Materials Science Forum Vol. 1183

Abstract: Rolled 7xxx-series Al alloys exhibit pronounced microstructural anisotropy (pancake grains and particle stringers) that can strongly affect fatigue initiation and-crack growth. In the current study, an AA7075-T6 plate was examined in three orthogonal machining orientations—L–S (longitudinal), L–T (long-transverse) and T–S (short-transverse)—using high-cycle three-point bending fatigue at room temperature (R = 0, f = 25 Hz, σ_max = 360–400 MPa, i.e., ~0.79–0.88σ_y). Optical/SEM observations reveal elongated grains and a grain-density gradient through thickness, accompanied by orientation-dependent distributions of intermetallic particles. Despite only small differences in monotonic response, fatigue performance is strongly orientation-dependent: the T–S specimens exhibit the longest lives in the S–N curves. Fractography and striation-based kinetics show the lowest Paris-regime crack-growth rate for T–S (da/dN ≈ 1.85×10⁻⁷ m/cycle at ΔK ≈ 10.5 MPa√m), while L–S shows the fastest growth (da/dN ≈ 4.3×10⁻⁷ m/cycle at ΔK ≈ 13.0 MPa√m). The improved T–S fatigue resistance is discussed in terms of crack-path interaction with grain boundaries and particle populations (coherent/penetrable vs non-coherent/coarse particles), which can either deflect/retard cracks or act as initiation sites. The results provide a compact microstructure–mechanics map for rolling-induced anisotropy in AA7075-T6 under bending fatigue.