PSI - Issue 66

Nur Mohamed Dhansay et al. / Procedia Structural Integrity 66 (2024) 87–101 Author name / Structural Integrity Procedia 00 (2025) 000–000

98

12

in the DA condition than the AF and SR conditions and give rise to a larger Δ K th . Both Aswath et al (Aswath et al., (1991)) and Davidson et al (Davidson et al., (1991)) reported the crack tip blunting is caused by the ductile β phase (Saxena and Radhakrishnan, (1998)). Considering the XZ and XY orientation for the AF and SR conditions, we observe a convergence behaviour for increasing R-ratio. In the DA condition, anisotropic behaviour is more pronounced when R > 0.3 i.e. more towards the global “closure-free” region initially. With regards to anisotropy observed in the bi-modal condition, the XY orientation had the largest Δ K th FCGRs while the ZX orientation has the lowest with the XZ somewhat in-between the two. However, we do observe the ZX and XZ orientations have the same final Δ K th but for different R-ratios. There are two main observable differences between these orientations on their fracture surfaces: (i) The ZX orientation has larger amounts of faceted fracture and (ii) PBG orientation. As will be discussed in section 4.3, faceted fracture is associated with lower resistance to crack initiation. It is likely that the observed increased faceted fracture in the ZX orientation causes the lower Δ K th than the XY orientation. In light of the above, both extrinsic and intrinsic influences are discussed in terms of morphological texture. However, different mechanisms are employed to describe the morphological texture’s influence on extrinsic and intrinsic regimes. In the extrinsic regime, the morphological texture results in different levels of crack tortuosity and RICC between the AF/SR and DA condition. whereas in the intrinsic regime, it is the difference in plastic flow properties and energy dissipation which causes different Δ K th . However, it should be noted that for low R-ratios, both extrinsic and intrinsic mechanisms are being experienced, while at high R-ratios, the effect of extrinsic mechanisms on crack propagation has largely reduced. 4.3. Comparison to conventional Ti-6Al-4V Chemically, CM and LPBF produced Ti-6Al-4V material are the same. However, their beginning states i.e. microstructure, residual stress and porosity levels will be the main highlighted difference. Based on literature, it is commonly understood that porosities exacerbate the crack initiation process. The previous authors (Becker et al., (2020)) showed that residual stress reduces crack closure. Therefore, the main comparison we can make is related to the material’s microstructure. When considering investigations which implement various R-ratios (Becker et al., (2020); Nalla et al., (2002); Oberwinkler, (2011)), it is observed at low R-ratios that Δ K th has larger scatter bands than when observing Δ K th at larger R-ratios, for various microstructural conditions. It is likely that for large variations in R-ratio for near-threshold FCGRs, morphological texture is more prominent, but as R-ratio becomes sufficiently high the influence of morphology diminishes. This is more evident in the works of Nalla et al (Nalla et al., (2002)). In Nalla et al (Nalla et al., (2002)), the R-ratio goes beyond 0.97. They use forged Ti-6Al-4V and compare bi-modal microstructure with lamellar microstructure. It was found that the differences observed in Δ K th observed at low R-ratios had been eliminated at very high R-ratios. It is likely that by using the same forged material, but with different heat treatments, the crystallographic texture remains similar in both microstructures, resulting in similar near-threshold FCGR behaviour at high R-ratios. However, an in-depth investigation into this is required to confirm or deny this. When comparing the bi-modal of this investigation and with Nalla et al (Nalla et al., (2002)), at highest R-ratio, we obtain ~ 2.7 MPa.m 0.5 while Nalla et al (Nalla et al., (2002)) obtains ~ 1.9 MPa.m 0.5 . However, factors such as grain size, morphology, colony size, lath size, α p content (volume fraction), amongst others, affect the fatigue properties of Ti-6Al-4V (Wu et al., (2013)) (Kumar et al., (2018)). Furthermore, Oberwinkler et al (Oberwinkler et al., (2010)) showed that the connectivity of the α regions also affected fatigue behaviour. More specifically, the solution treaded microstructure, which has separated α grains had a lower Δ K th than the globular equiaxed microstructure. Oberwinkler et al (Oberwinkler et al., (2010)) defines the connectivity of α regions as the ratio of mean size of interconnected α regions , α ic , and the mean size of the primary α grain size, α p . The near-threshold Δ K th obtained by Oberwinkler (Oberwinkler et al., (2010)) in their two forged conditions were At R = 0, Δ K th ~ 5.1 MPa.m 0.5 for pancake solution treated and ~ 4.8 MPa.m 0.5 for V-shape solution treated. At R ~ 0.7, Δ K th ~ 2.3 MPa.m 0.5 for both solution treated shapes. Although the results obtained by Oberwinkler et al (Oberwinkler et al., (2010)) is lower than that of the current investigation, it is still somewhat comparable. It is likely that the differences in grain size, shape, volume fraction, amount of basal slip regime and α grain connectivity all play an influential role in causing the differences observed.