A new modeling framework for anisotropic yield strength of Al-Li alloy sheet with inhomogeneous plate-like T1 precipitates

Abstract

The high strength of Al-Li alloy is mainly attributed to precipitates, which has attracted extensive investigations on precipitation dynamics and interaction with dislocations. However, the influence of plate-like T1 precipitate on mechanical anisotropy is still not fully clarified, and an accurate corresponding modeling framework considering inhomogeneous distribution and various configurations of T1 precipitate is also needed. Here, the evolution of mechanical anisotropies in 2198 Al-Li alloy sheets is systematically studied with different distributions and thicknesses of T1 precipitates obtained via changing pre-deformation direction and artificial aging temperature. A new modeling framework based on visco-plasitic self consistant (VPSC) is proposed to predict the anisotropic yield strength under different processing conditions. The resistance of four variants of T1 precipitates which form a 'T1 precipitate forest' to dislocation movement is considered. According to the geometric relationship with slipping systems, T1 precipitates can be divided into three types: type I is parallel to slipping plane, type II is parallel to slipping direction but not slipping plane, and type III has a certain angle to both slipping plane and direction. The critical resolved shear stress (CRSS) of each slipping system can be calculated using a linear mixture law considering volume fraction and shear strength of these three types. It is found that T1 precipitate of type I plays an important role in the strengthening of 2198 Al-Li alloy sheet. The shear strengths of types I, II and III are determined to be 400 MPa, 8 MPa and 8 MPa, respectively, at artificial aging temperature of 155 °C, and reach 400 MPa, 40 MPa and 40 MPa due to the increased T1 thickness when aging temperature increases to 162 °C. The calculated anisotropy of yield strength is in good agreement with experimental results. Furthermore, a reasonable correlation between slipping activation and T1 precipitate is revealed. That is, the activation fractions of different slipping systems exhibit how the varying distribution and thickness of T1 precipitate lead to different mechanical anisotropy of 2198 Al-Li alloy sheet.