PSI - Issue 52

Marie Kvapilova et al. / Procedia Structural Integrity 52 (2024) 89–98 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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more exacting demands on its microstructure stability after long-term thermal exposure and high creep resistance. Several authors have studied the microstructural features of the GTD 111 superalloy, notably, Wangyao et al. (2008), Wongnawapreechachai et al. (2012), Lee et al. (2012), Berahmand and Sajjadi (2012) and Turazi et al. (2015). It has been frequently observed that the GTD 111 superalloy in as-cast state has a multi-phase microstructure consisting of fcc γ matrix, bimodal γ´ precipitates, γ - γ´eutectics, which are mostly distributed at dendrite boundaries, M 23 C 6 carbides and a small number of deleterious phases such as σ, η, δ and Laves phase. Further, Wangyao et al. (2008), Berahmand and Sajjadi (2012), Lee et al. (2012), Turazi et al. (2015), Wang et al. (2015) and Keyvanlou et al. (2020) have studied the resulting microstructure of the GTD 111 superalloy after long term thermal exposures. It appeared that, the main degrading effect of various thermal expositions at elevated or high temperatures on the microstructure of the GTD 111 superalloy was the coarsening and the dissolution of γ´ precipitates. The complete characterisation of high temperature components operating in advanced gas turbines is important in establishing actual operating conditions, degradation factors and optimum design and/or remaining lives. Fail-safe designs are based on the ability to predict the response of a microstructure to applied loads and ensuing creep deformation. However, few creep data on the GTD 111 alloy and the studies of the link between microstructure and cr eep have been published in the last two decades in the open literature. To the author’s knowledge, the first study that measured short-term creep data as well as microstructure characteristics of this superalloy was reported by Daleo and Wilson (1998). Microstructural characteristics and creep behaviour of the GTD 111 superalloy were examined by Sajjadi and Nategh (2001), Sajjadi et al. (2002), Nategh and Sajjadi (2003), Sajjadi et al. (2003), Ibanez et al. (2006), Shigeyama et al. (2017) and Asadi et al. (2020). However, the operating creep deformation mechanisms in high temperature creep of the GTD 111 are not clearly resolved. Furthermore, the creep damage processes often limit the lives of components operated at high temperature. Analogous to creep deformation mechanisms the creep damage development in the GTD 111superalloy remains poorly understood and this is due to the extremely small number of relevant studies that have been carried out to present. Therefore, the present study aims to identify operating mechanisms of creep deformation, damage, and fracture and to clarify the decisive factors governing the creep resistance and performance of the GTD 111 superalloy. 2. Experimental 2.1 . Material The as-cast nickel- based GTD 111 superalloy was provided by foundry PBS Velká Bíteš a.s., Velká Bíteš, Czech Republic, in the form of bars made by investment casting. The bars have a diameter of 15 mm and were given to a HIP (hot isostatic pressing) treatment to eliminate of casting microporosity and to reduce the scatter of creep properties (Kim et a. (2008)). The chemical composition of the GTD 111 superalloy is in Table 1. 2.2 . Experimental methods Cylindrical creep specimens with a gauge of 50 mm in length and 3.5 mm in diameter were cut out of bars. Constant load creep tests in tension were carried out in an argon atmosphere until the final fracture of the specimen following standard ASTM E139 (Book of ASTM Standards (2009)). The creep testing was carried out at 800, 900, and 950°C with the testing temperature maintained to within ± 0.5°C of the desired value. The initial applied tensile stress σ varied from 125 to 700 MPa. The creep elongations were continuously measured using a linear variable differential transducer (the strain was measured with a sensitivity 5 x 10 -6 ) and they were recorded digitally and then computer processed. The microstructure and creep damage after creep exposures were observed on the longitudinal metallographic cross-sections and creep fracture surfaces by scanning electron microscopy (SEM) using Tescan Lyra 3 XMH Table 1. Chemical composition of cast GTD 111 nickel-based superalloy (in wt. %). element wt. % Ni C Cr Al Ti Ta Mo 1.44 Co W B V Mn 0.02 Si Cu bulk 0.09 14.3 3.18 4.61 2.77 9.33 3.84 0.008 0.01 0.08 0.01