Microstructure evolution and mechanical performance of ternary Zn-0.8Mg-0.2Sr (wt. %) alloy processed by equal-channel angular pressing
Introduction
The demand for biodegradable implants increases all over the world. The main reason for that is the demand for the highest possible standards of healthcare quality and patient's comfort. Biodegradable materials should be able to fulfil some of the basic requirements (mechanical properties, non-toxic behaviour, optimal degradation rate in the specific environments) to avoid complications associated with the insufficient mechanical properties or negative reactions of the organism [[1], [2], [3]]. Despite the various possibilities of applications, biodegradable materials are considered as materials predominantly for orthopaedic and cardiovascular applications [[4], [5], [6], [7]]. The biodegradable implants for orthopaedic use should be able to mimic the mechanical properties of the supported bone, and to hold the properties stable for a relatively long time (12–18 weeks [8]), despite the gradual material degradation [9]. Based on these requirements, metallic materials seem to be ideal candidates for such applications. The group of biodegradable metallic materials consists of three basic metal elements being magnesium, iron, zinc, and other less-studied metals like pure tungsten or molybdenum [10]. It is known that zinc participates in a large number of processes in the human body and shows almost ideal degradation characteristics compared to other biodegradable metals [[11], [12], [13], [14], [15]]. From orthopaedic aspects, the participation of zinc on the processes concerning new bone growth is crucial [16]. All this information suggests that zinc is promising material for the preparation of orthopaedic biodegradable implants.
It is well known that pure zinc is not suitable for some applications, as its mechanical behaviour especially does not fulfil the basic criteria for implantology [17]. In addition, concerns about the amount of released Zn2+ ions during degradation are often mentioned in the literature. To minimize the impact of those issues, zinc is often alloyed with essential elements from 2nd group of the periodic table of elements, i.e. with magnesium, calcium or strontium. The appropriate addition of those elements leads to an improvement of the mechanical properties and biological interactions with the tissues. Alloying of zinc with magnesium influences both biological and mechanical behaviour. Magnesium is considered to be a biodegradable and essential element. Besides, Mg participates in bone metabolism and supports the healing process during the degradation of the alloy [[18], [19], [20]]. Moreover, the formation of MgxZny based intermetallic phases leads to the desired increment of mechanical properties [20]. Another element which can be used for alloying of Zn-based biodegradable materials, is strontium. The main role of strontium in zinc alloys is connected predominantly with tuning their mechanical performance [21,22]. Although strontium is not considered to be an essential element for human body, it was found that it plays a role in anabolic processes in the skeletal system [23]. Toxic symptoms due to overdosing of strontium have not been reported in human body [24]. A strontium salt – strontium ranelate is even used as a drug for the treatment of postmenopausal osteoporosis [25].
Equal-channel angular pressing (ECAP) belongs to the group of severe plastic deformation (SPD) techniques allowing the preparation of bulk materials with sub-micrometre or even nanoscale microstructure [[26], [27], [28]]. Among all SPD techniques, ECAP is the most applied method which enables to impose large deformation while the final shape after processing remains almost identical to the initial sample [[29], [30], [31]]. The principle of ECAP is generally very simple and comprises the passing of billets through a die with geometrically equal channels which are intersected at a certain angle, often 90° [32,33]. This causes an intense plastic strain [33] and the imposed shear deformation leads to the accumulation of stress and to the promotion of the dynamic recrystallization [34]. The optimum microstructure and grain size are achieved by repeated ECAP [9,27,34]. Because the grain refinement also occurs during the process, mechanical properties are significantly improved. The reduction of the grain size by several orders of magnitudes is often observed [35]. The grain size can subsequently vary from a few tens to hundreds of nanometres in some cases (Al alloys [36,37], Ti alloys [38] or pure Cu [39]), and high angle boundaries are usually formed [40]. Besides many factors, the formation of the microstructure is affected by the ECAP parameters including die angle, corner angle, processing temperature, ram speed, and processing route [[41], [42], [43]]. The processing routes can be divided into four categories labelled as A, BA, BC, and C which differ in the rotation angle (0, 90, and 180°, respectively) and direction between individual passes [32,40,42].
In the field of biodegradable applications, ECAP has been studied mainly as a promising technique for preparation of high-performance ultrafine-grained Mg alloys until now [[44], [45], [46], [47]]. ECAPed zinc and its alloys have been investigated as well; however, only in a few studies [31,48,49]. It is well known that recrystallization of pure zinc takes place already at only −12 °C [50] and therefore, it is difficult to obtain an ultra-fine grained microstructure at ambient temperature [38,41]. Despite this limitation, a relatively low average grain sizes (2–3 μm) were reached in the case of the ECAPed zinc and its alloys [48,49,51]. Material properties obtained by ECAP are often compared with those obtained by the extrusion process to determinate the efficiency of the process. Bednarczyk et al. [48] compared pure zinc and low-alloyed Zn–Ag, Zn–Cu, Zn–Mn alloys prepared by ECAP and by hot extrusion. They revealed that the ECAP process generally leads to finer microstructure and paradoxically to a decrement of material strength. This was ascribed to the activation of creep-like deformation mechanisms, such as grain boundary sliding, in low-alloyed zinc-based materials processed via ECAP thus resulting into ineffective strengthening [48]. Similar behaviour was also observed in other studies of Zn-based materials processed by ECAP [47,48,50]. However, it is not possible to generalize those observations due to the used various possibilities of processing (routes, temperature, etc.) leading to different textures and subsequently to different deformation mechanisms.
This work is focused on preparation and characterization of a Zn-0.8Mg-0.2Sr (in wt. %) alloy by the ECAP process. To the best of our knowledge, only a few publications concerning ECAPed zinc for bio-applications were published [31,52]. Moreover, the ternary alloy Zn–Mg–Sr was prepared in this way for the first time. The prepared materials were analysed from the microstructural and mechanical point of view and our results were supported by calculations. The study describes the influence of ECAP on the material properties.
Section snippets
Experimental
A Zn-0.8Mg-0.2Sr (wt. %) alloy was prepared by malting of pure zinc (99.995 wt %, commercial availability), magnesium (99.95 wt %, Magnesium Elektron), and strontium (99.9 wt %, Strem chemicals) in a MgO crucible in an electrical resistance furnace. The melting was performed under the air atmosphere at 520 °C. The mixture was stirred using a graphite rod during the melting in order to homogenize the composition of the resulting material. After 20 min of homogenization, the mixture was cast into
Microstructure
The microstructures of the as-cast, annealed, and ECAPed samples are shown in Fig. 3. The as-cast samples (Fig. 3a) consisted of zinc dendrites (matrix), lamellar eutectic mixtures, both stable Zn + Mg2Zn11 and metastable Zn + MgZn2, and particles of the SrZn13 phase occurring predominantly inside the eutectic mixtures. More details about the structure of the as-cast alloy were published in our previous work [55]. The phase composition and the distribution of individual phases were confirmed by
Microstructure
The microstructure of the as-ECAPed alloy (Fig. 3, Fig. 6a) was significantly refined compared to the initial material (Fig. 3b and e). Besides, the intermetallic phases were aligned in rows perpendicular to TD and tilted by about 45° from ED (Fig. 3d) as a consequence of the mass flow during the ECAP process, which took place along the shear plane tilted by about 45° to ED and ND.
Besides the intermetallic phases (Mg2Zn11 and SrZn13), fine recrystallized Zn grains (~100 nm) and coarser
Conclusions
The evolution of the microstructure and mechanical properties of the Zn-0.8Mg-0.2Sr alloy prepared by ECAP were comprehensively studied and discussed in detail. Results of this study can be summarized into several points listed below.
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The distribution of the intermetallic phases was similar in ED and ND, while it significantly differed in TD.
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The intermetallic particles were disrupted by the ECAP process and aligned in rows tilted by about 45° in respect to ED and ND.
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The intermetallic particles
CRediT authorship contribution statement
Jan Pinc: Conceptualization, Methodology, Formal analysis, Investigation, Data curation, Writing – original draft, Visualization. Andrea Školáková: Formal analysis, Investigation, Writing – original draft. Petr Veřtát: Investigation, Writing – review & editing. Jan Duchoň: Investigation. Jiří Kubásek: Investigation. Pavel Lejček: Writing – review & editing. Dalibor Vojtěch: Writing – review & editing. Jaroslav Čapek: Conceptualization, Methodology, Formal analysis, Data curation, Writing –
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors would like to thank the Czech Science Foundation (project no. 18-06110 S) for the financial support. This study was also supported by the Operational Programme Research, Development and Education financed by European Structural and Investment Funds and the Czech Ministry of Education, Youth and Sports (Project No. SOLID21 – CZ.02.1.01/0.0/16 019/0000760), and by the LNSM Research Infrastructure supported by MEYS CR (LM2018110). The authors would also like to thanks to Stanislav Habr
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