A Study of Crack Initiation Mechanism in the Aluminum Alloy 7475-T7351 when Tensile Loading

Abstract A study of 3D surface topography of the aluminum alloy 7475-T7351 after advanced machining and its material and mechanical properties were investigated in situ herein the scanning electron microscope TESCAN MIRA 3, equipped with a brand new combined tensile fixture MT1000 (NewTec, France). Advanced characterizations of material structures and electron backscattered diffraction mapping (EBSD) of selected chemical elements were made, as well as the energy dispersive X-ray analyses of the surface and distributions of the material inclusions. Some very thin cross-sections of the material have been produced by the focus ion beam technique also. The tensile tests confirmed that the cracks nucleated at the interfaces of brittle particles and metal material matrix. Consequently, the cracks spread under the increasing tensile loading and the samples were broken finally before reaching the standard tensile strength limits even. The fatigue results displayed an evident dispersion of the data, but the mechanism of fracture was similar. The surfaces topographies that have been made under selected cutting conditions by face milling operation have not proved to have any decisive impact on the fatigue properties. On contrary, a very evident decohesion between the alumina matrix and brittle inclusions have been observed frequently affecting the strength of the material. The main conclusion is that the phases the complex Al7Cu2Fe and Al-Cr-Fe-Cu-Si intermetallic inclusions have the crucial effect on the tensile mechanical and fatigue properties of the material, suppressing the expected effect of machining and surface topography.