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

Jelena Horky,Katharina Werbach,Jörg F Löffler,Peter J Uggowitzer,Bernhard Mingler,Robin Schäublin,Michael J Zehetbauer,Abdul Ghaffar,Daria Setman,Stefan Pogatscher

doi:10.3390/ma12152460

Abstract

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.

Highlights

The first observations of the corrosion and degradation behavior of magnesium in the human body were made over a hundred years ago [1]
The hardness in the transverse direction (TD) was slightly different for the initial samples (68.8 ± 3.3 HV), indicating a texture that developed during the extrusion process
The investigations of two biodegradable low-alloyed Mg-Zn-Ca alloys showed that the hardness and strength of the materials were increased through high pressure torsion (HPT)-induced grain refinement

Summary

Introduction

The first observations of the corrosion and degradation behavior of magnesium in the human body were made over a hundred years ago [1]. Materials 2019, 12, 2460 can have detrimental effects on the surrounding tissue [9,10]); and (iii) material release, which is not harmful to the human body. One way to increase the latter is alloying, but alloying elements usually form precipitates that are nobler than the Mg matrix. These particles act as cathodic sites for micro-galvanic corrosion and thereby strongly increase the degradation rate [13,14]. A possible solution to this dilemma is to use only limited amounts of highly biocompatible alloying elements and to strengthen the alloy using plastic deformation, i.e., grain boundary and dislocation strengthening mechanisms

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Discussion

Conclusion