Nanograined Zinc Alloys with Improved Mechanical Properties Prepared by Powder Metallurgy

Researchers are currently developing new biodegradable metals to address the limitations of traditional implant materials. The goal is to create materials that support tissue regeneration and completely dissolve after the regenerative process has occurred. Additionally, it is important that the by-products of these materials can be metabolized by the body. Research has primarily focused on essential elements such as iron, zinc, magnesium, and their alloys. Zinc achieves excellent biocompatibility and corrosion properties making him a promising candidate for biodegradable devices (stents, screws, etc). However, pure zinc and its alloys suffer from poor mechanical properties (yield, ultimate strength and low-temperature creep at 37 °C). Improvements can be achieved by suitable alloying or the selection of various preparation techniques [1]. We have focused on the synthesis of binary Zn-Mg alloy from pure powders by mechanical alloying (MA) and compaction by spark plasma sintering (SPS) in combination with extrusion. Magnesium was selected as the alloying element due to its positive effect on the mechanical properties of zinc. This is related to the strengthening by the secondary phase (Mg2Zn11) and solid solution. Concentration 1 wt. % was chosen to reach a compromise between strength and elongation, based on the literature data [2, 3]. MA was selected as the technology enabling the formation of fine-grained, homogeneous microstructures consisting of solid solutions and metastable intermetallics. The selected powders were compacted by the SPS, a fast compaction technique that prevents grain coarsening and extrusion for enhancing properties by redistributing microstructure [4]. In our study, Zn-1Mg alloy was prepared from pure powders in combination with 0.03g stearic acid to prevent the agglomeration of powder particles during milling. The milling was performed in ZrO2 vessels under an argon protective atmosphere (purity 99.95%) in a Retsch E-Max milling machine (800 rotations per minute) with a water-cooling system for 8 h. The milling balls were also composed of ZrO2 in various sizes. The process temperature was maintained between 30–50 °C and the direction of rotation was changed every 10 min. The powders that were mechanically alloyed and had ideal properties were then consolidated using the Spark Plasma Sintering (SPS) method (FCT System HP-D 10) under an argon atmosphere (purity 99.95%) at a temperature of 300 °C and a pressure of 80 MPa for 10 min using a graphite tool. The consolidated samples had a cylindrical shape with a diameter of 20 mm. These were further extruded. The parameters chosen for the extrusion process were a temperature of 200 °C and an extrusion ratio (ER) of 11. The microstructure of the prepared materials was characterized using optical microscopy (Olympus PME3) and scanning electron microscopy (SEM – TESCAN VEGA 3 LMU) equipped with an EDS analyzer (OXFORD Instruments AZtec). The microstructure and mechanical properties of the prepared materials were studied and compared to zinc samples compacted also from powder. The results show that the combination of MA and compaction by SPS produced a fine, homogeneous microstructure consisting of zinc matrix, intermetallic phase Mg2Zn11 and partially dissolved magnesium in the form of a solid solution. However, oxide shells remain in the materials from the original powder particle surface (Figure 1A). This oxide net structure is typical for zinc and magnesium materials prepared by SPS and leads to the deterioration of the mechanical properties. By adding following thermomechanical processing these shells broke into small particles located mainly on the grain boundaries (Figure 1B). The nano-grained microstructure with intermetallics and dispersed oxides enhanced the hardness and compressive ultimate strength, but reduced elongation compared to pure metal. Due to the extrusion and generation of texture inside the material anisotropic properties were observed, which were most noticeable on performed compressive tests. Even though, the results show significant improvement in mechanical properties compared to pure zinc (Figure 2) [5]. Microstructure of the prepared materials – SEM: A) Zn-1Mg_SPS, B) Zn-1Mg_SPS+Ex Stress-strain curves for zinc and its alloys prepared by mechanical alloying