The Effect of High‐Pressure Torsion on Microstructure, Hardness and Corrosion Behavior for Pure Magnesium and Different Magnesium Alloys

Abstract

Severe plastic deformation by high pressure torsion (HPT) is used to process and refine the grain structure of commercial purity magnesium and AZ31, AZ91, and ZK60 magnesium alloys. Transmission electron microscopy shows that the microstructure of pure magnesium is characterized by a bi-modal grain size distribution with grains in the range of a few microns and ultrafine grains after HPT, whereas the magnesium alloys display a homogeneous ultrafine grain structure after processing. X ray diffraction analysis reveals that the AZ91 alloy displays the largest lattice microstrain and this alloy also exhibits the highest hardness after processing. The processed AZ31 and the ZK60 alloys show similar microstructures and maximum values of hardness. Contrary to earlier reports of significant improvements in the corrosion resistance of magnesium alloys in biological environments, the present results show that processing by HPT has no significant effect on the corrosion behavior of magnesium alloys in a 3.5% NaCl solution. By contrast, pure magnesium exhibits an increased corrosion resistance after HPT.

Electrochemical behavior of a magnesium ZK60 alloy processed by high-pressure torsion

“…It should be noted that in this study a 0.1 M Cl -, with relatively high ohmic resistance, was used as the aggressive ion whereas earlier research involved more corrosive environments such as 0.62 M [Cl] -(3.5 wt% NaCl) which may persistently attack the surface and break down and change the chemistry corrosion product [9,10,13,43]. On the other hand, using passivating rather than corrosive solutions such as a simulated body fluid leads to an improvement in the protective oxide layer and corrosion behavior in each turn of the HPT process compared both to earlier turns and to the original extruded condition [22,23,82].…”

Thermal Stability of an Mg–Nd Alloy Processed by High‐Pressure Torsion

“…Nevertheless, it is necessary in practice to increase the performance of Mg–RE alloys to extend their use in the automotive and aerospace industries or for use in biomedical applications. Severe plastic deformation (SPD) by equal‐channel angular pressing (ECAP) or high‐pressure torsion (HPT) appears to be an excellent strategy for improving the mechanical and superplastic properties of Mg‐based alloys through the production of excellent grain refinement, down to the sub‐micrometer level, and by introducing a high density of defects …”

Using High-Pressure Torsion to Achieve Superplasticity in an AZ91 Magnesium Alloy

“…This showed that the grain structure was not stable in this temperature range but all grain sizes remained within the conventional upper limit of~10 µm for superplasticity [4]. An earlier report showed that the grain size obtained in a similar alloy processed by HPT using similar processing parameters was 100 nm [31]. Therefore, significant grain growth took place even at the lowest testing temperature of 423 K. The highest elongations providing evidence for high-strain-rate superplasticity were observed at 523 K. Precipitation of second-phase particles was observed at this temperature.…”

“…The rotation rate was 2 rpm, and discs were processed to 10 turns under an applied pressure of 6.0 GPa. A detailed characterization of the processed material is given elsewhere [31].…”