Scripta METALLURGICA Vol. 19, pp. 73-78, 1985 Pergamon Press Ltd. Printed in the U.S.A. All rights reserved MICROSTRUCTURAL EVOLUTION IN AL DEFORMED TO STRAINS OF 60 AT 400°C H.J. McQueen*, O. Knustad, N. Ryum and J.K. Solberg Physical Metallurgy Inst., Norwegian Institute of Technology, Trondheim *Mech. Eng., Concordia University, Montreal, H3G IM8, Canada {Received October I, 1984) ~Revised October 18, 1984) INTRODUCTION Deformation of aluminum up to strains of 4 at elevated temperature results in a steady state regime in which the substructure remains equiaxed and approximately constant in dimension, misorientation and dislocation density (I-5). Recent observations of specimens strained to 40 showed that the substructure consisted of crystallites of normal size but with a much higher frequency of high angle boundaries than at 4 (6,7). It was therefore suggested that the Al had undergone dynamic recrystallization, either discontinuous or continuous (gradual increase in sub-boundary misorientation)(6,7). The present research was undertaken to clarify the micro- structural evolution by following the change in grain shape and in size and misorientation of subgrains as ~ is deformed at 400°C to a series of strains as high as 60. In torsion, the equiaxed surface grains are sheared into spirals appearing on a tangential section as elongated and reduced in axial thickness to dn~(= dQo/J'~ ~). Two initial grain sizes (dan) were used so that at a strain where da~ of the ~alle~ grains approaches the subgrain diam~Zer d~, that of the larger is still greateC~y an order of magnitude. EXPERIMENTAL PROCEDURE Aluminum (99.7%) containing 0.18% Fe, 0.07% Si was annealed to grain sizes of 0.1 and 2 mm. The torsion specimens had a gage section 20 mm long and 10 mm diameter and shoulders of 16 mm diameter. The specimens were heated to 400°C for 5 minutes before and then during deforma- tion by induction and were quenched after deformation. They were deformed at a strain rate of 0.2 s -I on a computer controlled, hydraulically powered torsion machine to equivalent surface strains of I, 3, 10, 20, 40 and 60. The equivalent stress B and strain E were calculated by ~tandard methods (8). For metallography, in normal and polarized light, tangential flats 2 mm wide were ground mechanically, electropolished and anodized (9). The specimens were positioned to expose the elongated grains as narrow bands of bright almost uniform shade. Upon rotation of 45° into the extinction condition, the subgrains of slightly different orientation stood out with high contrast (10). Specimens were prepared for TEM from tangential slices about 0.1 mm thick by double jet thinning. They were examined at 160 kV at 0 ° t i l t to observe subgrain sizes and diffraction patterns but individual sub-boundaries were observed at various tilts. Electropolished tangential flats were examined by channelling mode contrast in SEM. X-ray pole figures were obtained from the outer annular shells of several specimens. RESULTS The flow curves for the large and fine grained metals (Fig. i) are essentially similar. They rise to the steady state regime by a strain of about i but the stress declines from about 18 MPa at ~ = 3 to 12 MPa at 60. At strains of I and 3 the grains, as predicted from the geom- etry, have elongated in the tangential direction and narrowed in the axial direction (Fig. 2). The grains have broken up into patches of different shades indicative of subgrain formation (10). The boundaries have developed serrations which result in the subgrains of one grain pro- truding into a neighbour (1,11). The original grains are completely distinguishable for either size since the variations in shading and the boundary ripples are limited. 73 0036-9748/85 $3.00 + .00 Copyright {c} 1985 Pergamon Press Ltd.