Effects of Extrusion and Rolling Processes on the Microstructure and Mechanical Properties of Zn-Li-Ag Alloys

Abstract

In this work, a novel Zn-0.5%Li-0.1%Ag alloy was cast and extruded into rods, which were rolled into a plate, and the effects of extrusion and rolling on the microstructure and mechanical properties of the Zn-0.5%Li-0.1%Ag alloy were evaluated. The results show that grain strengthening occurs in all of the alloys because of the presence of nano-LiZn₄ precipitates. The extrusion and rolling processes promote grain size refinement and orientation order, and the microstructure and mechanical properties of the Zn-0.5%Li-0.1%Ag alloy can be significantly improved by secondary processing. The elastic modulus and tensile strength of the processed alloy increased to 83.1 GPa and 251.6 MPa, respectively, compared to 75.6 GPa and 185.8 MPa, respectively, for the as-cast Zn-0.5%Li-0.1%Ag alloy. More importantly, elongation was greatly improved, from 16.9% to 92.6%, which is an increase of up to 448%, and there were transgranular cleavage planes and intergranular cleavage planes in the fracture surfaces. The intergranular cleavage planes were dominant, and they showed ductile fracture characteristics.

1. Introduction

Compared with Fe-based and Mg-based medical implant materials, Zn-based alloys have better comprehensive properties. First, the self-corrosion potential of Zn-based alloys is −0.76 V, which is much higher than the Mg value of −2.37 V and lower than the Fe value of −0.44 V. Moreover, the degradation rate of Zn is in between those of Fe and Mg [1,2]. The degradation products of Zn are completely bioabsorbable without the evolution of transitional H₂ gas [3,4]. Zinc is an essential nutrient in the human body; it is a second transition metal and has a number of roles in organisms, such as nucleic acid metabolism, signal transduction, retention, bone mass, stimulation of new bone formation, apoptosis regulation, and gene expression [5,6,7]. Zn shows great potential for applications in the field of biodegradable implant materials.

Pure Zn does not meet the mechanical requirements because of its low mechanical properties, and elemental strengthening is an effective way to improve its mechanical properties [8,9,10,11,12]. Li is one of the elements that increases the mechanical properties of pure Zn. Li-doped Zn-based alloys have been previously prepared in our past work, and the alloys showed a fine grain structure and good mechanical properties [13,14]. Ag is another important element in Zn-based alloys; it can significantly refine grain size and improve the physical properties of Zn-based alloys. More importantly, Ag can preserve the biocompatibility of Zn, and Ag-containing materials have already been used in the field of implants [14,15,16].

Casting technology can achieve large-scale production of Zn-based alloys and produce alloy ingots with good structures. To create product shape, however, the secondary processing of the ingot casting billet body is often needed, and the secondary mechanical properties and its processed products tend to be better. This is mainly because the machining process destroys the as-cast organization by activating the plastic deformation mechanism. This improves the mechanical properties of the as-cast groups, leading to fault slipping and twinning. This process can be divided into hot processing and cold processing [17,18,19]. In general, as-cast Zn alloys are homogenized at temperatures in the range of 250 °C to 350 °C for 30–180 min. Extrusion and rolling processes can be used to process Zn alloy sheets. Extrusion molding effectively improves the microstructure and mechanical properties of alloy materials. Therefore, conventional processing based on extrusion and rolling is considered to be an effective method for producing high-performance Zn alloys [20,21].

However, the mechanical properties of the developed Zn-based alloys significantly limit their further clinical applications in the field of medical degradable metal materials. Therefore, further improving the mechanical properties of Zn alloys through secondary processes is a key scientific problem, and there is an urgent need to solve it for Zn alloys to be used in large-scale clinical applications. Zheng et al. [22] used three different commercial Zn alloys to conduct hot extrusion welding at 200 °C, and they found that Zn alloys had better mechanical properties, elongation, and corrosion rate. It has been reported that the strength of the alloy increases because of a combination of grain refinement and work hardening after cold deformation. Moreover, the solution-strengthening effect of adding elements can improve the mechanical properties of the alloys. After plastic deformation, the grain size is further reduced and the distribution is more dispersed; this reduces the degree of stress concentration. Moreover, dynamic recrystallization will eliminate a dislocation plug and the lattice distortion caused by cold deformation; thus, the elongation of the alloy increases. Hence, severe plastic deformation can improve the hardness and plasticity of Zn alloys under certain working conditions [23,24,25]. However, the effects that severe plastic deformation has on the tensile properties of Zn alloys have not been widely studied.

In this paper, we successfully prepared a new Zn-based alloy, and the effects that extrusion and rolling processes have on the microstructure and mechanical properties of Zn-Li-Ag alloys were assessed. Our newly prepared Zn-Li-Ag alloy has improved mechanical properties, and this means that the Zn-Li-Ag alloy has wide applications.

2. Material and Experimental Procedure

In this study, the raw materials are pure Zn (99.99%, ingots) (Huludao Zinc Industry Co., Huludao, China), pure Li (99.9%, particles), and pure Ag (99.9%, foils). The pure Li and Ag were purchased from Hunan Rare Earth Metal Material Research Institute (Changsha, China). Figure 1 shows a schematic diagram of the fabrication process of the Zn-0.5%Li-0.1%Ag alloy. First, the initial Zn-0.5%Li-0.1%Ag alloy was prepared at 600 °C via molten casting under an Ar atmosphere to obtain an ingot (Ø40 mm × 50 mm) [19]. Second, the as-cast Zn-0.5%Li-0.1%Ag alloy ingot was homogenized at 400 °C for 24 h, quenched in water, and hot-extruded at 300 °C to form a rod that had a diameter of 10 mm. Finally, the as-extruded Zn-0.5%Li-0.1%Ag alloy was rolled using multipass rolling to obtain a rolled plate that had a thickness of 1 mm. Before every rolling procedure, the sheets were placed in a box furnace at 200 °C for several minutes until they reached the temperature of the furnace.

Different specimens of the Zn-0.5%Li-0.1%Ag alloy were used to investigate the effects that the extrusion and rolling processes have on the microstructure and mechanical properties of Zn-0.5%Li-0.1%Ag alloys. The sample was sanded, then polished, and finally cleaned with alcohol several times before being dried. First, a metallurgical corrosion solution was prepared with the following composition: 100 mL of distilled water, 20 g of chromium oxide, and 5 g of sodium sulfate. After etching with the corrosive solution, the sample was washed with clean water and dried. The surface of polished specimens was studied using an optical microscope (OM, Leica DMi8, Germany), and the high magnification microstructures were studied using a scanning electron microscope (SEM, FEI Quanta 200, Hillsboro, America) that was equipped with an energy dispersive spectrometer (EDS). To analyze the phase characteristics, phase identification was carried out using X-ray diffraction (XRD, Bruker D8, Rheinstetten, Germany) with Cu Kα radiation. For TEM observation, specimens were mechanically thinned to 80 μm and then reduced by electrolytic jet polishing in a solution of 10% HClO₄ + 90% C₂H₅OH at 20 V between −30 °C and −20 °C. The detailed microstructure was examined using transmission electron microscopy (TEM, F20). The modulus, tensile strength, and percent elongation (ε) were measured according to the GB/T 228.1-2010 standard using an INSTRON 3369- type mechanical testing machine (INSTRON, Boston, America)with a loading speed of 0.5 mm min−1 at room temperature. At least three samples were tested for each condition.

3. Results and Discussion

3.1. Microstructure

To study the effects that the extrusion and rolling processes have on the microstructure of the Zn-0.5%Li-0.1%Ag alloy, the optical microstructure of the Zn-0.5%Li-0.1%Ag alloy was characterized in different treatment states. The results are shown in Figure 2. As seen in Figure 2a, the as-cast grains of the ternary alloy show a slender island morphology with a grain width of approximately 10 μm and an uneven distribution of lengths that range between 20 and 50 μm. Figure 2 also shows the optical structure of the Zn-0.5%Li-0.1%Ag alloy in the extruded and rolled states. As seen in Figure 2b, the grain size of the alloy is significantly smaller than that of the as-cast alloy, and this can help to further improve the mechanical properties of the alloy. At the same time, the extrusion process generates deformation heat, and this heat emission promotes dynamic recrystallization. This phenomenon effectively hinders the secondary growth of grains, which also improves the mechanical properties of the alloy. The extruded bar was rolled to obtain the required size of the plate, and it can be seen that the rolling behavior promotes sliding and elongating of grains along the rolling direction (Figure 2c). The grain size decreases and presents a slender strip structure, and a large number of secondary precipitates are distributed in the gap. Compared with the extruded state, the number of dark secondary phases that are distributed in the metallurgical structure is significantly increased, and this also indicates that the rolling process promotes the generation of a large number of secondary phases of LiZn₄.

To further study the effects that different treatment processes have on the microstructure of the Zn-0.5%Li-0.1%Ag alloy, the Zn-0.5%Li-0.1%Ag alloy in different treatment states was characterized using SEM and EDS. The results are shown in Figure 3. As seen in Figure 3, the as-cast structure of the alloy presents an irregular island structure, and the dark field is evenly distributed in the matrix. Then, the material was extruded. The grains were elongated to a certain extent, the island grains gradually became long island grains, and the grain size decreased to a certain extent. The rolled microstructure has a regular fringe structure, and the grain size becomes very small. Corresponding EDS analysis of the local microstructure of different treated alloys shows that both the light-colored matrix ring structure and the dark-colored ring edge structure are basically composed of the Zn matrix. The Ag content is approximately 0.1%, which is basically consistent with the amount of Ag added according to the alloy ratio. It can be considered that Ag atoms are completely dissolved into the Zn matrix in the process of casting and are evenly distributed. Different post-processing processes have little effect on the distribution of Ag atoms in the alloy.

The XRD patterns of the Zn-0.5%Li-0.1%Ag alloy in different treatment states are shown in Figure 4, and the results show that the Zn-Li-Ag ternary alloy is composed of a Zn matrix (PDF#04-0831) and a small amount of a second phase of LiZn₄ (PDF#03-0954). The addition of Ag does not produce a new second phase, and this is mainly because of the low content of Ag, which is mainly in a solid solution state in the Zn-based alloy. The phase composition of the Zn-Li-Ag ternary alloy in different treatment processes does not change, but the peak strength and peak position of the second phase of LiZn₄ have significant changes in the rolled state. This is mainly because the rolling process promotes the precipitation of the second phase of LiZn₄ in the alloy, and this increases the content of the second phase.

TEM images (Figure 5) show that the spherical secondary phase of LiZn4 is embedded in the matrix. This effectively improves the mechanical performance of the alloy. However, no diffraction peaks or spots were detected for Ag or Zn-Li-Ag secondary phases in the Zn-0.5%Li-0.1%Ag alloy since the amount of Ag that was added was relatively low.

3.2. Mechanical Properties

To study the influence of extrusion and rolling on the mechanical properties of Zn-0.5%Li-0.1%Ag with an initial strain rate of 0.001/s, the mechanical properties of Zn-0.5%Li-0.1%Ag in different treatment states were tested. The results are shown in Figure 6. As seen in Figure 6, the modulus and tensile strength of the as-cast Zn-0.5%Li-0.1%Ag alloy are high (reaching 75.6 GPa and 185.8 MPa, respectively), whereas the elongation is only 16.9%. After the extrusion process, the elastic modulus, tensile strength, and elongation of the Zn-0.5%Li-0.1%Ag alloy increase slightly (92.8 GPa, 378.5 MPa, and 27.8%, respectively). At the same time, the extrusion process improves the porosity of alloys. This is mainly because the extrusion process promotes grain refinement. To obtain the desired sample size and shape, the sample was further rolled to obtain a corresponding plate. The mechanical properties of the rolled alloy show significant changes, and the elastic modulus and tensile strength decreased to 83.1 GPa and 251.6 MPa, respectively. This is mainly because the rolling process promotes an increase in the LiZn₄ phase content, which decreases the amount of Li in solid solution in the matrix. Meanwhile, the dynamic recrystallization in the rolling process resulted in the formation of new distortion-free equiaxed crystals, and these weakened the dislocation stacking behavior during processing. Therefore, there is no obvious hardening phenomenon during the secondary processing of the alloy. More importantly, the elongation of the Zn-0.5%Li-0.1%Ag alloy greatly improved; specifically, it increased from 16.9% to 92.6%, which is an increase of up to 448%. This is mainly because the rolling process further refines the grain size, and dynamic recrystallization can eliminate dislocation accumulation and lattice distortion during hot extrusion. This can significantly improve the elongation of the material. At the same time, the second phase in the alloy is more evenly distributed in the matrix during processing, and this reduces the stress concentration in the alloy and reduces the source of cracks in the deformation process. This behavior can also significantly improve the elongation of the alloy.

Figure 7 shows the tensile fracture morphology of the Zn-0.5%Li-0.1%Ag alloy in different treatment states. By observing the fracture morphology, an in-depth understanding of the fracture mechanism of the sample can be gained. The reasons for the differences in mechanical properties of the sample can be further explained. As seen in Figure 7, there were obvious differences in the fracture morphology of different treatment states. The fracture of the as-cast sample is relatively rough, the grain size of the as-cast sample is large, and there is no obvious orientation. Moreover, most of the samples show cleavage fracture (Figure 7a) with brittle cleavage fracture characteristics overall but ductile fracture characteristics locally. After the extrusion process, the grain has obvious orientation with a shear lip and deep dimpled holes after crack propagation. Furthermore, it shows typical ductile fracture characteristics. Therefore, the mechanical properties of extruded specimens are better than those of as-cast specimens. In contrast, the fracture morphology of the rolled alloy has an obviously small grain size. There were transgranular cleavage planes and intergranular cleavage planes in the fracture surfaces; the intergranular cleavage planes were dominant. Therefore, the samples that were rolled showed ductile fracture characteristics and high elongation.

4. Conclusions

Zn-0.5%Li-0.1%Ag alloys that were subjected to different treatment processes were successfully prepared. The microstructure and mechanical properties of the Zn-0.5%Li-0.1%Ag alloys were investigated. First, the addition of alloying elements effectively improves grain refinement, and the pinning effects that are caused by nano-LiZn₄ precipitates improve the mechanical properties. Second, the extrusion and rolling processes further improve the grain size refinement and orientation order, and the microstructure of the Zn-0.5%Li-0.1%Ag alloy can be significantly improved by secondary processing. The grain size of the Zn-0.5%Li-0.1%Ag alloy decreases from a large size in the as-cast alloy (20–50 μm) to a small size in the rolled alloy (2–3 μm). Above all, the elastic modulus and tensile strength decreased from 75.6 GPa to 83.1 GPa and decreased from 185.8 MPa to 251.6 MPa. More importantly, the elongation was greatly improved from 16.9% to 92.6%, which is an increase of up to 448%, and it showed ductile fracture characteristics. In conclusion, the rolled Zn-0.5%Li-0.1%Ag alloy can be considered for use as a potential absorbable stent material.