The effect of inhomogeneous microstructures on strength and fatigue properties of an Al-Cu-Li thick plate

The inhomogeneity of the microstructure of an Al-Cu-Li thick plate in the thickness direction results in inhomogeneity in the tensile property and fatigue crack propagation (FCP) rate. The grain size gradually increases from the surface layer to the central layer. The contents of low-angle grain boundaries (LAGBs) were characterized by an electron backscattered diffraction (EBSD) technique, and the LAGBs, which act as nucleation sites, induced a greater number of T1 precipitates in the central layer. In the surface layer, the random texture resulted from the greater recrystallization degree due to the high degree of cold deformation during the rolling process. The 1/8 layer contained a rotated cube texture, which is a typical shear texture. A strong β fiber rolling texture was found in the central layer, including S, brass and copper texture components, because the adequate recovery during hot rolling decreased the recrystallization driving force. Both the yield strength (YS) and ultimate tensile strength (UTS) gradually increased from the surface layer to the central layer, i.e., UTS from 479.7 MPa to 546.0 MPa, YS from 455.9 MPa to 514.0 MPa. The inhomogeneity of the tensile properties was primarily caused by the different textures, LAGB content and T1 precipitate density. However, the surface layer had a distinctively lower FCP rate than the central layer. The disparity in the FCP rate between the surface layer and the central layer could be attributed to the roughness and plasticity, which are related to the LAGBs, the T1 precipitates and the grain size.