Evolution of microstructure and microtexture in fcc metals during high-pressure torsion

Abstract

Pure nickel and commercially pure (CP) aluminium were selected as model fcc materials for a detailed investigation of the experimental parameters influencing grain refinement and evolution of microstructure and microtexture during processing by highpressure torsion (HPT). Samples were examined after HPT using microhardness measurements, transmission electron microscopy and orientation imaging microscopy. Processing by HPT produces a grain size of 170 nm in pure Ni and~1 lm in CP aluminium. It is shown that homogeneous and equiaxed microstructures can be attained throughout the samples of nickel when using applied pressures of at least~6 GPa after 5 whole revolution. In CP aluminium, a homogeneous and equiaxed microstructure was achieved after 2 whole revolutions under an applied pressure of 1 GPa. For these conditions, the distributions of grain boundary misorientations are similar in the centre and at the periphery of the samples. It is shown that simple shear texture develops in fcc metals subjected to highpressure torsion. Some grain growth was detected at the periphery of the Al disk after 8 revolutions. The factors influencing the development of homogeneous microstructures in processing by HPT are discussed.

Microstructural Evolution in Pure Aluminum in the Early Stages of Processing by High-Pressure Torsion

“…24) A critical and important feature after N ¼ 1=8 in Fig. 1 is that, similar to the results reported for several different materials, [4][5][6][7][8][9][10][11][12][13]15,16,25,26) the measured hardness in the center of the disk is lower than in the surrounding areas. However, this result differs from earlier measurements on aluminum of 99.99% purity where the measured hardness was consistently higher in the centers of the disks after small numbers of whole revolutions.…”

“…The processing by HPT was conducted at room temperature (298 K) under an applied pressure, P, of 1.25 GPa with a constant rotational speed of 1 rpm using a quasiconstrained processing condition 8) and the processing facility described earlier. 14) Because of the low pressure used in these experiments and the requirement to rotate through relatively small fractions of whole revolutions, no lubricant was applied around the edges of the samples prior to HPT 6) in order to reduce any slippage between the sample and the die.…”

“…4) The gradual evolution towards a more homogeneous structure in disks of Ni was later demonstrated by constructing plots of the hardness values over the disk surfaces after processing through different numbers of torsional revolutions and different applied pressures and these plots demonstrated greater homogeneity at larger numbers of revolutions and higher applied pressures. 5) To date, several reports are now available on the variations of hardness across HPT disks for a range of metals including Al and aluminum alloys, [6][7][8] Cu, 9-11) Ni 12) and steel. 13) It is possible to achieve an excellent pictorial representation of the hardness distributions introduced in HPT by taking large numbers of individual hardness measurements following rectilinear grid patterns on the disk surfaces and then plotting these data in the form of color-coded contour maps.…”

Comparing Plastic Deformations Produced by HPT and ECAP Processes Using the Finite Element Analysis Method

“…7(a) to 7(d), some flash is formed in the die parting line, which increases as the process continues. According to the simulation results, and consistent with the existing experimental reports [3][4][5][6][7], as the process continues and the number of rotations increases, the plastic deformation at every point of the disc continuously increases. In Fig.…”

“…In general, a fine-grained material structure has many advantages over a coarse-grained one, and besides improving the mechanical properties, it helps raise the possibility of applying superplastic deep drawing processes to that material [1][2][3][4][5][6]. Such structures in polycrystalline materials are known as Ultra Fine Grained (UFG) [7,8].…”