In situ tensile investigations on AA 7020-T6 using synchrotron diffraction for texture, lattice strain and defect density studies

Abstract

The present work deals with the texture, dislocation and lattice strain evolution under uniaxial tension. The in situ experiments were carried out using high-energy X-ray diffraction and a 20 kN universal tensile machine. The anisotropic mechanical behavior is revealed through the reflex-dependent lattice strain evolution of flat samples under uniaxial tension. The as-received material is a 29.5 mm thick aluminum block with strong texture gradient along the block thickness. In the first experiment, two tensile samples from 7.5 mm below the surface were prepared along the RD (rolling direction). The two samples have the deformation texture (copper type) with sharpness of 10 mrd (multiples of random distribution). One sample is used to study the texture evolution and the other sample is used to determine the lattice strain evolution and dislocation evolution during uniaxial tension. During the tensile test, the texture changes slightly due to the low amount of deformation from initial state to sample fracture. In contrast, significant changes in reflex-dependent lattice strains are observed. In the elastic region, the {111} lattice planes are the stiffest, as in theory, but the {200} lattice planes are not the most compliant ones due to the interaction with neighboring grains having other orientations. Moreover, the Young’s modulus of {311} lattice planes is less affected by the texture of 10 mrd. The work hardening during plastic deformation shows a clear dependence on the grain orientations, as shown by the reflex-dependent lattice strains. The essential difference of work hardening as a function of grain orientation is based on two types of dislocation arrangements, the so called [111] tension stress state and [100] tension stress states. Load release first starts on the {111} and {222} lattice planes before macroscopic UTS (ultimate tensile strength). After UTS all lattice planes experience load release. The dislocation density evolution indicates that in the elastic region, dislocation density decreases as the tensile stress increases. From YS (yield strength), the increase of dislocation density causes the work hardening. Near UTS region, the dislocation density becomes constant. Further beyond UTS, some grain orientations show further work hardening while other grain orientations show softening. It leads to a slight increase in the dislocation density. In the fracture region, a significant increase of dislocation density is observed. It results from the reversal stress (compressive stress) which is formed outside the necking zone, caused by the return of elastic strain. In a second experiment the anisotropic behavior of AA 7020-T6 sample with maximum orientation density of 29.7 mrd was investigated. For this purpose, three flat samples from the center of the Al-block were prepared, with the orientations of 0° to RD, 45° to RD and 90° to RD. The uniaxial tensile tests were carried out till UTS, which shows three stress-strain curves with different yield strengths, ultimate tensile strengths and elongations. The in situ experiments were performed in HEMS@PetraIII/DESY. Due to the low elastic anisotropy of aluminum, the yield strengths, which were determined from the reflex-dependent lattice strain evolution, show only small variation. The plastic anisotropy is much stronger, which can be observed from the lattice strain evolution of all the three samples. The tensile test of the 45° to RD sample shows lowest values of YS and UTS. The variation of lattice strain shows that differently oriented grains undergo different stresses. This effect is greatest in the 45° to RD sample. Overall, the 45° to RD sample shows the most specific characteristics, because less (111) and more (200) grain orientations are available.