Strain-induced grain evolution in polycrystalline copper during warm deformation
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I.
INTRODUCTION
STUDIES on grain refinement of metallic materials by thermomechanical processing are of great practical importance because of the improvement of mechanical properties of the products. One of the important mechanisms for new grain evolution is the dynamic recrystallization (DRX) occurring under hot working at temperatures above half of the melting point (0.5Tm). The mechanisms of structure evolution under DRX have been fairly clarified.[1–5] In contrast, those operating during deformation at lower temperatures below 0.5 Tm have remained unresolved. It has been shown in previous works[6–11] that the structure evolution taking place around or below 0.5 Tm is characterized by the formation of deformation-induced dislocation boundaries followed by new grain development at high strains. Hansen and coworkers[12,13,14] classified such deformation-induced subboundaries as dense dislocation walls (DDWs) and microbands. Rybin[15] termed such structures the fragmented structure in which the fragment subboundaries have high misorientations. The dynamic evolution of new grains under ambient temperature is sometimes termed a low temperature DRX.[16] Recently, the authors have studied the DDWs formation followed by the DRX appearance during compression of a copper at 0.4 to 0.5 Tm.[17] The new grains are evolved by a bulging mechanism taking place at serrated grain boundaries that usually operates at high temperatures, i.e., above 0.5Tm. In addition, the DDWs evolved near grain boundaries can assist the nucleation in the separation of nuclei from parent grains. The DRX, however, hardly takes place at strains of around 1.2 below 523 K. Questions concerning the microstructure formation taking place at high strains are often complicated by some A. BELYAKOV, Postdoctoral Fellow, W. GAO, Graduate Student, H. MIURA,Associate Professor, and T. SAKAI, Professor, are with the Department of Mechanical and Control Engineering, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan. Manuscript submitted February 5, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
limited workability of most metallic materials at low-tomoderate temperatures. More recently, high strains have been applied by some deformation modes involving shear strain, such as torsion under high hydrostatic pressure and equal channel angular pressing.[6–9,15,16] These methods, however, have some limitations for scientific analyses and industrial application. The main difficulties are associated with the calculation of both true strain and true flow stress during deformation. The aim of the present work is to study the microstructural changes associated with new grain formation at severely high strains above 4 and at temperatures below 0.5Tm. For this purpose, multiple compression tests with changing of the loading direction in 90 deg were carried out on a polycrystalline pure copper. This is a kind of multiple forging.[10] The dynamic evolution of dislocation microstructures and new grains and the mechanisms operating under warm deformation a
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