METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 29A, DECEMBER 1998—2957 Strain-Induced Grain Evolution in Polycrystalline Copper during Warm Deformation A. BELYAKOV, W. GAO, H. MIURA, and T. SAKAI The evolution mechanisms of dislocation microstructures and new grains at high strains of above 4 were studied by means of multiple compression of a polycrystalline copper (99.99 pct). Deformation was carried out by multipass compression with changing of the loading direction in 90 deg in each pass at temperatures of 473 K to 573 K (0.35 to 0.42 T m ) under a strain rate of 10 -3 s -1 . The flow stresses increase to a peak followed by a work softening accompanied mainly by dynamic recrys- tallization (DRX) at 523 K to 573 K. In contrast, the steady-state-like flow appears at 473K accom- panied with the development of fine grains at strains as high as 4.2. The relationship of flow stress to the new grain size evolved can be expressed by a power law function with a grain size exponent of about -0.35, which is different from -0.75 for high-temperature DRX at above 0.5 T m . At 473 K, misorientations of deformation-induced dislocation subboundaries increase with increasing strain, finally leading to the evolution of new grains. It is concluded that the dynamic grain formation at 473 K cannot result from DRX, but from the evolution of deformation-induced dislocation sub- boundaries with high misorientations and, concurrently, the operation of dynamic recovery. I. INTRODUCTION STUDIES on grain refinement of metallic materials by thermomechanical processing are of great practical impor- tance because of the improvement of mechanical properties of the products. One of the important mechanisms for new grain evolution is the dynamic recrystallization (DRX) oc- curring under hot working at temperatures above half of the melting point (0.5T m ). The mechanisms of structure evo- lution under DRX have been fairly clarified. [1–5] In contrast, those operating during deformation at lower temperatures below 0.5 T m have remained unresolved. It has been shown in previous works [6–11] that the structure evolution taking place around or below 0.5 T m is characterized by the for- mation of deformation-induced dislocation boundaries fol- lowed by new grain development at high strains. Hansen and coworkers [12,13,14] classified such deformation-induced subboundaries as dense dislocation walls (DDWs) and mi- crobands. Rybin [15] termed such structures the fragmented structure in which the fragment subboundaries have high misorientations. The dynamic evolution of new grains un- der ambient temperature is sometimes termed a low tem- perature 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 T m . [17] The new grains are evolved by a bulging mechanism taking place at ser- rated grain boundaries that usually operates at high tem- peratures, i.e., above 0.5T m . In addition, the DDWs evolved near grain boundaries can assist the nucleation in the sep- aration 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 tak- ing 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. limited workability of most metallic materials at low-to- moderate 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 microstruc- tural changes associated with new grain formation at se- verely high strains above 4 and at temperatures below 0.5T m . 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 mul- tiple forging. [10] The dynamic evolution of dislocation mi- crostructures and new grains and the mechanisms operating under warm deformation are discussed in detail compared with the DRX at high temperatures. II. EXPERIMENTAL PROCEDURE The testing machine, equipped with a water quenching apparatus [17] , was enabled true strain rates constant. The samples of a polycrystalline copper of 99.99 pct purity were previously strained to 1.2 at 623 K and at 10 -3 s -1 followed by quenching in water. They had a DRX microstructure with an average grain size of about 11 m. For subsequent compressions, the deformed samples were machined in a rectangular shape with the dimension ratio of 1.5: 1.22: 1. This ratio did not change during multipass compression 0.3 to 0.45 in each pass. The starting dimension of samples was about 9: 7: 6 mm. Multipass compression tests were carried out with changing of the loading direction in 90 deg in each pass at 473 to 573K (0.35 to 0.42 T m ) under a strain rate of 10 -3 s -1 . The deformed samples were quenched in water, machined, polished, and then reheated to a test tem- perature within 0.6 to 0.9 ks in each compression. The metallographic analysis was carried out using an op-