PSI - Issue 47

Aleksandr Inozemtsev et al. / Procedia Structural Integrity 47 (2023) 705–710 Author name / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction A cold dwell fatigue of titanium alloys was studied starting from the 1970s, when significant decrease in-service fatigue life of titanium fan disks in Rolls  Royce RB211 engines was established and attracted both fundamental and applied research (Bache (2003), Cuddihy (2017), Evans and Bache (1994), Song and Hoeppner (1989), Evans and Gostelow (1979), Hasija (2003), Qiu (2014)). Dwell fatigue failure occurs in titanium alloys subjected to cyclic loading as a result of load holds at maximum stress (dwell period) and low temperatures (e.g. T < 0.4T m ) during each loading cycle. The fatigue and cold dwell fatigue lifetime of aircraft engine components depend on a number of chemical and physical factors, such as metal composition, processing conditions, the resulting microstructure, and particular crystallographic texture. The applied processing techniques (hot-working, rolling) should comply with a set of specific requirements to be able to produce optimal microstructures for different engine components ensuring their reliable and safe operation. Dwell fatigue cyclic loading of titanium alloys is supposed to favor the creation of conditions that trigger the mechanisms of fatigue facet nucleation associated with clusters of neighboring grains that have similar crystallographic orientations at the unfavorable orientation with respect to the loading direction. The latter are referred to as macrozones, microtextured regions (MTRs) or effective structural units. However, it is to be noted here that cyclic and pure creep deformations can also act as the drivers of facet nucleation. In comparison to the microstructure observable by optical microscopy, the macrozones formed and developed within the microstructure during thermo mechanical processing of titanium alloys proved to have a detrimental impact on the processes of crack nucleation and growth, and the resulting lifetime reduction under dwell fatigue. The formation of macrozones is supposed to be caused by variant selection and the resulting crystallographic orientation relationship between HCP (secondary or primary ) and BCC grains during phase transformation under conditions of thermo-mechanical processing by Stanford (2004), Germain (2005). These local texture heterogeneities are of frequent occurrence in and + titanium alloys including Ti-6Al-4V by Le Biavant (2002), Kasemer (2017), Lunt (2018). The crystallographic and morphological features of the macrozone are distinguished from those of the observed microstructure. Macrozones often have one or more very large morphological length scales ~1 to 5 mm and sometimes up to 10 mm, compared to an apparent average grain size of ~20 to 100  m. The stages of material fracture under conditions of low- and high cycle loading are classified based on the signs of structural damage for a broad spectrum of spatial scales, including persistent slip bands (PSBs), fatigue striations, microcracks (formed as a result of PSB crossing), and grain-boundary defects. The main damage occurs on the defect scales within 0.1 μm–1 mm, which are substantially smaller than those detected by the standard methods of nondestructive testing, which are commonly used to monitor, for example, the lifetime of buildings during their exploitation. Among new advances in the field of topographic characteristics of the fracture surface, the quantitative fractography is widely regarded as highly effective analytical tool for studying the role of initial structural heterogeneity, monitoring the accumulation of defects on different scales (dislocation ensembles, micropores, microcracks), and determining critical conditions for the transition from dispersed to macroscopic fracture. This technique made it possible to determine the characteristic stages of fracture (crack nucleation and propagation), which provides the basis for evaluating the lifetime of materials and structures subjected to low- and high cycle loading. The description of fracture surface morphology in terms of spatial-temporal invariants was first proposed by Mandelbrot (1983). This approach is based on the analysis of the relief of fracture surface showing the self-affinity property, which is evidenced by the invariant characteristics of the surface relief (roughness) over a broad spectrum of spatial scales. Furthermore, these characteristics reflect a correlated behavior of defects on different scaling levels.