PSI - Issue 23

Jan Pinc et al. / Procedia Structural Integrity 23 (2019) 21–26 Jan Pinc et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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Nomenclature AAS

atomic absorption spectroscopy scanning electron microscope energy dispersive spectroscopy

SEM EDS

XRD X-ray diffraction HV Vickers hardness F5, F30, F50

diameters of cast samples (5, 30 and 50 mm)

Introduction

Zinc and its alloys have been intensively studied due to almost optimal properties for the preparation of biodegradable implants. Biodegradable materials have to fulfill some basic conditions such as optimal corrosion rate, mechanical properties sufficient for the applications in human body and the removal of corrosion products without a negative response of organism, Zheng et al. (2014). The advantages of zinc based materials are mainly connected with the corrosion process and biological function of zinc in the organism, Vojtěch et al. (2011) . The corrosion process of zinc materials is not connected with hydrogen evolution under physiological conditions and the corrosion rate is optimal in comparison with other materials (Mg, Fe) intended for the preparation of biodegradable implants, Li et al. (2014). In the human body, zinc has various functions, for example it acts as a part of enzymes and it participates in many biological processes such as brain activity or bone mineralization, Plum et al. (2010). The limitations of Zn-based materials are connected to high density and mechanical properties insufficient for some applications. The density of zinc is 7.14 g/cm 3 and it is approximately four times higher compared to magnesium and three times compared to the bone. However, the function of the implant is temporary and the weight of the implant decreases rapidly in just several months (depending on the particular application) due to the corrosion process. The tensile strength of pure zinc is approximately 30 MPa and the elongation to fracture 0.3% which are values insufficient for a bone replacement, Liu et al. (2016). One of the possible ways, how to solve this problem is the alloying of zinc with light elements and creation of phase system which leads to an enhancement of mechanical properties. Magnesium and calcium belong to the group of essential elements and play an important role in the bone metabolism. As alloying elements of zinc, they can significantly affect its mechanical and corrosion properties. The influence of magnesium strongly depends on its amount in the alloy, Dambatta et al. (2017). It was found that the best mechanical performance was achieved by the addition of magnesium in amounts up to 1 wt.%, Kubasek et al. (2014). Calcium mainly affects the hardness of the material, Finkel et al. (2006). This paper focuses on the microstructural characterization of the ZnMg0.8Ca0.2 alloy and on the investigation of the influence of different cooling rates on the hardness and micro-hardness of the prepared materials. 1. Experimental The ZnMg0.8Ca0.2 alloys were prepared by the melting of pure metals in a MgO crucible at 520 °C in a weight ratio 99:0.8:0.2. The melt was cast into brass molds with a diameter of 5, 30 and 50 mm. Parts of the ingots were dissolved in nitric acid and analyzed using an AGILENT 280 FS atomic absorption spectrometer (AAS) in order to measure the exact chemical composition. The ingot with the diameter of 50 mm was also analyzed using a PANalytical X'Pert PRO powder diffractometer with a Co anode (λ = 0.1789 nm) in order to evaluate the phase composition. A part of the ingot with the diameter of 50 mm was subjected to a homogenization annealing (350 °C, 24 h, quenching in water) and was characterized in a similar way as the as-cast materials. All the prepared samples (5, 30, 50 mm) were ground using sand papers P120-P4000 and subsequently polished using a diamond paste D2 and an Etosil E suspension (Al 2 O 3 , 0. 06 µm). Etching was performed in a chromium oxide solution (200g CrO 3 , 15g Na 2 SO 4 , 50 ml HNO3 and 1000 ml of distilled water) and led to the revelation of the microstructure. Microstructure of the materials was observed using an optical microscope Olympus PME3 (LM) and a scanning electron microscope TESCAN VEGA 3 LMU (SEM) equipped with an energy dispersive spectroscopy