Micro-orientation Evolution And Deformation Mechanism Of Al-Li Alloy5A90during Superplastic Deformation

ABSTRACT:A novel method is needed to solve the difficulty in forming at room temperature of Al-Li alloy with severe local plastic deformation by superplastic forming. Up to now, some superplastic deformation (SPD) mechanisms have been proposed. However, the SPD mechanism depends on the microstructures, that is to say, each of the SPD mechanisms mainly deals with one certain microstructure so that it can only interpret specific experimental result. Grain size, misorientation, texture and grain boundaries always change during superplastic deformation. Therefore, high temperature tensile test, optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD) are used to investigate the micro-orientation evolution and deformation mechanism of Al-Li alloy5A90during static recrystallization and superplastic deformation. Main conclusions are as follows:(1) The influence of different annealing temperatures on the micro-orientation evolution of Al-Li alloy5A90was obtained. When annealing in the temperature range of450-500℃, with increasing temperature, the grain size increased a little and there was no obvious change in misorientation. After annealing, the type of rolling texture was unchanged, and it began to deviate from the initial orientation as the intensity decreased with increasing temperature.(2) The optimum condition for superplastic deformation was investigated, and the effects of deformation temperatures and strain rates on mechanical character of superplastic deformation were explored. It was shown that recrystallized alloy had elongation of310%～1050%when deformed at temperature range of450-500℃and strain rate range of3×10-4～1.8×10-3S-1.The maximum elongation of1050%was achieved at temperature of475℃and at strain rate of8×10-4s-1. The strain rate sensitivity index m was obtained by slope method, and the value was between0.27-0.63, which showed that the main mechanism was grain boundary sliding during the whole superplastic deformation.(3)The superplastic mechanisms of Al-Li alloy5A90were explored by investigating the micro-orientation evolution during the whole superplastic deformation. It was shown that each stage had its own mechanism. During the initial plastic deformation stage (ε≤0.59), the grain size increased along the normal direction, the misorientation increased while deformation texture was weakened, cavities nucleation was formed and dislocation activity was the main mechanism at the stage. With increasing strain, dynamic recrystallization began to occur while the grain shape changed to be equiaxed, which led to the larger misorientation and lower rolling texture intensity. With recrystallization, grain boundary sliding (GBS) was possibly operative and the grain rotation occured as an accommodation to GBS. When the true strain reached1.55, the grains all appeared to be equiaxed, and the value of larger angle grain boundary achieved92.7%. With larger true strains (ε≥1.55), grain growth continued but the shape kept stable, and the texture appeared dispersion. Meanwhile, cavities began to link and grew larger, which led to fracture. Thus, at the last stage, GBS was the main mechanism with grain boundary migration as the accommodation process.