*4.8. Tensile tests*

When comparing the stress–strain curves of the alloys in the IS (furnace-cooled)/low-temperature HPT-processing for 0.5 rotations (Figure 13), it can be seen that ductility was markedly reduced in all the HPT-processed samples. After two or more rotations, the material became very brittle, as the failure occurred right after the elastic limit. After additional heat treatment, the samples remained very brittle. Since thermal treatments primarily launched the formation of vacancy agglomerates, it is evident that those were responsible for this brittleness observed.

The values of ultimate tensile strength (*UTS*) derived from the tensile tests suggest comparing them with the Vickers microhardness values (*HV*) as, according to literature [70,73], the *UTS* can be related to *HV* as

$$HV = m \times \text{lITS} \tag{10}$$

with *m* as the so-called "Tabor factor." For continuum materials or at least highly isotropic ones, *m* = 3, as the result of the theory of stress distribution of a force on a semi-infinite body [73]. Inserting now our measured data for *UTS* and *HV*, *m* turns out to be *m* = 4.2 with a relatively small standard deviation of ± 0.5 confirming the validity of (11). The strong deviation to the continuum value *m* = 3 can be explained by the fact that Mg alloys are typical examples of highly anisotropic materials. Therefore, another constant value *m* of Equation (10) is expected at least unless the materials exhibit strong textures [74].

### **5. Summary and Conclusions**

The present study showed methods for the optimization of various MgZnCa alloys concerning their strength and corrosion properties. Several Mg alloys (Mg5Zn0.3Ca, Mg5Zn0.15Ca, Mg5Zn0.15Ca0.15Zr) have been investigated with respect to Vickers hardness, Young's modulus, ductility and corrosion properties, and the results were compared to investigations of two binary alloys, Mg5Zn and Mg0.3Ca. The combined application of severe plastic deformation by HPT and heat treatments at low temperatures provided the production of both special intermetallic precipitates and vacancy agglomerates and thus tremendous increases of the alloy's strength. Investigations were done using electron microscopes, microhardness and nanoindentation testing, a microtensile testing facility, XRD and DSC.

Homogenization of all materials was followed by furnace cooling; the fraction of primary precipitates could be reduced to below 1%, their size being close to 1 nm. The main results of our investigations are the following:


**Author Contributions:** Conceptualization, M.V.; formal analysis, A.O.; investigation, A.O., J.H., M.F. and B.S.; data curation, A.O., J.H., M.J.Z., E.S., D.O. and M.F.; writing—original draft preparation, A.O.; writing—review and editing, M.J.Z., D.O., J.H., B.M. and M.F.; supervision, E.S., M.J.Z. and D.O.; project administration, D.O., M.J.Z., M.V. and S.G.; funding acquisition, D.O., M.J.Z., M.V. and S.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Austrian Science Fund FWF (Fonds zur Förderung der Wissenschaftlichen Forschung Österreich), grant number I2815-N36, and the Slovenian Research Agency (Agencija Raziskovalno Republike Slovenije ARRS), grant number J2-7157.

**Acknowledgments:** The authors gratefully appreciate financial support from the projects J2-7157 and research program P2-0412 of the Slovenian Research Agency ARRS, and I2815-N36 of the Austrian Science Fund FWF. B.S., A.O. and M.Z. thank the Austrian and Slovenian Exchange Services for mutual visits within projects PL 14/2017 and PL 14/2019.

**Conflicts of Interest:** The authors declare no conflict of interest.
