PSI - Issue 71

T. Vivekananda Swamy et al. / Procedia Structural Integrity 71 (2025) 111–117

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1. Introduction Titanium alloys demonstrate high specific strength and corrosion resistance at moderately high temperatures. TiAl-based alloys are directly competing with Ni-base superalloys and well-known high temperature steels. Among the titanium alloys, Ti-6Al-4V is the most popular and widely used alloy in aerospace gas turbine engine components like discs and fan blades, according to Leyens et al (2003). Such aerospace components are usually subjected to high-frequency vibrational stresses at different temperatures throughout their operating lives. The lamellar microstructure typically exhibits a stronger fatigue crack-growth (FCG) resistance. In contrast, an equiaxed microstructure shows a higher resistance to fatigue crack-initiation (FCI) as specified by Zhihong Wu et al. (2020). As the FCI is the key factor in deciding the HCF strength, equiaxed microstructures will hold good HCF resistance. G.Q. Wu et al. (2013) reviewed many data sets from the literature and investigated the microstructure types and parameters of the RT HCF strength of Ti-6Al-4V alloy. For equiaxed microstructures, this author analysed the data from HCF tests conducted at different R values of -1, 0, 0.1, 0.2, and 0.5, and frequencies of 10, 25, 30, 90, 130 and 20000 Hz. Wanhill et al. (1989) conducted fatigue crack growth experiments and examined microstructural and fractographic analysis at RT. Zhihong Wu et al. (2020) performed HCF tests and examined microstructural variations of this alloy and fracture behaviours at RT and 400 ℃. Ding et al. (2005) executed fatigue crack growth tests at RT and 350 ℃ and studied the effect of temperature and stress ratio. Tokaji (2006) studied crack initiation, small crack growth, and fracture surface analysis by performing HCF loading at RT and elevated temperatures (350 ℃ and 450 ℃) . The R value of -1 is used at a frequency of 60 Hz. In his work, the fatigue strength of Ti-6Al-4V alloy was reduced with an increase in temperature, indicating lower crack initiation resistance at elevated temperatures. As stated by Jiang et al. (2007), a temperature of 150 ℃ is suitable for some Ti–6Al–4V fan components. The literature revealed that no data for HCF behaviour at 150 ℃ have been observed. The data for R values of 0.7 and cyclic frequency of 78Hz was also unavailable. Hence, assessing this alloy's performance under high-frequency stress-controlled cyclic loads at intermediate operating temperatures like 150 ° C is crucial. This article studied the HCF behaviour and fractographic analysis of Ti-6Al-4V alloy with equiaxed microstructure (Fig. 2). These tests have been performed at RT and 150 ℃ to study the effect of intermediate operating temperature on the HCF behaviour of this alloy. The R value is 0.7 and a cyclic frequency of 78 Hz. 2. Materials and methods The material under study in this work was Ti-6Al-4V alloy. Specimens used in this study were machined from hot rolled bars and subjected to annealing heat treatment. The 6mm gauge diameter specimens were produced as per ASTM E466-15 2015 standard. The same was used for high-cycle fatigue (HCF) testing. Fig. 1(a) shows the hourglass specimen. Dimensions of the specimen, as shown by Sandeep Kale et al (2022), are presented in Fig.1 (b). Optical microscopic microstructural image of the as-received material is presented in Fig. 2. This shows an equiaxed microstructure with a few lamellae of only one particular crystallographic orientation, called micro textured regions (MTRs) as described in Alexander et al (2022).