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 purity aluminum with an initial grain size of similar to 1.0 mm. The results show that, under ECA pressing conditions giving a strain of similar to 1.05 on each passage through the die, the microstructure is reasonably homogeneous after a single pressing and consists of parallel bands of elongated subgrains, having an average length of similar to 4 mu m, and these subgrains are further divided by boundaries with very low angles of misorientation. Repetitive pressings were conducted on the same samples, up to a total of 10 passages through the die, with the samples pressed either without relation (route A) or after rotating through 180 degrees between each pressing (route C). It is demonstrated that the misorientations of the subgrain boundaries increase with repetitive pressings until ultimately both routes lead to a similar equiaxed ultra-fine grain size of similar to 1 mu m after 10 pressings, but the microstructural evolution is enhanced using route C where there is a more rapid transition into an array of high angle grain boundaries. The results suggest that, at least for high purity aluminum, an ultra-fine microstructure close to optimum may be obtained after only 4 pressings provided the sample is rotated through 180 degrees between each pressing. (C) 1997 Acta Metallurgica Inc.