AN INVESTIGATION OF MICROSTRUCTURAL EVOLUTION DURING EQUAL-CHANNEL ANGULAR PRESSING

Experiments were conducted to investigate the development of an ultra- fine grain size during equal-channel angular (EGA) pressing of high pu rity aluminum with an initial grain size of similar to 1.0 mm. The res ults show that, under ECA pressing conditions giving a strain of simil ar to 1.05 on each passage through the die, the microstructure is reas onably homogeneous after a single pressing and consists of parallel ba nds of elongated subgrains, having an average length of similar to 4 m u m, and these subgrains are further divided by boundaries with very l ow angles of misorientation. Repetitive pressings were conducted on th e same samples, up to a total of 10 passages through the die, with the samples pressed either without relation (route A) or after rotating t hrough 180 degrees between each pressing (route C). It is demonstrated that the misorientations of the subgrain boundaries increase with rep etitive pressings until ultimately both routes lead to a similar equia xed ultra-fine grain size of similar to 1 mu m after 10 pressings, but the microstructural evolution is enhanced using route C where there i s a more rapid transition into an array of high angle grain boundaries . The results suggest that, at least for high purity aluminum, an ultr a-fine microstructure close to optimum may be obtained after only 4 pr essings provided the sample is rotated through 180 degrees between eac h pressing. (C) 1997 Acta Metallurgica Inc.