Materials Science and Engineering A 410–411 (2005) 213–216 Strain hardening during high pressure torsion deformation F. Wetscher a,b,∗ , A. Vorhauer a,b , R. Pippan a,b a Erich Schmid Institute of Material Sciences, Austrian Academy of Science, Jahnstraße 12, A-8700 Leoben, Austria b Christian Doppler Laboratory for Local Analysis of Deformation and Fracture, Jahnstraße 12, A-8700 Leoben, Austria Received in revised form 18 March 2005 Abstract Severe plastic deformation has been applied to Armco iron and copper by means of high pressure torsion. The evolution of the shear stress during deformation was measured under different hydrostatic pressures. By applying a simple model for a strain hardening, the shear stress–shear strain curves are fitted and the influence of the hydrostatic pressure is studied. These results are compared to microhardness measurements, tensile strength and the microstructural evolution. © 2005 Elsevier B.V. All rights reserved. Keywords: High pressure torsion; Strain hardening; Hydrostatic pressure; Tensile test 1. Introduction Recent investigations [1–3] have shown for various materials deformed by different severe plastic deformation (SPD) meth- ods like high pressure torsion (HPT) or equal channel angular pressing (ECAP), a saturation of both the microstructural refine- ment and the yield strength. The question always arises, are these changes already present during the deformation or are they the result of post-deformation recovery or recrystallisation. If there are no changes of the developed microstructure after the defor- mation, then in situ measurements of the flow stress during the deformation should reflect the characteristics of the results from mechanical tests after the deformation. The aim of this study is to compare the mechanical behaviour during and after the defor- mation by HPT. The in situ measurement permits, furthermore, to estimate the effect of the hydrostatic pressure on the flow stress during HPT. 2. Experimental 2.1. Test equipment Fig. 1 shows a sketch of our tool. The sample lies inside the cavity, in the gap between the two anvils a burr is formed during ∗ Corresponding author. Tel.: +43 3842 804310; fax: +43 3842 804116. E-mail address: wetscher@unileoben.ac.at (F. Wetscher). the compressive loading. The torque is measured directly above the sample on one of the anvils by means of strain gauges. This measured torque M consists of the torque necessary to deform the sample M d plus a torque arising in the area of the burr M b . 2.2. Samples To investigate the influence of the hydrostatic pressure on flow stress of severely plastic deformed samples during and after deformation, Armco iron and pure copper have been deformed by HPT at different hydrostatic pressures at room temperature and the torque versus angle of twist have been measured in situ. The hydrostatic pressure was varied between 800 MPa and 7 GPa for copper and between 1.6 GPa and 7 GPa for Armco iron. The shear strain for a number of turns, n, can be calculated by γ (r) = 2πnr t (1) The samples for HPT had a diameter of 8 mm and a thickness, t, of 0.8 mm, the number of turns per minute was 0.2. Samples were deformed to strains γ of 60, 120 and 180 in the case of Armco iron and to 220 in the case of copper at a radius r of 2 mm. From each of these samples, two sub-sized tensile test spec- imens were produced. The longitudinal axis of the tensile specimens was situated at a radius of 2 mm of the HPT sam- 0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.08.027