PSI - Issue 43

Ivo Šulák et al. / Procedia Structural Integrity 43 (2023) 209–214 Author name / Structural Integrity Procedia 00 (2022) 000 – 000

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engines (Sims, 1970). The superior high-temperature properties derive from the microstructure, which habitually consists of a face-centered cubic (fcc) γ matrix with coherent γ´ strengthening precipitates (L1 2 type structure), carbides, and exceptionally adverse topologically close-packed (TCP) phases (Reed, 2008). Due to the decomposition of primary MC carbides (M = metal) to M 23 C 6 carbides associated with the formation of undesirable TCP phases, the carbon can be substituted by boron, as a grain boundary strengthening element (Burke et al., 1984; Li et al., 2021). Boron is considered to be capable of improving the strength and cohesivity of grain boundaries and inhibiting embrittlement which is closely related to its positive grain boundary segregation tendency (Li et al., 2021). Wang et al. reported (Wang et al., 2015) that only a small addition of boron (60 ppm) significantly improved the stress rupture life and elongation of GH984 alloy at 700 °C under the load of 350 MPa. Despite the interesting characteristics of boron-doped superalloys, there is a lack of information in the literature about the high-temperature mechanical properties of the B1914 superalloy. During service, high-temperature facilities are often subjected to temperatures of about 1000°C with short - term peaks above 1100°C. These high temperatures can be maintained through advanced cooling techniques (Yeranee and Rao, 2021) and the introduction of thermal barrier coatings (Obrtlík et al., 2017) , which can together significantly reduce the temperature of components made of nickel- based superalloys. Kvapilová e t al. (Kvapilova et al., 2021) recently published, that B1914 has superior creep behavior over IN713LC and has the potential to replace IN713LC in service. Horník et al. (Horník et al., 2 019, p. 19) reported high cycle fatigue behavior of the B1914 superalloy up to 950 °C. A change in the fatigue crack propagation mode was documented as a function of temperature. However, the main issue of high-temperature facilities remains the low cycle fatigue (LCF). Generally, isothermal LCF experiments are performed to obtain relevant fatigue data together with the cyclic stress-strain response (Polák, 1991; Šulák et al., 2022; Šulák and Obrtlík, 2020) . It has been shown, that the LCF behavior of nickel-based superalloys is intensely dependent on temperature (Brien and Décamps, 2001; Šulák and Obrtlík, 2020; Vasseur and Rémy, 1994) . With an increase in the temperature from room to intermediate and then high, additional mechanisms such as cross-slip or dislocation climb enable dislocations to change their slip planes, and thus, different fatigue behavior can be observed (Pineau and Antolovich, 2009). In this work, we report on the LCF behavior of superalloy B1914 at 800 °C and 900 °C. Special attention is paid to the stress response of the material, fatigue life, and the influence of the microstructure on the failure mechanism. 2. Material and methods The studied material was polycrystalline cast nickel-based superalloy B1914 supplied by PBS, Velká Bíteš, a.s in the form of conventionally cast rods. The casting temperature was ( 1360 ± 10 ) °C. The as -cast rods were subjected to two- stepped heat treatment consisting of solution annealing at the temperature 1080 °C for 4 hours with cooling on the air and precipitation annealing at the temperature 900 °C for 10 hours w ith cooling on the air. The material is typical of a coarse dendritic structure (Fig. 1a) and a small amount of shrinkage pores with linear size up to 130 µm. The average grain size of ( 1.05 ± 0.5 ) mm was determined using the linear interception method. The typical microstructure is shown in Fig. 1b. It consists of γ matrix, γ´ strengthening precipitates, γ/γ´ eutectic, MC carbide , and Mo 3 B 2 borides (Horník et al., 2019; Kvapilova et al., 2021) . Since boron extends the solidification-freezing range, borides are formed mainly in the final stages of solidification and they usually accompanies γ/γ´ eutectic (Fig. 1b and Fig. 1c). The chemical composition by wt. % of the B1914 superalloy is 9.63 Co, 9.99 Cr, 5.28 Ti, 5.51 Al, 2.90 Mo, 0.08B, 0.009C and Ni balance. LCF experiments were carried out in a computer-controlled servo-hydraulic testing machine MTS 810 with a maximum capacity of ± 100 kN. The solid round LCF specimens have a gauge length and diameter of 15 and 6 mm, respectively. All LCF tests were performed in a push-pull cycle under total strain control conditions using a fully reversed triangular waveform (R ε = -1) at 800 °C and 900 °C. Specimens were heated in a resistance furnace with a temperature gradient along the specimen length within 0.3 °C /cm. The strain rate of 2 × 10 -3 s -1 and total strain amplitude were kept constant in each test. The strain was measured and controlled with a sensitive MTS extensometer equipped with 120 mm long ceramic tips. In addition, the extensometer was cooled by airflow to prevent it from being affected during the test. The cyclic stress-strain data were recorded. The hysteresis loops obtained at a pre-selected number of cycles were analysed in separate analytic software to evaluate plastic strain amplitude and stress amplitude. The details of LCF data evaluation can be found elsewhere (Obrtlík et al., 2017) .