FEM simulation of large deformation of copper in the quasi-constrain high-pressure-torsion setup

High pressure-torsion (HPT) technique is widely used to introduce severe plastic deformation in order to obtain ultrafine-grained materials with improved properties or to enhance the phase transformation which may result in an appearance of new phases. The behavior of the copper sample under pressure and torsion in a quasi-constraint configuration is studied using finite element method (FEM). A complete system of equations for small elastic and large plastic deformations and rotations is presented. Contact friction conditions include combined Coulomb and plastic sliding. The evolution of the distribution of fields of components of the stress tensor, hydrostatic pressure, and equivalent plastic strain are studied. The effects of the critical strain for saturation of strain hardening, m, and different friction conditions are investigated. Strong heterogeneity of the plastic strain both along the radius and thickness of the sample is found, which is not well described by known approximate expressions. Torque-rotation angle response of the sample is not sensitive to the value of m but strongly depends on the friction coefficient between anvil and flash. The main conclusion is that the value of m obtained in literature from HPT (from 4.8 to 20) is highly overestimated and m = 1.57 obtained in a homogeneous compression test should be used.