Magnesium-based composites reinforced with graphene nanoplatelets as biodegradable implant materials Mohammad Shahin , Khurram Munir , Cuie Wen , Yuncang Li * School of Engineering, RMIT University, Melbourne, Victoria, 3001, Australia article info Article history: Received 7 January 2020 Received in revised form 20 February 2020 Accepted 21 February 2020 Available online 22 February 2020 Keywords: Biocorrosion Graphene Magnesium metal matrix composite Strengthening mechanism abstract Magnesium (Mg) and its alloys are considered promising biodegradable implant materials because of their strength and ability to degrade naturally in the body. However, pure Mg degrades rapidly in the physiological environment which adversely affects its mechanical integrity before sufficient bone heal- ing. In this study, a high energy ball mill was used to disperse 0.5 wt% zirconium (Zr) and 0.1 wt% GNPs in Mg powders. Ball milled powder mixtures were then cold pressed under 760 MPa into green compacts and sintered in an argon atmosphere at 610  C for 2 h. Results indicated that the addition of Zr and GNPs to the Mg matrix significantly enhanced its compressive yield strength by 91% and reduced the corrosion rate by 48% and 68% in electrochemical polarization test and hydrogen evolution test, respectively, compared to pure Mg. The contributions of various strengthening mechanisms to the compressive yield strength of MMNCs were quantitatively predicted in conjunction with validation via experimental re- sults. This study demonstrates the potential of GNPs as effective reinforcement in fabrication of MMNCs with improved mechanical and corrosion properties. © 2020 Elsevier B.V. All rights reserved. 1. Introduction The clinical success of biodegradable implants such as bone plates, screws, pins, etc., relies on their natural degradation in the physiological environment without compromising their mechani- cal integrity before sufficient bone healing [1 ,2]. In bone-tissue engineering, bone repair and regeneration are promoted by me- chanical loading. However, existing implant materials such as stainless steels (SS), cobalt-chromium alloys, and titanium alloys exhibit higher elastic moduli than that of natural bone. This mismatch in elastic modulus between the metallic implant and bone triggers stress-shielding in the host bone tissue which leads to bone resorption and implant loosening, thus requiring additional complicated revision surgery [3]. Magnesium (Mg) and its alloys have emerged as promising candidate materials in recent years for applications in bone-tissue engineering due to their ability to naturally degrade in the body, which eliminates the requirement of revision surgeries. Further- more, the tensile strength (135e285 MPa), elastic modulus (40e45 GPa), and elongation (2e10%) of pure Mg [3,4] are closer to those of cortical bone (96e200 MPa, 5.6e14 GPa, and  2%, respec- tively [5,6]) than other conventional metallic biomaterials. The elastic moduli of the conventional metal implants such as cobalt- chromium alloys (210e240 GPa), stainless steels (190e200 GPa) and titanium alloys (60e110 GPa) [7] are much higher than that of cortical bone. This may cause stress shielding to the surrounding bone, leading to bone resorption and loosening of the implant [8]. However, pure Mg exhibits inadequate mechanical and corrosion properties, which impedes its application in load-bearing implants [9]. For this reason, Mg is generally alloyed with other elements such as aluminum (Al), zinc (Zn), zirconium (Zr), and rare earth elements (REE) to improve its mechanical properties and corrosion resistance. However, conventional Mg alloys are not necessarily biocom- patible because they usually contain toxic alloying elements such as Al. Furthermore, Al-containing Mg alloys such as AZ91 (Mge9Ale1Zn), AZ61 (Mge6Ale1Zn), AZ31 (Mge3Ale1Zn), and AJ62 (Mge6Ale2Sr) exhibit pitting and stress corrosion in the physiological environment by forming galvanic couples in Mg matrices [10e15]. In addition, an excess amount (6.0 wt%) of Zn in Mg alloys causes embrittlement, which reduces the mechanical integrity of Mg alloys [16e18]. The surface properties of the implant materials modulate the biological response at the interface of the implant and tissue, leading to bone growth along with the implant * Corresponding author. E-mail address: yuncang.li@rmit.edu.au (Y. Li). Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom https://doi.org/10.1016/j.jallcom.2020.154461 0925-8388/© 2020 Elsevier B.V. All rights reserved. Journal of Alloys and Compounds 828 (2020) 154461