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Abstract:It is well established that magnesium has a considerable potential for use as a biodegradable material. This report describes the effect of processing by severe plastic deformation (SPD) on the grain refinement, mechanical behavior, biocompatibility and corrosion behavior of commercial purity (CP) magnesium. The material was received as cast slabs and processed by rolling, equal-channel angular pressing and high-pressure torsion to produce samples with average grain sizes in the range of ~0.5 -300 m. The results show that severe plastic deformation does not affect the biocompatibility. However, the corrosion behavior is affected by the processing route. Specifically, SPD processing leads to general corrosion as opposed to localized corrosion in the as-cast and hot-rolled condition.

It is well‐known that processing by severe plastic deformation using high‐pressure torsion (HPT) promotes grain refinement and increases the strength of magnesium and its alloys. The present research is conducted to evaluate the effect of such processing on cytotoxicity and corrosion behavior in Hank's solution by using samples of commercial purity magnesium and AZ31, AZ91, and ZK60 magnesium alloys. All samples are subjected to electrochemical testing and hydrogen evolution testing before and after processing by HPT and the results show that this processing improves the corrosion resistance of pure magnesium, has no significant effect on the AZ31 and AZ91 alloys but reduces the corrosion resistance of the ZK60 alloy. The observations support the conclusion that grain refinement improves the corrosion resistance of metals with a tendency for passivation but impedes the resistance of metals without passivation. In addition, in vitro cytotoxicity tests are performed on the processed materials and show cell viability in all samples. The results demonstrate that HPT processing may be used to improve the performance of magnesium and its alloys as biodegradable implants.

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.

High pressure torsion provides an opportunity to process materials with low formability such as magnesium at room temperature. The present work shows the microstructure evolution in commercially pure magnesium processed using a pressure of 6.0 GPa up to 10 turns of rotation. The microstructure evolution is evaluated using electron microscopy and the hardness is determined using dynamic hardness testing. The results show that the grain refinement mechanism in this material differs from materials with b.c.c. and f.c.c. structures. The mechanism of grain refinement observed at high temperatures also applies at room temperature. The hardness distribution is heterogeneous along the longitudinal section of the discs and is not affected by the amount of deformation imposed to the material.

Reports in the literature show that severe plastic deformation can improve mechanical strength, ductility, and corrosion resistance of pure magnesium, which suggests good performance for biodegradable applications. However, the reported results were based on testing of small samples on limited directions. The present study reports compression testing of larger samples, at different directions, in pure magnesium processed by hot rolling, equal channel angular pressing (ECAP), and high pressure torsion (HPT). The results show that severe plastic deformation through ECAP and HPT reduces anisotropy and increases strength and strain rate sensitivity. Also, scaffolds were fabricated from the material with different processing histories and immersed in Hank’s solution for up to 14 days. The as-cast material displays higher corrosion rate and localized corrosion and it is reported that severe plastic deformation induces uniform corrosion and reduces the corrosion rate.