PSI - Issue 66

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

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(2016)). The primary α ’ has a width of 1 – 3 μ m and this reduces down the hierarchy to below 10 nm for the quartic α ’, as measured by Ter Haar and Becker (Ter Haar and Becker, (2018)). The orientations of the primary laths for the AF and SR conditions have a ± 45° orientation affinity in Z, which is observed in the XY and XZ crack planes while the ZX crack shows α laths having a stronger affinity to 0° and 90° (Becker et al., (2020); Ter Haar and Becker, (2021)). The ± 45° orientation affinity in Z is caused by the combination of 〈 001 〉 _ β //Z-axis and the {334} β , {344} β habit plane formation of martensitic laths. The bi-modal microstructure, after the first anneal, initially consists of α in a matrix of α ’ after the water quench. The second annealing step decomposes the α ’ into α + β lamellar. The final result is a near equiaxed primary α phase and lamella secondary α (Ter Haar and Becker, (2018)). Ter Haar and Becker (Ter Haar and Becker, (2018)) also found that it is the primary α ’ grains which transforms into lamella α and the tertiary and quadric α ’ transform into a β matrix (before water quenching). This led to the suggestion that transformation favours higher orders of the martensitic hierarchy and, therefore, texture. In the bi-modal condition, the morphological texture now changes from a needle like primary α ’ to a more globular to coarse elongated lamellar α p grain. The primary α ’ width of 1 – 3 μ m has now increased to ~ 10 μ m. Furthermore, the ± 45° orientation affinity in Z observed in AF/SR, does not seem to have an obvious change in the bi-modal condition, as seen in Fig. 3. In addition, the PBG in the bi-modal condition still has the 〈 001 〉 _ β //Z-axis affinity due to the heat treatment temperature remaining below the β -transus. The tensile properties for the AF condition are as follows: yield stress σ o = 1072 ± 21 MPa, ultimate tensile strength σ u = 1210 ± 8 MPa and percentage elongation at break of ε f = 8.9 ± 0.8 %. The tensile properties for the SR condition are as follows: σ o = 1180 ± 21 MPa, σ u = 1290 ± 4 MPa and ε f = 6.6 ± 0.1 %. The tensile properties for the bi-modal condition are as follows: σ o = 900 MPa, σ u = 950 MPa and ε f = 15 %. 3.2. ∆ Kth The near fatigue crack growth rate threshold results for AF, SR and DA are shown in Fig. 4. The general threshold behaviour, where Δ K th decreases with increasing R-ratio, is observed in the three orientations and conditions, except for the ZX and XZ orientation in the AF condition. The Δ K th as a function of R-ratio for both conditions are shown in Fig. 5(a). The AF condition is discussed in detail in the works by the previous authors(Becker et al., (2020)). The following observations can be made: (i) For the SR condition, the ZX orientation has a lesser reliance on R-ratio compared to the remaining oreintations. There is an approximate 30% decrease in Δ K th which is almost reached at R= 0.3. Whereas the XZ and XY orientation has an approximate 30% and 50% decrease, respectively, but over the entire span of R-ratio reductions. (ii) The XZ and XY orientations in the SR condition have a convergence behaviour where they both tend towards ~ 1.5 MPa √ m while the ZX orientation results in ~ 1.8 MPa. √ m. (iii) The DA condition for both the ZX and XZ orientation has approximately 50% decrease in Δ Kth from the lowest to highest R-ratio, while the XY orientation has approximately 60% decrease in Δ K th . The Δ K th for the ZX and XZ orientation reduces to ~ 2.7 MPa √ m and ~ 3.5 MPa √ m in the XY orientation. Considering the Δ Kth versus K max envelope in Fig. 5(b), the authors (Becker et al., (2020)) previously found an intrinsic Δ K th of ~ 1.6 ± 0.2 MPa √ m and critical K max of ~ 3 MPa √ m. in the DA condition, the ZX and XZ orientation have an intrinsic Δ K th ~ 2.7 MPa √ m and the XY orientation ~ 3.5 MPa √ m. The intrinsic critical K max for all three orientations in the DA condition is ~ 6 MPa √ m. 3.3. Fractographic analysis With regards to the AF and SR microstructural conditions in Fig. 6, we observe the same as in the previous work by the authors (Becker et al., (2020)). Fracture mechanisms are predominantly transgranular quasi-cleavage faceted fracture through the laths. The crack path is deflected at primary laths specifically oriented at ± 45°, resulting in crack tortuosity on a localised scale. Furthermore, the PBG boundaries are outlined on the fracture surfaces, highlighting the different orientations perceiving different geometries of the PBG, resulting in varying amount of RICC being experienced for the respective orientations.