Analysis of the influence of alloying additions on the structure of Al-Li alloys and deformation mechanisms under superplastic conditions (an analytical review)

The use of structural superplasticity is promising in the development of production technologies with complex shapes and improved physical, mechanical and operational characteristics. Deformation in the superplasticity regime is characterized by reduced (compared to conventional plastic processing) loads on tools and decreased number of finishing operations. It seems preferable to use the superplasticity regime at relatively moderate homologous temperatures (less than 0.7) and high strain rates (on the order of 10-2 s-1), in which the equiaxed grain shape can be preserved with an insignificant change in its size. Under these conditions, staged (bell-shaped) tension curves are observed in experiments on uniaxial tension with access to the structural superplasticity regime for many alloys preliminarily prepared by severe plastic deformations. The latter is associated with the action and interaction of various physical mechanisms, the change in their roles during the deformation and evolution of defective material structures. The above factors are influenced by the initial temperature and strain rate conditions and characteristics of material structures after the pretreatment, in particular, grain shapes and sizes, fraction of high-angle boundaries, degree of recrystallization of the structure, presence of alloying additives that can form various phases in materials. This review attempts to systematize experimental data on superplasticity of aluminum alloys 1420 and 1421 with a focus on the main characteristics of material structures before and during the superplastic deformation tests, as well as its effect on the acting mechanisms. This will make it possible to form a more complete understanding the physical nature of deformation with a transition to structural superplasticity regimes for aluminum alloys and to develop a scenario for the action and interaction of mechanisms taking the influence of the evolving material structure into account. The above will be the concept basis for development the multilevel constitutive models of inelastic deformations of alloys to describe the material structure evolution and change in deformation regimes, which is necessary to improve superplastic forming technologies.