Cleavage of S′ (Al2CuMg) needles in slip bands of a cyclically strained aluminium alloy

TEM examination of deformation mechanisms of low cycle fatigued 8090 alloy shows that at high cyclic plastic strain (Δεp/2=0.5×10−2), the semi-coherent (Al2CuMg) S′ needles, intercepting slip bands, are cut. A previous crystallographic study has shown the absence of slip continuity between S′ orthorhombic phase and the FCC matrix α. The origin of this fracture phenomenon is associated with stress concentration favoured by strain localisation induced by the shearing of the L12 coherent phase (Al3Li) δ′. A cleavage fracture model of S′ needles is proposed here. It seems that cleavage fracture is quite probable because the local level stress reached at head of dislocation pileups is of the order of the estimated cleavage fracture stress of S′ needles.