Grain boundary sliding is an important deformation mechanism, and therefore its description is essential for modeling different technological processes of thermomechanical treatment, in particular the superplasticity forming of metallic materials. For this purpose, we have developed a three-level statistical crystal plasticity constitutive model of polycrystalline metals and alloys, which takes into account intragranular dislocation sliding, crystallite lattice rotation and grain boundary sliding. A key advantage of our model over the classical Taylor-type models is that it also includes a consideration of grain boundaries and possible changes in their mutual arrangement. The constitutive relations are defined in rate form and in current configuration, which makes it possible to use additive contributions of intragranular sliding and grain boundary sliding to the strain rate at the macrolevel. In describing grain boundary sliding, displacements along the grain boundaries are considered explicitly, and changes in the neighboring grains are taken into account. In addition, the transition from displacements to deformation (shear) characteristics is done for the macrolevel representative volume via averaging, and the grain boundary sliding submodel is attributed to a separate structural level. We have also analyzed the interaction between grain boundary sliding and intragranular inelastic deformation. The influx of intragranular dislocations into the boundary increases the number of defects in it and the boundary energy, and promotes grain boundary sliding. The constitutive equation for grain boundary sliding describes boundary smoothing caused by diffusion effects. The results of the numerical experiments are in good agreement with the known experimental data. The numerical simulation demonstrates that analysis of grain boundary sliding has a significant impact on the results, and the multilevel constitutive model proposed in this study can be used to describe different inelastic deformation regimes, including superplasticity and transitions between conventional plasticity and superplasticity.

Superplastic forming is promising for the manufacturing of complex-shaped metal parts and components. Superplastic conditions allow one to produce unwelded parts, to reduce the number of...

In recent decades, multilevel constitutive models of polycrystalline metals and alloys have been developed intensively. These models consider the structure of the material and physical mechanisms of deformation at the crystallite level explicitly and allow the description of changes in the internal structure and of physical and mechanical properties of the material during thermomechanical processing that depend on the state of the material. Models of micro- and nanomechanics (for example, elements of molecular dynamics, dislocation dynamics) can be used either as submodels in a multilevel model of materials or can be used for clarification of its parameters. In the present paper, we present a modification of multilevel models, which was previously developed by a team of authors, by taking into account the mechanism of grain boundary sliding and changes of the grain structure as a result of breaking and fragmentation of grains. The earlier proposed two-level model of inelastic deformation of polycrystalline metals is used as a reference model. In describing the grain boundary sliding, viscoplastic shears at grain boundaries are explicitly considered with account for changes in the critical shear stresses: an increase due to intrusion as a result of intercrystalline slip, and a decrease due to increasing in energy as a result of inflow of the intragranular dislocations and diffusion processes. In order to describe the fragmentation associated with changes in mutual orientations of crystallite parts, the couple stresses that cause the fragment rotation due to nonsimultaneous dislocation sliding in it and in the neighboring fragments, are considered. The process of crystallite breaking is described by analogy with the ductile fracture. For finding the most probable plane, an optimization problem is formulated: the plane and the direction where the largest shears are accumulated are determined with account for grain elongation in the direction perpendicular to the plane. The proposed model allows the description of "regular" inelastic deformation of polycrystalline materials and deformation under the conditions of structural superplasticity (with specific effects and the state of grain structure), the transitions between these conditions of deformation (including grain structure refinement on preparation of material to superplastic deformation). The test calculation results illustrating the capacities of the proposed model are presented.

Material behavior description in a wide range of thermomechanical effects is one of the topical areas in mathematical modeling. Inclusion of grain boundary sliding as an important mechanism of polycrystalline material deformation at elevated temperatures and predominant deformation mechanism of metals and alloys in structural superplasticity allows to simulate various deformation regimes and their transitions (including superplasticity regime with switch-on and switch-off regimes). The paper is devoted to description of grain boundary sliding in structure of two-level model, based on crystal plasticity, and relations for determination the contribution of this mechanism to inelastic deformation. Some results are presented concerning computational experiments of polycrystalline representative volume deformation using developed model.

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.

The paper deals with three-level model of polycrystal inelasticity based on crystal plasticity. This model allows to regard the most important inelastic deformation mechanisms of polycrystals including grain boundary sliding. The inflow of intragranular dislocations, changing of the boundary structure under realization of grain boundary sliding and diffusion processes are taken into account in equations for grain boundary sliding. Consistency conditions of constitutive relations at the different scale levels are used in constructing model. The results of computational experiments under uniaxial tension of a representative volume are obtained with developed model. The results show that grain boundary sliding is important and must be taking into account.