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

Abstract

Measurements were taken to evaluate the evolution of homogeneity in disks of high-purity aluminum in the early stages of processing by high-pressure torsion. The results demonstrate that samples processed through 1/4 or more whole revolutions exhibit microhardness values which are generally higher in the centers of the disks than at the edges whereas after 1/8 turn the hardness is higher at the edge than in the center. It is shown that all of the hardness measurements are mutually consistent and they scatter around a unique curve when plotted against the equivalent strain. The measurements of hardness are supplemented by microstructural observations which provide evidence of a gradual evolution in the microstructure with increasing strain from an initial formation of subgrains to an array of ultrafine grains separated by boundaries having high angles of misorientation.

Microstructure and Hardness Evolution in Magnesium Processed by HPT

“…Therefore, a large fraction of low angle boundaries are expected in the early stage of HPT processing and a gradual transition towards a larger fraction of high angle boundaries is expected at later stages. This has been confirmed by experiments in aluminium 45,46 and in pure iron 47 . However, the mechanism of grain refinement in magnesium and its alloys differs from f.c.c.…”

Synergetic strengthening far beyond rule of mixtures in gradient structured aluminum rod

“…1 (e-f) shows the subgrain boundary size approaches 3 μm. These subgrain boundaries are typical of the early stages of SMAT processing, and have been observed in samples subjected to small revolutions of High Pressure Torsion (HPT) [10,30]. However, the degree of grain refinement is quite mild at the surface of the SMAT sample which correlates well to the mild improvement in hardness at the surface, as seen in Fig.…”

Effect of Initial Annealing Temperature on Microstructural Development and Microhardness in High‐Purity Copper Processed by High‐Pressure Torsion

“…For example, in the early stage of straining high-purity (99.99%) Al exhibits a higher hardness at the center of the disk and this value decreases with increasing straining, until it reaches a homogeneous high hardness compared to the annealed condition. [31][32][33] Similar behavior was also reported in high-purity (99.99%) Zn [34] and pure (99.9%) Mg. [35] By contrast, some metals such as the Zn-22% Al eutectoid alloy and the Pb-62% Sn eutectic alloy show different behavior where the hardness decreases significantly with increasing straining and ultimately reaches a homogeneous or saturation hardness, that is, lower than in the annealed condition. [36] These three different types of hardening behavior may be effectively illustrated as in Figure 1, [37] where the separate models delineate hardening without recovery, hardening with subsequent recovery, and an overall weakening.…”

“…[30] In the second model, the strain hardens rapidly in the early stages of straining, then softens with rapid recovery to reach a lower saturation microhardness value. This model applies to some high purity metals such as Al, [31][32][33] Mg, [35] and Zn. [34] As shown in Figure 1a and b, both of these models have final saturation microhardness values, which lie above the annealed or as-received values.…”