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Article

Exceptional Strengthening of Biodegradable Mg-Zn-Ca Alloys through High Pressure Torsion and Subsequent Heat Treatment

1
Physics of Nanostructured Materials, Faculty of Physics, University of Vienna, 1090 Vienna, Austria
2
Center for Health & Bioresources, Biomedical Systems, AIT Austrian Institute of Technology GmbH, 2700 Wiener Neustadt, Austria
3
Department of Physics, GC University, 54000 Lahore, Pakistan
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Laboratory of Metal Physics and Technology, Department of Materials, ETH Zurich, 8093 Zurich, Switzerland
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Institute of Nonferrous Metallurgy, Montanuniversität Leoben, 8700 Leoben, Austria
*
Author to whom correspondence should be addressed.
Materials 2019, 12(15), 2460; https://doi.org/10.3390/ma12152460
Received: 8 July 2019 / Revised: 25 July 2019 / Accepted: 30 July 2019 / Published: 2 August 2019
(This article belongs to the Special Issue Bioabsorbable and Permanent Materials for Highly Loaded Implants)
In this study, two biodegradable Mg-Zn-Ca alloys with alloy content of less than 1 wt % were strengthened via high pressure torsion (HPT). A subsequent heat treatment at temperatures of around 0.45 Tm led to an additional, sometimes even larger increase in both hardness and tensile strength. A hardness of more than 110 HV and tensile strength of more than 300 MPa were achieved in Mg-0.2Zn-0.5Ca by this procedure. Microstructural analyses were conducted by scanning and transmission electron microscopy (SEM and TEM, respectively) and atom probe tomography (APT) to reveal the origin of this strength increase. They indicated a grain size in the sub-micron range, Ca-rich precipitates, and segregation of the alloying elements at the grain boundaries after HPT-processing. While the grain size and segregation remained mostly unchanged during the heat treatment, the size and density of the precipitates increased slightly. However, estimates with an Orowan-type equation showed that precipitation hardening cannot account for the strength increase observed. Instead, the high concentration of vacancies after HPT-processing is thought to lead to the formation of vacancy agglomerates and dislocation loops in the basal plane, where they represent particularly strong obstacles to dislocation movement, thus, accounting for the considerable strength increase observed. This idea is substantiated by theoretical considerations and quenching experiments, which also show an increase in hardness when the same heat treatment is applied. View Full-Text
Keywords: Mg alloy; Mg-Zn-Ca; severe plastic deformation (SPD); high pressure torsion (HPT); dislocation loops; precipitates; vacancies Mg alloy; Mg-Zn-Ca; severe plastic deformation (SPD); high pressure torsion (HPT); dislocation loops; precipitates; vacancies
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MDPI and ACS Style

Horky, J.; Ghaffar, A.; Werbach, K.; Mingler, B.; Pogatscher, S.; Schäublin, R.; Setman, D.; Uggowitzer, P.J.; Löffler, J.F.; Zehetbauer, M.J. Exceptional Strengthening of Biodegradable Mg-Zn-Ca Alloys through High Pressure Torsion and Subsequent Heat Treatment. Materials 2019, 12, 2460. https://doi.org/10.3390/ma12152460

AMA Style

Horky J, Ghaffar A, Werbach K, Mingler B, Pogatscher S, Schäublin R, Setman D, Uggowitzer PJ, Löffler JF, Zehetbauer MJ. Exceptional Strengthening of Biodegradable Mg-Zn-Ca Alloys through High Pressure Torsion and Subsequent Heat Treatment. Materials. 2019; 12(15):2460. https://doi.org/10.3390/ma12152460

Chicago/Turabian Style

Horky, Jelena, Abdul Ghaffar, Katharina Werbach, Bernhard Mingler, Stefan Pogatscher, Robin Schäublin, Daria Setman, Peter J. Uggowitzer, Jörg F. Löffler, and Michael J. Zehetbauer. 2019. "Exceptional Strengthening of Biodegradable Mg-Zn-Ca Alloys through High Pressure Torsion and Subsequent Heat Treatment" Materials 12, no. 15: 2460. https://doi.org/10.3390/ma12152460

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