Microstructure evolution and mechanical performance of ternary Zn-0.8Mg-0.2Sr (wt. %) alloy processed by equal-channel angular pressing

https://doi.org/10.1016/j.msea.2021.141809Get rights and content

Highlights

  • The average grain size of the ECAPed Zn-0.8Mg-0.2Sr alloy was 2.5 μm.

  • Continuous dynamic recrystallization is the prevailing recrystallization mechanism.

  • Intermetallic regions possess a different texture and grain size compared to the Zn matrix.

  • Material texture and alignment of intermetallic regions have a substantial impact on resulting mechanical properties.

Abstract

In this study, we prepared a Zn-0.8Mg-0.2Sr (wt. %) alloy and processed it by ECAP. The evolution of the microstructure during the processing was observed and discussed in detail. The obtained results revealed the continuous dynamic recrystallization as the prevailing recrystallization mechanism. It affected all the aspects of the microstructure, namely the grain size, residual stresses, and dislocation arrangement. The obtained grain size was in good agreement with both empirical and theoretical relations predicting the minimal (0.4–0.6 μm) and average (2.5 μm) grain size. The compressive tests revealed the relations between alignment of the intermetallic regions, texture of the Zn matrix, and resulting mechanical performance of the material. The compressive yield strength of the material ranged from 230 to 250 MPa in the individual directions, and the tensile yield strength reached the value of approximately 200 MPa. The resulting mechanical properties were almost isotropic in the individual directions and fulfilled the basic requirements for applications in implantology, particularly, for maxillofacial, cranial or orthopaedic implants.

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.

  • The distribution of the intermetallic phases was similar in ED and ND, while it significantly differed in TD.

  • The intermetallic particles were disrupted by the ECAP process and aligned in rows tilted by about 45° in respect to ED and ND.

  • 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

References (107)

  • X. Liu et al.

    Microstructure, mechanical properties, in vitro degradation behavior and hemocompatibility of novel Zn–Mg–Sr alloys as biodegradable metals

    Mater. Lett.

    (2016)
  • X. Liu et al.

    Effects of alloying elements (Ca and Sr) on microstructure, mechanical property and in vitro corrosion behavior of biodegradable Zn–1.5Mg alloy

    J. Alloys Compd.

    (2016)
  • S. Pors Nielsen

    The biological role of strontium

    Bone

    (2004)
  • L. Wang et al.

    A critical review of Mg-based hydrogen storage materials processed by equal channel angular pressing

    Metals

    (2017)
  • A. Abbas et al.

    Investigation of severe plastic deformation effects on microstructure and mechanical properties of WS2/AZ91 magnesium metal matrix composites

    Mater. Sci. Eng., A

    (2020)
  • M.S. Dambatta et al.

    Processing of Zn-3Mg alloy by equal channel angular pressing for biodegradable metal implants

    J. King Saud Univ. Sci.

    (2017)
  • R.Z. Valiev et al.

    Principles of equal-channel angular pressing as a processing tool for grain refinement

    Prog. Mater. Sci.

    (2006)
  • L. Tong et al.

    Development of high-performance Mg–Zn–Ca–Mn alloy via an extrusion process at relatively low temperature

    J. Alloys Compd.

    (2020)
  • L.S. Toth et al.

    Ultrafine-grain metals by severe plastic deformation

    Mater. Char.

    (2014)
  • R. Valiev et al.

    Structure and properties of ultrafine-grained materials produced by severe plastic deformation

    Mater. Sci. Eng., A

    (1993)
  • R.Z. Valiev et al.

    Plastic deformation of alloys with submicron-grained structure

    Mater. Sci. Eng., A

    (1991)
  • V. Polyakova et al.

    Influence of grain boundary misorientations on the mechanical behavior of a near-α Ti-6Al-7Nb alloy processed by ECAP

    Mater. Lett.

    (2017)
  • H. Wang et al.

    Effective grain refinement of pure Cu processed by new route of equal channel angular pressing

    Mater. Sci. Eng., A

    (2019)
  • I.J. Beyerlein et al.

    Texture evolution in equal-channel angular extrusion

    Prog. Mater. Sci.

    (2009)
  • B. Xu et al.

    Microstructure and anisotropic mechanical behavior of the high-strength and ductility AZ91 Mg alloy processed by hot extrusion and multi-pass RD-ECAP

    Mater. Sci. Eng., A

    (2020)
  • Z. Shan et al.

    Microstructure evolution and mechanical properties of an AZ61 alloy processed with TS-ECAP and EPT

    Mater. Sci. Eng., A

    (2020)
  • H. Torabi et al.

    Microstructure, mechanical properties and bio-corrosion properties of Mg-HA bionanocomposites fabricated by a novel severe plastic deformation process

    Ceram. Int.

    (2020)
  • W. Bednarczyk et al.

    Can zinc alloys be strengthened by grain refinement? A critical evaluation of the processing of low-alloyed binary zinc alloys using ECAP

    Mater. Sci. Eng., A

    (2019)
  • W. Bednarczyk et al.

    Determination of room-temperature superplastic asymmetry and anisotropy of Zn-0.8 Ag alloy processed by ECAP

    Mater. Sci. Eng., A

    (2019)
  • W. Bednarczyk et al.

    Achieving room temperature superplasticity in the Zn-0.5 Cu alloy processed via equal channel angular pressing

    Mater. Sci. Eng., A

    (2018)
  • H. Huang et al.

    A high-strength and biodegradable Zn–Mg alloy with refined ternary eutectic structure processed by ECAP

    Acta Metall. Sin.

    (2020)
  • J. Čapek et al.

    ZnMg0.8Ca/Sr0.2 ternary alloys – the influence of the third element on material properties

    Procedia Structural Integrity

    (2019)
  • S.K. Sahoo et al.

    Texture and microstructure evolution of pure zinc during rolling at liquid nitrogen temperature and subsequent annealing

    Mater. Char.

    (2017)
  • C. Zhao et al.

    Strain hardening of as-extruded Mg-xZn (x= 1, 2, 3 and 4 wt%) alloys

    J. Mater. Sci. Technol.

    (2019)
  • J.D. Robson et al.

    Particle effects on recrystallization in magnesium–manganese alloys: particle-stimulated nucleation

    Acta Mater.

    (2009)
  • K. Wang et al.

    Achieving enhanced mechanical properties in Mg-Gd-Y-Zn-Mn alloy by altering dynamic recrystallization behavior via pre-ageing treatment

    Mater. Sci. Eng., A

    (2020)
  • H. Jia et al.

    Texture evolution of an Al-8Zn alloy during ECAP and post-ECAP isothermal annealing

    Mater. Char.

    (2019)
  • M. Suresh et al.

    Effect of equal channel angular pressing (ECAP) on the evolution of texture, microstructure and mechanical properties in the Al-Cu-Li alloy AA2195

    J. Alloys Compd.

    (2019)
  • W. Lei et al.

    Analysis of microstructural evolution and compressive properties for pure Mg after room-temperature ECAP

    Mater. Lett.

    (2020)
  • J.C. Tan et al.

    Dynamic continuous recrystallization characteristics in two stage deformation of Mg–3Al–1Zn alloy sheet

    Mater. Sci. Eng., A

    (2003)
  • S. Yi et al.

    Microstructural evolution during the annealing of an extruded AZ31 magnesium alloy

    J. Alloys Compd.

    (2010)
  • T. Sakai et al.

    Dynamic and post-dynamic recrystallization under hot, cold and severe plastic deformation conditions

    Prog. Mater. Sci.

    (2014)
  • V.G. Sursaeva

    Effect of faceting on twin grain boundary motion in zinc

    Mater. Lett.

    (2010)
  • S. Liu et al.

    Dynamic recrystallization of pure zinc during high strain-rate compression at ambient temperature

    Mater. Sci. Eng., A

    (2020)
  • P.G. Partridge et al.

    The formation and behaviour of incoherent twin boundaries in hexagonal metals

    Acta Metall.

    (1964)
  • F.A. Mohamed et al.

    On the minimum grain size obtainable by high-pressure torsion

    Mater. Sci. Eng., A

    (2012)
  • F.F. Lavrentev

    The type of dislocation interaction as the factor determining work hardening

    Mater. Sci. Eng.

    (1980)
  • M. Shahzad et al.

    Influence of extrusion parameters on microstructure and texture developments, and their effects on mechanical properties of the magnesium alloy AZ80

    Mater. Sci. Eng., A

    (2009)
  • J. Čapek et al.

    Extrusion of the biodegradable ZnMg0.8Ca0.2 alloy–The influence of extrusion parameters on microstructure and mechanical characteristics

    Journal of the Mechanical Behavior of Biomedical Materials

    (2020)
  • B. Derby

    The dependence of grain size on stress during dynamic recrystallisation

    Acta Metall. Mater.

    (1991)
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