The Requirements for Superplasticity with an Emphasis on Magnesium Alloys

Abstract

Superplasticity is defined formally as a tensile elongation of at least 400% and a strain rate sensitivity of %0.5. It is now well established that superplasticity occurs only in materials having very small grain sizes, typically less than %10 mm. These small grains may be achieved in two different ways in magnesium alloys: either by thermomechanical processing or through the occurrence of recrystallization in the early stages of deformation in alloys tested with coarse grain sizes.

Principles and Technical Parameters of High‐Pressure Torsion

“…However, the following discussion suggests that these results are universal. It is worthwhile noting that shear-strain patterns similar to those reported here were also observed in different materials that should have different crystallographic textures, including evidence for single swirls in pure Al [34,57], shear vortices in a Cu-Ag alloy [44] and a pearlitic steel [50], and a reorientation of phase domains along the torsional direction at the peripheries of disks of the Zn-Al eutectoid alloy [49]. In fact, the two phases were strain hardened at almost the same rate.…”

“…Earlier experiments noted a gradual evolution towards homogeneity in HPT testing, but the experiments were discontinued at 5 turns so that a full homogenization was not achieved [29,57]. First, the measurements show that, for any selected experimental condition of HPT processing, the distribution of microhardness values throughout disks of high-purity aluminum seems to be independent of the plane of sectioning.…”

Processing Magnesium and Its Alloys by High‐Pressure Torsion: An Overview

“…Superplasticity denotes the very high elongations which may be achieved in many metals when testing in tension at elevated temperatures. Recent reports documented a formal definition of superplasticity as the ability of a material to exhibit an elongation of at least 400% with a strain rate sensitivity of ≈0.5. It is now accepted that superplasticity is observed in fine‐grained metals deformed at high temperatures under conditions such that grain boundary sliding is the rate controlling mechanism .…”

Achieving room-temperature superplasticity in an ultrafine-grained Zn-22% Al alloy

“…Finally, it is worthwhile to mention that, although the influence may not be as critical as the threshold stress, the lower value of m is also caused in part by the level of GBS attributed to the microstructural homogeneity of Al-and Zn-rich phases in the sample. Early GBS measurement by TEM showed there is different levels of contributions of sliding to the entire plastic strain at individual interfaces during high temperature deformation and a maximum contribution was observed on the Zn-Zn interfaces and slightly less on the Zn-Al interfaces while the Al-Al interfaces exhibited a minimum contribution of GBS in a conventional Zn-22% Al alloy [51] and after ECAP [17,18,20]. In a very recent report, room-temperature plastic deformation by nanoindentation examinations revealed the homogeneity in distributions of the Zn and Al phases is strongly correlated with the change in the m value [40].…”

“…There have been numerous trials of grain refinement for improving superplastic properties of Zn-22 % Al alloys by means of the techniques of cross-channel extrusion (CCE) [11], equal-channel angular pressing (ECAP) [12], friction stir processing (FSP) [13], high-pressure torsion (HPT) [14] and thermo-mechanical controlling process (TMCP) [15]. These processing techniques led to refine grains of Zn-22 % Al alloys to the submicrometer range so that the alloys exhibited superplastic characteristics including excellent ductility at elevated temperature [16][17][18][19][20][21] and superplastic flow behavior even at room temperature [22][23][24][25][26][27][28][29][30][31]. Recent reports examined the capability of room-temperature superplastic properties in Zn-22 % Al alloys as tuned mass dampers to reduce seismic vibrations in building structures [32,33].…”