PSI - Issue 82

Hang Su et al. / Procedia Structural Integrity 82 (2026) 131–137 H. Su et al. / Structural Integrity Procedia 00 (2026) 000–000

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1. Introduction Fatigue failure is one of the most common and critical failure modes for metallic structural materials used in aerospace, transportation and energy industries, where long service life and cyclic reliability are required (Suresh, 1998; Schijve, 2009). In traditional design practice, the fatigue strength of metallic materials was empirically correlated with their tensile strength through the so-called “fatigue ratio,” suggesting a proportional relationship between the fatigue limit and ultimate tensile strength (UTS) at 10⁷ cycles under fully reversed loading (Hong et al., 2009). However, subsequent studies revealed that fatigue fracture can still occur far beyond this conventional limit, giving rise to the concept of very-high-cycle fatigue (VHCF), which extends to 10⁸–10¹⁰ cycles or more (Mayer, 2016; Sakai, 2023). In this regime, the classical proportionality between UTS and fatigue resistance breaks down because the failure process is governed not only by the material strength but also by the internal microstructural and defect characteristics (Murakami, 2002; Furuya and Takeuchi, 2014). For conventionally processed (CP) titanium alloys, fatigue behavior is strongly dependent on microstructural morphology and phase distribution. According to the type and stability of α and β phases, titanium alloys can be categorized into near-α, α+β, metastable β and β grades (Leyens and Peters, 2003; Lütjering and Williams, 2003). Among these, α+β alloys such as Ti-6Al-4V have been extensively studied due to their wide engineering applications and well-defined microstructural varieties, including equiaxed (EM), bimodal (BM), lamellar (LM), and basketweave (BW) morphologies. Each type exhibits distinct combinations of strength, ductility and fatigue resistance (Crupi et al., 2017; Cui et al., 2021). EM and BM alloys, produced by annealing, generally show excellent resistance in high-cycle and very-high-cycle regimes due to their homogeneous slip distribution and fine α grains (Heinz et al., 2013; Heinz and Eifler, 2016). In contrast, LM and BW microstructures, typically generated through forging, exhibit higher UTS but reduced VHCF resistance owing to limited plastic accommodation and increased anisotropy (Nikitin et al., 2016; Wu et al., 2013). Extensive VHCF investigations have demonstrated that titanium alloys can exhibit dual-slope or step-wise S–N curves, where surface-initiated failure transitions to internal crack initiation beyond 10⁷ cycles, often forming fish-eye (FiE) and fine granular area (FGA) features on fracture surfaces (Pan et al., 2018; Pan and Hong, 2019; Pan et al., 2020a; Su et al., 2017). These observations align with the mechanisms reported in high-strength steels (Abe et al., 2004; Furuya, 2021), confirming that VHCF failure in titanium alloys is governed by the interplay of microstructural heterogeneity, inclusion type, and local plasticity exhaustion (Atrens et al., 1983; Neal and Blenkinsop, 1976). Recent studies have further revealed that fatigue crack initiation in VHCF regimes is accompanied by local nanograin formation and facet evolution within the FGA (Pan et al., 2021, 2024d; Du et al., 2023), highlighting the importance of micro- and nano-structural characterization for understanding long-life fatigue mechanisms in α+β titanium alloys (Wu et al., 2024; Sakai, 2023). The emergence of additive manufacturing (AM), especially laser-based powder bed fusion (PBF-LB), has profoundly altered the microstructure–property relationship of titanium alloys (Gibson et al., 2021; Yadroitsev et al., 2021; Toyserkani et al., 2021). The rapid thermal cycles inherent to this process induce hierarchical microstructures characterized by fine acicular α′/α″ martensitic laths and directionally solidified prior-β grains (Thijs et al., 2010; Xu et al., 2017). Although such features lead to high tensile strength, often exceeding 1100 MPa, the fatigue performance of AM Ti-6Al-4V alloys remains inferior and more scattered than that of CP counterparts (Molaei et al., 2020; Pegues et al., 2020). This discrepancy primarily arises from metallurgical defects such as lack-of-fusion (LoF) voids, gas pores and keyholes, which act as severe stress concentrators and dominate crack initiation in both HCF and VHCF regimes (Du et al., 2021; Qian et al., 2020; Pan et al., 2025). Even after post-treatments like hot isostatic pressing (HIP), certain submicron defects and local heterogeneities persist, leading to early internal crack nucleation and FiE-type failures (Pan et al., 2020a; Pan et al., 2024c; Pan et al., 2024d). Despite extensive research on AM and CP titanium alloys, most existing studies only focus on either the mechanical response of AM alloys or the microstructure-controlled fatigue of CP ones, lacking a unified framework that captures the transition from microstructure-dominated to defect-dominated failure mechanisms (Pan and Hong, 2019; Pan et al., 2024b). In engineering practice, this gap leads to uncertainty in fatigue design and life prediction, especially for components expected to endure billions of cycles under low stress amplitudes. Understanding how intrinsic microstructural features interact with extrinsic defects to govern fatigue resistance is thus essential for both performance optimization and process qualification of AM components (Furuya et al., 2022; Gao et al., 2024). Therefore, this study conducts a direct and systematic investigation and comparison on AM and CP titanium alloys