||Equal channel angular extrusion (ECAE) has attracted a substantial attention for it provides the opportunity to introduce large plastic strain into the material in the bulk form. Both die angles and processing routes have been recognized as the important parameters in applying ECAE to fabricate ultrafine-grained materials. Unfortunately, studies of different group provided inconsistent conclusions on the effectiveness of processing routes, which are believed to be due to the incomplete microstructural information obtained in each investigation. In the present work, quantitative analysis of the microstructure developed by different processing conditions were conducted using transmission electron microscopy (TEM), in which the morphology, size, and shape of subgrains as well as boundary misorientation were fully characterized.|
A commercial pure aluminum (AA 1050) was deformed by ECAE to strain of ~ 8 with different routes (A, Bc and C, in terms of reorientation angle 0o, 90o, and 180o respectively of the billet between two extrusion passes) and die angles. The results show that the effectiveness of high angle boundary (HAB) formation is in the sequence of route A≒Bc>C. However, in terms of grain refinement, the effectiveness is in the order of route Bc>A>C. In addition, route A produces subgrains with the most elongated shape, while route Bc produces subgrains with the most equiaxed shape. These results may be attributed to the different shear pattern introduced in each route.
ECAE die angle determines both the strain per pass and the shear plane orientation. In route C, the shear is maintained in the same plane and the effect of strain per pass can be studied. With route C, both the 90o and 120o die produce microstructure with similar HAB proportions, but they result in different arrangement of HABs. The 120o die produces subgrains with larger size and higher aspect ratio than the 90o die does in route C. Generally speaking, for the die angle range studied, the different values of strain per pass used in ECAE mainly affect the morphology of the subgrains. On the other hand, the effect of die angle is weakened with route Bc as compared to route C, which may be attributed to the intersection of shear planes involved in route Bc.