Hardening and Softening Behavior by Cyclic High-Pressure Torsion

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INTRODUCTION

FOR the past several years, severe plastic deformation (SPD) is an important field in material science.[1] With the method of SPD, it is possible to produce ultrafine-grained bulk materials with outstanding mechanical properties in terms of ductility and tensile strength.[2,3] For a further use of bulk nanostructured materials in industrial applications, it will be necessary to apply additional forming operations such as rolling or extrusion. Therefore, the very homogeneous ultrafinegrained microstructure will be further deformed. In most cases, this will comprise a deformation along a new strain path. To investigate the effect of a further deformation of ultrafine-grained material, a combination of monotonic and cyclic high-pressure torsion was applied to pure nickel. The aim of this study is to investigate the effect of a cyclic deformation of a ultrafine-grained structure on microstructure and mechanical strength and to determine what happens at the very beginning of a deformation along a reversed strain path. II.

EXPERIMENTAL

The material used in this study was pure nickel (99.99 pct), and the initial grain size was 50 lm. Disks of the material with a diameter of 8 mm and an initial thickness of 0.8 mm were deformed by a combination of conventional high-pressure torsion (HPT) and cyclic high-pressure torsion (CHPT); the details of this technique are given elsewhere.[4] The total deformations at a given radius r for each deformation mode were calculated by Eq. [1] (HPT) and Eq. [2] (CHPT), respectively. In these equations, t is the thickness of the sample (average value of the thickness before and after the F. WETSCHER and R. PIPPAN are with the Erich Schmid Institute of Materials Science of the Austrian Academy of Sciences and the Christian Doppler Laboratory of Local Analysis of Deformation and Fracture, Jahnstrasse 12, 8700 Leoben, Austria. Contact e-mail: [email protected] Manuscript submitted April 2, 2008. Article published online October 17, 2009 3258—VOLUME 40A, DECEMBER 2009

deformation by torsion), n is the number of turns, and N is the number of cycles. 2prn eeq ¼ pffiffiffi t 3

½1

2prn eeq;total ¼ N pffiffiffi ¼ NDe t 3

½2

The number of turns n was calculated to get a strain increment De of 0.5, 1, and 2 for CHPT experiments and 32 for HPT experiments, respectively, at a radius r = 3 mm, where the structural analyses were performed. This radius was chosen to exclude influences of the edge of the tool, where a variation in strain in the axial direction may occur. The number of cycles N for the CHPT experiments was chosen to obtain total strains of 32 at r = 3 mm. Two sets of experiments were performed. In one set, the sequence of experiments was HPT-CHPT-HPT-CHPT, and in the other set, the sequence was CHPT-HPT-CHPT-HPT. This was done with all De. The experiments were performed at room temperature with a rotation speed of 0.2 turns per minute, for both HPT and CHPT. The torque during the deformation was measured by means of strain gages and is proportional to the shear stress of the mater

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