The significance of strain reversals during processing by high-pressure torsion

In processing by high-pressure torsion (HPT), it is possible to continuously strain in a forward direction in monotonic HPT (m-HPT) or to reverse the direction of straining in cyclic HPT (c-HPT). Experiments were conducted to compare the effects of torsionally straining high-purity Al using m-HPT and c-HPT through up to 4 turns at room temperature under a pressure of 6.0 GPa. The appearance of the microstructural damage was examined on the surface of each disk and values of the Vickers microhardness were recorded both along disk diameters and over the total surfaces to permit the construction of color-coded contour maps. Although the inhomogeneities in the microstructures decreased with increasing numbers of turns in m-HPT, the experiments show that microstructural evolution is slower when using c-HPT. It is concluded that reversals in the direction of straining during HPT processing provide an opportunity for manipulating the hardness values attained in HPT.