Unconfined Twist: a Simple Method to Prepare Ultrafine Grained Metallic Materials

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Unconfined Twist: a Simple Method to Prepare Ultrafine Grained Metallic Materials Yonghao Zhao, Xiaozhou Liao and Yuntian T. Zhu* Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM-87545, U.S.A. ABSTRACT A new simple method – unconfined twist was employed to prepare ultrafine grained (UFG) Fe wire. A coarse grained (CG) Fe wire with a diameter of 0.85 mm was fixed at one end, and twisted at the other end. After maximum twist before fracture, in the cross-sectional plane, concentrically deformed layers with a width of several micrometers formed surrounding the center axis of the wire. The near-surface deformed layers consist of lamella grains with a width in submicrometer range. In the longitudinal plane, deformed bands (with a width of several micrometers) formed uniformly, which were composed of lamella crystallites (with a width in submicrometer range). The tensile yield strength and ultimate strength of the twisted Fe wire are increased by about 150 % and 100 % compared with the values of its CG counterpart. INTRODUCTION Ultra-fine grained (UFG) metals and alloys have been the subject of considerable research in recent years. Such interest has been spurred by the recognition that these materials possess some appealing mechanical properties, such as high yield and fracture strengths, superior wear resistance, increased resistance to tribological and environmentally-resisted damage, increased strength and/or ductility with increasing strain rate and potential for enhanced superplastic deformation at lower temperature and higher strain rates. Nevertheless, the industrial application of the UFG materials has not been realized yet because of the difficulty to fabricate them into dense and bulk dimensions. Among the numerous UFG material synthesis methods, such as mechanical alloying and compaction [1,2], inert-gas condensation and consolidation [3], electrodeposition [4], crystallization of amorphous solids [5,6] and severe plastic deformation (SPD) [7], the SPD method, such as equal channel angular pressing (ECAP) [8,9] is especially attractive because it can economically produce relatively large bulk dense UFG samples without changing their cross-sectional dimensions. Another SPD method, high-pressure torsion (HPT) [7], can effectively refine coarse grains into several tens nanometers, but it can only process very thin disk (about 1 mm thick). Moreover, both HPT and ECAP methods need very high pressure (up to several GPa). Such high pressure brought difficulties for the industrial applications of these SPD preparations. Therefore, simpler SPD method would be attractive. In the present work, we developed a new simple SPD method – unconfined twist to process UFG metallic materials. In this method, an initial rod-shaped sample was fixed at one end, and was twisted at the other end. During the twist, severe plastic deformation occurred within the sample and formed the UFG structures. The twisted UFG sample is found to have greatly enhanced mechanical strength compared with