An investigation of microstructure and grain-boundary evolution during ECA pressing of pure aluminum

Abstract

High-purity aluminum (99.99 pet) was processed by equal-channel angular pressing (ECAP) at room temperature through a die with a 90 deg angle between the die channels. Samples were examined by transmission electron microscopy (TEM) and orientation imaging microscopy (OIM) methods after one, four, and 12 passes through the die. Repetitively pressed samples were rotated by 90 deg in the same sense between successive pressing operations (route Be). After one pressing, TEM showed a subgrain structure which was elongated in the shearing direction. Corresponding OIM data illustrated an inhomogeneous microstructure in which bandlike features were also aligned with the shearing direction. The lattice orientation varied from location to location in the material. The boundary disorientation distribution determined from the OIM data exhibited a peak at 2 to 5 deg, in agreement with a predominance of subgrains in the microstructure. After four pressings, the microstructure data obtained by TEM and OIM were mutually consistent. The disorientation data revealed a decrease in the population of 2 to 5 deg boundaries accompanied by an overall upward shift in the distribution. Two orientations were generally apparent in the texture, although specific orientations varied with location. Often, a (111) orientation tended to align with the shear direction. Following 12 EGA passes, the grain size was reduced further to about 1.0 fim. The populations of high-angle boundaries (^15 deg) increased in the disorientation distribution. A texture characteristic of shear deformation of fee metals became apparent, although the orientations and particular components varied with location. Microstructural refinement during severe straining includes the development of large fractions of high-angle boundaries.

The microstructural characteristics of ultrafine-grained nickel

“…There is also evidence in all three distributions for the presence of two peaks at the higher angles, with these peaks lying in the vicinity of misorientation angles of ∼35 • and ∼60 • , respectively. A similar effect was reported in experiments on pure Al processed by ECAP [33] and the second peak corresponds to the twin misorientation.…”

The effects of severe plastic deformation on some properties relevant to Ti implants

“…For instance 7 it was shown that grain refinement mainly takes place after the first pass, saturation being reached by the sixth and eighth passes. This confirms previous findings by Langdon 8 which were obtained on Al samples. It is important to point out that this effect of the first pass on grain size appears to be a fundamental occurrence, not confined to this or that material.…”

Effect of strain path on evolution of deformation bands during ECAP of pure aluminum

“…Full details of the materials and processes of this work have been given previously [12,13]. Briefly, the pure aluminum material (99.99%) was swaged to a rod 10 mm in diameter and 60 mm in length for pressing in the 90 • die.…”

“…The symmetry of an ECAP billet is monoclinic, and so the senses of the coordinate axes in the flow plane were carefully ascertained. Transmission electron microscopy (TEM) foil preparation has been described previously [13]. Fig.…”