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 (Delta epsilon (p)/2=0.5x10(-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 alpha. The origin of this fracture phenomenon is associated with stress concentration favoured by st rain localisation induced by the shearing of the L1(2) coherent phase (Al3L i) delta '. A cleavage fracture model of S ' needles is proposed here. It s eems that cleavage fracture is quite probable because the local level stres s reached at head of dislocation pileups is of the order of the estimated c leavage fracture stress of S ' needles. (C) 2001 Elsevier Science Ltd. All rights reserved.