Issue 78

T. Nasri et alii, Frattura ed Integrità Strutturale, 78 (2026) 275-297; DOI: 10.3221/IGF-ESIS.78.17

Microstructural characterisation by EBSD The microstructure was characterised using electron backscatter diffraction (EBSD) along the TL and TC directions. The surfaces were electropolished prior to acquisition. Orientation maps and pole figures were obtained with a step size of 0.5 µm. Orientation maps are based on Euler angle colour coding ( φ 1, Φ , and φ 2 represented in red, green, and blue, respectively). High-angle grain boundaries (misorientation > 10°) and low-angle boundaries (> 2°) are represented by thick and thin black lines, respectively. Recrystallisation maps are based on internal misorientation: recrystallised grains are shown in blue, while deformed grains are shown in red. Fig. 1 presents the resulting EBSD maps for the TL direction. The alloy is partially recrystallised. It consists of randomly oriented recrystallised grains and a minority fraction of strongly textured, elongated non-recrystallised grains with ⟨ 111 ⟩ aligned along TL. Average grain sizes along the three principal directions are given in Tab. 3. Direction L (rolling) TL (transverse) TC (thickness) Mean grain size (µm) 395 ± 203 154 ± 47 133 ± 45 Anisotropy ratio / 2.56 L TL d d  / 1.16 TL TC d d  — Table 3: Average grain sizes along the three principal directions (EBSD measurements). of the enriched model (see "Enriched Model Formulation" below). The grain boundary density-defined as the number of boundaries per unit crack length-differs by a factor of 2.56 between the perpendicular-to- D R direction ( f  = 6.5 boundaries/mm, type A) and the parallel-to- D R direction ( f  = 2.5 boundaries/mm, type B). This ratio governs the experimentally observed propagation anisotropy. Fig. 2 shows the corresponding EBSD maps for the TC direction. X-ray tomography indicates that the intermetallic particle distribution along the crack path is similar for both specimen types [28]. The inclusions-mainly  phase (Al ₂ Cu) and Al ₇ Cu ₂ (Fe,Mn)-are not identified as the primary factor differentiating the two orientations. The difference in behaviour between types A and B is therefore attributed to morphological and crystallographic grain anisotropy, consistent with the findings of Chen et al. [14,15]. Morphological anisotropy is pronounced. The ratio / L TL d d  2.56 reflects grain elongation along the rolling direction. The TL-TC plane is nearly equiaxed ( / TL TC d d  1.16 ). These values enter directly into the normalised variable ξ = ( / ) f a d

Figure 2: EBSD characterisation of AA2024-T351 -TC direction. (a) Grain orientation map. (b) Recrystallisation map. (c) Pole figures {111} and {100}.

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