Inhomogeneous work hardening in multiphase and precipitation strengthened materials

Project manager: Paulina Lisiecka- Graca

Project financing: National Science Centre Poland

Project number: 013/09/N/ST/00250

The better understanding of the phenomena accompanying real metal forming processes was a basic motivation for the research work focused on the role of strain path changes with particular attention paid to the dislocation strengthening mechanisms. Evolution of microstructure of the deformed material and observed effects in the dislocation systems are directly connected to the conditions of deformation processes. An accurate prediction of the microstructure development resulting from complex deformation modes is of paramount importance. Additionally, the increasing demand for materials with strictly defined and precisely controlled properties needs proper selection of the strengthening mechanisms. These types of the materials belong to the group of HSLA steels, steel whose production continues to grow.

Hence, the main purpose of the project was to propose a comprehensive rheological model of structural materials strengthened by dispersed second phase particles, with particular emphasis on the strengthening inhomogeneity resulting from complex history of deformation. The main goals of the project were: (1) proper justification of the existing work hardening models and (2) detailed microstructural analysis of the selected materials.

In the performed investigations, a series of tests with cyclic strain path changes were conducted Additionally, the complex metal forming processes including Accumulated Angular Drawing (AAD), Linear Wire Drawing (WD) and Wire Flattening (WF) processes were carried out. The experiments were supported by the full- field strain measurement methods of Digital Image Correlation. Next, based on the experimental data, numerical simulation and validation of the selected work hardening models were performed. The investigations were focused on the physically based models which include dislocation density and backstress as an internal variables. Furthermore, the rheological description of the studied materials including dislocation density in the cell walls and cell interiors was proposed. The proposed solution was validated based on data from the experimental part of the project.

The obtained results allow for better understanding and prediction of mechanical response of the materials subjected to metal forming processes with complex deformation modes. The developed models can be successfully adopted in the prediction process of mechanical behavior of structural materials under various loading conditions.