth rate for these types of cracks [1,2]. The existing methods of determination of the life between overhauls of TDs are based on monitoring the development of fatigue cracks. In this article we present the results of the microstructural analysis of fatigue cracks, which were originated beneath the sample surface and expanding without air access. Different electron microscopy methods, namely scanning, transmission and scanning-transmission electron microscopy (SEM, TEM and STEM, respectively) together with energy dispersive microanalysis were used. The cylindrical billets with a gauge length of 13 mm and a diameter of 4.37 mm, were formed from a high temperature nickel based EP741NP alloy, which is used for the manufacture of the aircraft GTE discs. This material is similar to AF115, AF21DA6, Alloy-10 and LSHR alloys [3]. The powder forging method was standard hydrostatic pressing of the powder, obtained by the plasma rotate electrode process (PREP). The size of the powder was less than 140 µm. The chemical composition of the powder is presented in table 1. Table 1 – The alloy element content (weight %) [3] Ni C Cr Mo W Al Ti Co Nb Hf B Zr Basis 0.04 9.0 3.9 5.5 5.1 1.8 15.8 2.6 0.25 ≤0.015 <0.015 The alloy after heat treatment exhibited typical microstructure [3 ]: a γ -Ni solid solution with an average grain size of 40 μm with a hardening intermetallic γ' -phase. The tests were carried out in the soft cycle condition (stress control), the maximum stress level in the cycle was σ max = 980 MPa, the cycle asymmetry parameter R = 0.1. The working conditions of the TDs determines, that if a fatigue crack starts growing inside the billet, the fracture mechanism corresponds to low-cycle fatigue. Two samples with fatigue cracks originated and grew beneath the surface were selected. Sample No.1: the final destruction occurred during the test. The fractured sample after testing was kept in an oven at a temperature of 650 ° С. Sample No.2 : the fatigue crack was of mixed type. The fatigue crack grew up to the surface and further development occurred with the air access. The microstructural and fractographic analysis were performed for the crack in sample No.1 and only fractographic - for the crack in sample No.2.The fractographic analysis was performed in a scanning electron microscopes (SEM) JSM-IT300LV (JEOL, Japan) and a dual-beam SEM-focus ion beam (FIB) Helios NanoLab 660 (ThermoFisher Scientific, USA). Cross-sections specimens were prepared by standard lift-out technique in a SEM/FIB Helios Nanolab (ThermoFisher Scientific, USA), equipped with a micromanipulator Omniprobe (Omniprobe, USA) for extracting thin lamella specimens. The lamellas were studied in a Titan 80-300 TEM/STEM (FEI, USA) at an accelerating voltage of 300 kV. The device is equipped with an EDX Si(Li) spectrometer (EDAX, USA), High Angle Annular Dark Field (HAADF) electron detector (Fischione, USA) and Gatan Image Filter (GIF) (Gatan, USA). 2. Material and methods