**The E**ff**ects of Severe Plastic Deformation and**/**or Thermal Treatment on the Mechanical Properties of Biodegradable Mg-Alloys**

**Andrea Ojdanic 1,2, Jelena Horky 3, Bernhard Mingler 3, Mattia Fanetti 4, Sandra Gardonio 4, Matjaz Valant 4,5, Bartosz Sulkowski 6, Erhard Schafler 1, Dmytro Orlov 4,7 and Michael J. Zehetbauer 1,\***


Received: 22 June 2020; Accepted: 1 August 2020; Published: 6 August 2020

**Abstract:** In this study, five MgZnCa alloys with low alloy content and high biocorrosion resistance were investigated during thermomechanical processing. As documented by microhardness and tensile tests, high pressure torsion (HPT)-processing and subsequent heat treatments led to strength increases of up to 250%; as much as about 1/3 of this increase was due to the heat treatment. Microstructural analyses by electron microscopy revealed a significant density of precipitates, but estimates of the Orowan strength exhibited values much smaller than the strength increases observed. Calculations using Kirchner's model of vacancy hardening, however, showed that vacancy concentrations of 10−<sup>5</sup> could have accounted for the extensive hardening observed, at least when they formed vacancy agglomerates with sizes around 50-100 nm. While such an effect has been suggested for a selected Mg-alloy already in a previous paper of the authors, in this study the effect was substantiated by combined quantitative evaluations from differential scanning calorimetry and X-ray line profile analysis. Those exhibited vacancy concentrations of up to about 10−<sup>3</sup> with a marked percentage being part of vacancy agglomerates, which has been confirmed by evaluations of defect specific activation migration enthalpies. The variations of Young's modulus during HPT-processing and during the subsequent thermal treatments were small. Additionally, the corrosion rate did not markedly change compared to that of the homogenized state.

**Keywords:** Mg alloy; severe plastic deformation (SPD); intermetallic precipitates; vacancy agglomerates; corrosion
