This paper considers the influence of subgrain coalescence at initial high-angle boundaries on the initiation and growth of recrystallization nuclei (subgrains) under thermomechanical treatment. With certain processing regimes, adjacent subgrains in polycrystalline materials can be assembled into clusters during coalescence. Subgrain clusters at high-angle boundaries are the preferred potential nuclei of recrystallization. Coalescence is one of the dynamic recovery mechanisms, a competing process to recrystallization. When intensive coalescence develops on both sides of the grain boundary, recrystallization slows down or even stops. The problem formulated is solved using a multilevel modeling apparatus with internal variables. Application of the statistical multilevel model modified to take into account the local interaction between crystallites makes it possible to explicitly describe dynamic recrystallization and recovery. The results of modeling the behavior of a copper sample are presented and the effects of temperature, deformation velocity and subgrain structure on the formation and growth of recrystallization nuclei at arbitrary and special grain boundaries during coalescence are analyzed.

This study employs a multilevel modeling approach to describe the deformation of specimens made from austenitic Wire Arc Additive Manufactured (WAAM) steel 308LSi. Two WAAM processing modes were investigated: (1) the Cold Metal Transfer (CMT) method and (2) Cold Metal Transfer combined with interlayer deformation strengthening (hammer peening/forging). Test specimens were cut from the deposited walls at 0° and 90° relative to the deposition direction. The grain and dendritic structures of the specimens were analyzed using optical stereomicroscopy. A statistical multilevel model has been developed, accounting for the features of the grain-dendritic and defect structures under various technological deposition modes. Parameter identification and model verification were conducted based on experimental data from uniaxial tensile tests of 308LSi steel specimens. The maximum deviation of the numerical results from the experimental data during the identification stage under uniaxial tensile loading did not exceed 3%, and during the verification stage it did not exceed 10%; the overall mean deviation did not exceed 1% for the identification stage and 2% for the verification stage. The model effectively captured the anisotropic mechanical behavior of WAAM-processed samples. The maximum calculated yield strength 360 MPa was obtained for specimens cut at an angle of 45°, while the minimum value 331 MPa was observed for vertically oriented specimens. Specimens subjected to interlayer forging (hammer peening) exhibited isotropic material properties. Explicit multilevel modeling, incorporating the presence of MnO oxide inclusions located within the austenite matrix, was performed. The results showed good correlation with experimental data and confirmed the localization of fatigue cracks at the phase boundary-matrix-oxide interface.

The problem of identifying the initial critical resolved shear stresses (CRSS) of slip systems is considered in the framework of a two-level statistical model of inelastic deformation. We propose a three-stage method for determining critical stresses. The problem is solved on the basis of an experimental data set that includes the ‘‘\({{{\sigma}}}-{{{\varepsilon}}}\)’’ diagrams for a polycrystal (subjected to uniaxial loading at different temperatures) and the anisotropic elastic moduli of single crystals. Analysis of the experimental loading diagrams makes it possible to determine the yield strength at the macrolevel. Using the two-level model, an optimization problem is formulated to determine the CRSS values corresponding to the experimentally established yield strength of a polycrystalline macrosample at a fixed temperature. The obtained CRSS are approximated by an exponential temperature dependence; there is good agreement between this approximation and the considered critical stresses. The problem parameters are determined for a copper polycrystal. The obtained results can be used to investigate inelastic deformation by means of multilevel models, in which the activation of slip systems is determined by the the Schmid law.

A hybrid multilevel model for a representative volume has been developed to describe the thermomechanical treatment of materials throughout the manufacturing–operation cycle. The model serves as an efficient tool for the digital design of functional materials and products. It quantitatively evaluates defects and grain structure formed under various thermomechanical treatment modes using statistical distributions of internal variables. These data are subsequently employed to construct polyhedral grain structure and to transfer defect-related information, expressed by using critical shear stresses and crystallographic lattice orientations in operational simulations. The model was calibrated, tested for Inconel 718 alloy, and applied to design alloy blanks during rolling and annealing, as well as to the analysis of low-cycle fatigue during operation.