PSI - Issue 52

Ivo Šulák et al. / Procedia Structural Integrity 52 (2024) 154–164 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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2. Material and methodology 2.1. Material

The material for the LCF tests was produced through an investment casting process in PBS Velká Bíteš, in the form of cylindrical rods with near-net-shape of fatigue specimens. The material was supplied in fully heat treated conditions consisting of hot isostatic pressing (1200 °C / 100 MPa / 4 hours) and subsequent solid solution annealing (1120 °C / 2 hours) followed by precipitation hardening (845 °C / 24 hours). The chemical composition of the material determined by the supplier using Glow Discharge Mass Spectrometry (GDMS) is given in Table 1.

Table 1. The chemical composition of the EEQ-111 superalloy determined by the GDMS technique. C Cr Mo Al Ti B Si Ta Mn Co W Cu V O Ni 0.09 14.3 1.44 3.18 4.61 0.008 0.08 2.77 0.02 9.33 3.84 0.01 0.01 0.0007 Bal.

2.2. LCF experiments The cylindrical fatigue specimens were prepared in the workshop of Institute of Physics of Materials and had a homogeneous diameter of 6 mm over a gauge length of 15 mm. The cyclic loading was performed using the MTS 810 electrohydraulic test system controlled by TestSuite digital electronics in strain control mode at 800 °C and 900 °C. Heating of the specimens to the specified temperatures was conducted in a three-zone electric resistance furnace. Temperature control in the furnace was carried out using a three-channel controller via K-type contact thermocouples. The deformation was measured and controlled by a sensitive extensometer MTS 632.53F-14 with a gauge length of 12 mm equipped with 140 mm long ceramic tips for use at high temperatures. The electrical part of the extensometer was placed outside the furnace and maintained at a constant temperature by a stream of compressed air. The loading was carried out at a constant strain rate of ̇ = 2×10 -3 s -1 in a symmetrical strain cycle (R ε = -1). Test control and recording of stress and strain values during the test were provided by MTS TestSuite TM MPE software. Digital data of selected hysteresis loops forming an approximately geometric sequence (20 values per decade) were recorded for post-experimental evaluation of the stress and plastic strain amplitudes. The stress amplitude and plastic strain amplitude were determined from the half-height and half-width of the hysteresis loop. The test termination criterion was set as a decrease in the ratio of the mean stress ( σ m ) to the stress amplitude ( σ a ): = −0.3 . (1) The number of cycles to fracture, N f , was determined as the number of cycles elapsed at the time the criterion given by equation (1) was met or at the time of fracture if it occurred before the test termination criterion was reached. 2.3. Observation The microstructural analyses before and after fatigue experiments were executed using a scanning electron microscope (SEM) LYRA 3 XMU FEG equipped with an x-ray energy-dispersive (EDX) spectrometer and electron backscatter diffraction (EBSD) detector. Grain size measurement was performed on five specimens using the linear interception method. The SEM images were taken either in secondary electron (SE) contrast or backscatter electron (BSE) contrast. 3. Results and discussion The microstructure of the polycrystalline EEQ-111 alloy in a section perpendicular to the specimen axis is shown in Fig. 1. The coarse dendritic grains (Fig. 1a) with an average grain size of (2.48 ± 0.87) mm are typical for cast superalloys. No preferential orientation was found (see the EBSD map in Fig. 1b). Fig. 1c is an SEM micrograph of the microstructure consisting of MC and M 23 C 6 carbides, γ/γ´ eu tectic, face-centred cubic γ matrix channels and