rus
Issue #7-8/2024
A.R.Sirazeeva, A.R.Khasanova, O.B.Kulyasova, D.A.Aksenov, B.O.Bolshakov
INFLUENCE OF PLASTIC DEFORMATION ON STRUCTURE AND PROPERTIES OF BIORESORBABLE ZINC ALLOY Zn-0.8Li-0.1Mn
INFLUENCE OF PLASTIC DEFORMATION ON STRUCTURE AND PROPERTIES OF BIORESORBABLE ZINC ALLOY Zn-0.8Li-0.1Mn
This paper presents the results of a study of the effect of plastic deformation on the structure and properties of zinc alloy Zn-0.8Li-0.1Mn. The evolution of the structure has been characterised by scanning electron microscopy (SEM) and EBSD methods. The stress-relative elongation diagrams obtained under uniaxial tension are discussed.
Теги: plastic deformation scanning electron microscopy stress напряжение пластическая деформация растровая электронная микроскопия
INTRODUCTION
Traditional orthopaedic metallic implants such as stainless steel, cobalt-chromium alloys and titanium-based alloys are primarily designed for osteosynthesis; their high strength and acceptable biocompatibility allow them to be used as primary load-bearing orthopaedic devices [1]. However, there are known limitations for such implants. Firstly, their prolonged stay leads to accumulation of harmful elements such as Ni, Co, Cr, Cr, Al and V in the human body, which cause chronic inflammation and eventually lead to revision surgeries [2]. Secondly, the mismatch of high elastic modulus between implants and human bone leads to bone resorption and implant failure after time [3]. The revolutionary concept of biodegradable implants aims to solve these problems [4]. Research on synthetic biodegradable polymers began in the 1960s. Biodegradable polymers such as polyglycolide (PGA), polylactide (PLA), and poly (L- or D, L-lactic acid) (PLLA/PDLLA) were approved by the FDA for use in orthopaedic fixation implants after 60 years of development [5]. However, their use was limited to skeletal sites with low load due to their insufficient strength. In addition, degradation products formed by volumetric erosion of biodegradable polymers can cause a noninfectious inflammatory response, which eventually leads to bone resorption [6]. More recently, development of magnesium (Mg)-based biodegradable implants has provided scientists and clinicians with new opportunities to address the shortcomings of existing commercial orthopaedic implants. As biodegradable metals, Mg-based materials provide good mechanical support compared to their polymer counterparts, demonstrating a desirable modulus of elasticity close to that of human bone. More importantly, degradation of Mg releases useful products such as Mg ions that promote bone regeneration and accelerate bone healing [7]. Tremendous efforts in basic research have brought the use of Mg to clinical trials and commercialisation in Europe [8], Korea [9], but their application is still limited to fixation of non-load bearing bone fractures and bone fragments. Meanwhile, more systematic studies are needed to slow down formation of gas cavities that are formed during the Mg dissolution in the physiological environment [10]. As a result, no single material can be used as a material for a bioresorbable implant used for high loads. Recently, a group of scientists conducted a systematic evaluation of biodegradable zinc (Zn) alloys as orthopaedic implants [11]. The results showed that Zn-Li, Zn-Mg, Zn-Ca and Zn-Sr alloys were the most favoured candidates for bone implants. Among them, Zn-Li alloys showed strength comparable to commercially pure titanium and stainless steel, indicating their great potential for use as implants with high load-bearing capacity. Recently, development of clinical needs requires improvement of the interaction between human tissues and implants to promote the process of bone regeneration and healing. Zn-Li alloy implants have been found to have a favourable effect on new bone tissue formation [12, 13]. However, alloying alone is often insufficient to achieve the required mechanical properties. Therefore, there is a need for deformation treatment of the alloy. From this point of view, application of severe plastic deformation and rolling is promising, which makes it possible to harden metals and alloys due to the formation of ultrafine-grained (UFG) structure. Thus, development of new ultrafine-grained medical materials based on zinc is of significant practical interest for application in clinical practice.
RESEARCH METHODS
Zn-0.8%Li-0.1%Mn (wt.%) alloy samples with a diameter of 20 mm and a length of 100 mm were taken as initial state and subjected to homogenisation in a Nabertherm muffle furnace at 300 °C for 72 hours with cooling in water.
The initial samples were rolled on a 2-roll Hankook M-Tech mill for section rolling in 2 stages: from the diameter of 20 mm to the diameter of 15 mm at temperature of 300 °C; with a change of cross-section from a circle to a square of 10 × 10 mm2 also at a temperature of 300 °C. The strain was 1.1.
The rolled specimens were subjected to equal-channel angular pressing (ECAP). Deformation was carried out at temperatures of 300 °C, 250 °C, 200 °C, 150 °C with 2 passes for each temperature. The total number of passes was 8. ECAP of initial billets was carried out in a round channel, rolled billets in a square channel. The angle of intersection of the channels was Ф = 120°, following the known route ВС (after each pass the billet was rotated around its longitudinal axis by the angle 90°). For 8 passes of the ECAP, the final accumulated strain of deformation was e = 5.6.
To reveal structure in the longitudinal direction, the samples were immersed for 30 seconds in an etchant with the following composition: 5 ml of nitric acid (HNO3) and 95 ml of ethyl alcohol (C2H5OH). The structure was analysed on a JEM-6390 scanning electron microscope (SEM) in secondary electron mode at an accelerating voltage of 30 kV. EBSD maps were obtained using a Thermo Scientific Q250 electron microscope of (FEI) at an accelerating voltage of 30 kV. The scanning step was 0.5 μm.
Mechanical tensile tests were performed on an Instron 5982 tensile testing machine on small specimens with working gauge dimensions of 0.6 × 1 × 4 mm3 at room temperature at a strain rate of 10–3 s–1. The test specimens were cut from deformed billets in longitudinal section.
RESULTS
Structural studies
In initial state, a complex dendritic structure is observed. The LiZn4 phase (Fig1a.) forms the base in the form of dendritic branches, the thickness of which can reach 100 μm. Between these branches, a eutectic of Zn+LiZn4 composition is observed, apparently crystallising a little later when the temperature reaches ~403 °C during cooling [14].
EBSD studies indicate formation of a large number of subboundaries in the branches of LiZn4 phase dendrites. The fraction of low-angle boundaries (LABs) reaches 87% (Fig.1c). In the eutectic mixture of LABs phases are practically not observed.
After hot rolling, the structure acquires a banded appearance and becomes refined. The average transverse size of bands of primary dendrites of LiZn4 phase is 35±16 μm, with grain shape elongation ratio ~1:3. The eutectic also undergoes transformation into a grain structure with grain size of Zn and ZnLi4 ~2 μm. EBSD analysis indicates preservation of sub-grain structure in the body of deformed grains. The fravtion of LABs is preserved and amounts to 86%. Low-angle boundaries are observed both in the bands of primary dendrites and in the eutectic grains.
The structure after rolling and ECAP retains a banded appearance, but the width of bands of primary dendrites of LiZn4 phase decreases and reaches an average value of 14±7 μm. The bands becomes elongated and the grain shape ratio is 1:6. EBSD analysis shows an increase in the fraction of high-angle boundaries (HABs), 54%. Grains of 2–6 µm in size are observed both in bands and in the fragmented eutectic phase. The increase in the fraction of HABs can be associated with the completion of the building of low-angle boundaries observed at the rolling stage.
Mechanical tests
Fig.4 and Table 1 show tensile curves and summary data on mechanical properties of Zn-Li-Mn alloy after various deformation treatments. In initial (after homogenisation) state, the curves were not obtained, as the samples are not ductile and fail before reaching the yield strength. It was shown in [14] that the fine-grained structure effectively prevents crack propagation and significantly increases ductility. Accordingly, the alloy was deformed by the severe plastic deformation method of ECAP. According to the results, it was found that this method did not lead to the ductility changes, the samples, just like in initial state, demonstrated brittle fracture. As a result of deformation by hot rolling method, the specimens acquired not only ductility, but also a high yield strength and a high ultimate tensile. However, the ductility values (8±2%) still do not reach the values required for implant fabrication. The best combination of strength characteristics (ultimate tensile strength 511±12 MPa) and ductility (47±7%) was shown by samples after combined deformation by rolling and ECAP methods. The highest strength in the alloy is observed after rolling and is is 530±17. Conducting ECAP after rolling leads to a slight decrease in strength characteristics to 511±12.
DISCUSSION
The constituent constituent phases of Zn-0.8Li-0.1Mn alloy have a weakly plastic hexagonal close-packed lattice (HCP lattice), and also the alloy was obtained by casting and even after homogenisation annealing retains a dendritic structure, which additionally embrittles the alloy. To increase the technological plasticity it is necessary to incorporate new slip systems, which is possible by increasing the deformation temperature. Carrying out rolling at high homologous temperature with a small deformation in 1 cycle, contributed to an increase in ductility up to 8%. It is most likely that rolling produces mainly normal stresses [15] rather than shear stresses, which lead to the crushing of the dendritic structure. Also, formation of a fine-grained structure in the rolling process allows realizing of shear deformation due to a larger number of grains favourably oriented to shear stresses during subsequent ECAP. After rolling, an increase in the alloy strength up to 530 MPa was observed, which is most likely due to form the fine-grained structure in the eutectic.
The analysis of the structure obtained after the additional operation of ECAP indicates a significant refinement of structural state, a significant increase in the
fraction of HABs, which can be explained by the completion of the building of LABs observed after rolling. However, there is a slight decrease in the ultimate tensile strength and yield strength by ~4% and an increase in ductility. The observed behaviour of the alloy can be explained by formation of a special texture during ECAP, which leads to some softening of the alloy and simultaneous increase in ductility due to orientation of the basal plane in the direction of the action of shear stresses [16]. Similar behaviour is demonstrated by samples of magnesium alloys subjected to ECAP [17, 18]. In addition, a more homogeneous structural state should exhibit higher corrosion resistance as it reduces the likelihood of galvanic corrosion. Homogeneity in grain size eliminates galvanic couple formation between large grains and adjacent small grains [19]. Reduction of the grain size due to decrease in the value of the free path of dislocations reduces inhomogeneity in the grain body by dislocation pileups, which can also cause galvanic corrosion [20]. The formed structural state in the course of combined processing provides the necessary characteristics required for the application of this alloy as a material for the fabrication of bioresorbable implants.
CONCLUSIONS
It is possible to improve the strength characteristics by plastic deformation methods, but the ductility to realise deformation by severe plastic deformation methods remains unsatisfactory. It is shown that pre-deformation by rolling method allows to increase technological plasticity of the material for subsequent realisation of severe plastic deformation. As a result of combined processing, namely rolling+ECAP, the ductility of the samples of the studied alloy was increased, the value reached from 0 (in initial state) to 47%, which is very important for the manufacture of bioresorbable implants. The strength characteristics as a result of the developed combined processing exceeded the values required for the fabrication of bioresorbable metallic implants.
ACKNOWLEDGEMENTS
This work was financially supported by the Russian Science Foundation (project No. 24-43-00154). The research part of the work was carried out using the equipment of the Nanotech Collective Use Centre (Ufa University of Science and Technology).
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
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.
Traditional orthopaedic metallic implants such as stainless steel, cobalt-chromium alloys and titanium-based alloys are primarily designed for osteosynthesis; their high strength and acceptable biocompatibility allow them to be used as primary load-bearing orthopaedic devices [1]. However, there are known limitations for such implants. Firstly, their prolonged stay leads to accumulation of harmful elements such as Ni, Co, Cr, Cr, Al and V in the human body, which cause chronic inflammation and eventually lead to revision surgeries [2]. Secondly, the mismatch of high elastic modulus between implants and human bone leads to bone resorption and implant failure after time [3]. The revolutionary concept of biodegradable implants aims to solve these problems [4]. Research on synthetic biodegradable polymers began in the 1960s. Biodegradable polymers such as polyglycolide (PGA), polylactide (PLA), and poly (L- or D, L-lactic acid) (PLLA/PDLLA) were approved by the FDA for use in orthopaedic fixation implants after 60 years of development [5]. However, their use was limited to skeletal sites with low load due to their insufficient strength. In addition, degradation products formed by volumetric erosion of biodegradable polymers can cause a noninfectious inflammatory response, which eventually leads to bone resorption [6]. More recently, development of magnesium (Mg)-based biodegradable implants has provided scientists and clinicians with new opportunities to address the shortcomings of existing commercial orthopaedic implants. As biodegradable metals, Mg-based materials provide good mechanical support compared to their polymer counterparts, demonstrating a desirable modulus of elasticity close to that of human bone. More importantly, degradation of Mg releases useful products such as Mg ions that promote bone regeneration and accelerate bone healing [7]. Tremendous efforts in basic research have brought the use of Mg to clinical trials and commercialisation in Europe [8], Korea [9], but their application is still limited to fixation of non-load bearing bone fractures and bone fragments. Meanwhile, more systematic studies are needed to slow down formation of gas cavities that are formed during the Mg dissolution in the physiological environment [10]. As a result, no single material can be used as a material for a bioresorbable implant used for high loads. Recently, a group of scientists conducted a systematic evaluation of biodegradable zinc (Zn) alloys as orthopaedic implants [11]. The results showed that Zn-Li, Zn-Mg, Zn-Ca and Zn-Sr alloys were the most favoured candidates for bone implants. Among them, Zn-Li alloys showed strength comparable to commercially pure titanium and stainless steel, indicating their great potential for use as implants with high load-bearing capacity. Recently, development of clinical needs requires improvement of the interaction between human tissues and implants to promote the process of bone regeneration and healing. Zn-Li alloy implants have been found to have a favourable effect on new bone tissue formation [12, 13]. However, alloying alone is often insufficient to achieve the required mechanical properties. Therefore, there is a need for deformation treatment of the alloy. From this point of view, application of severe plastic deformation and rolling is promising, which makes it possible to harden metals and alloys due to the formation of ultrafine-grained (UFG) structure. Thus, development of new ultrafine-grained medical materials based on zinc is of significant practical interest for application in clinical practice.
RESEARCH METHODS
Zn-0.8%Li-0.1%Mn (wt.%) alloy samples with a diameter of 20 mm and a length of 100 mm were taken as initial state and subjected to homogenisation in a Nabertherm muffle furnace at 300 °C for 72 hours with cooling in water.
The initial samples were rolled on a 2-roll Hankook M-Tech mill for section rolling in 2 stages: from the diameter of 20 mm to the diameter of 15 mm at temperature of 300 °C; with a change of cross-section from a circle to a square of 10 × 10 mm2 also at a temperature of 300 °C. The strain was 1.1.
The rolled specimens were subjected to equal-channel angular pressing (ECAP). Deformation was carried out at temperatures of 300 °C, 250 °C, 200 °C, 150 °C with 2 passes for each temperature. The total number of passes was 8. ECAP of initial billets was carried out in a round channel, rolled billets in a square channel. The angle of intersection of the channels was Ф = 120°, following the known route ВС (after each pass the billet was rotated around its longitudinal axis by the angle 90°). For 8 passes of the ECAP, the final accumulated strain of deformation was e = 5.6.
To reveal structure in the longitudinal direction, the samples were immersed for 30 seconds in an etchant with the following composition: 5 ml of nitric acid (HNO3) and 95 ml of ethyl alcohol (C2H5OH). The structure was analysed on a JEM-6390 scanning electron microscope (SEM) in secondary electron mode at an accelerating voltage of 30 kV. EBSD maps were obtained using a Thermo Scientific Q250 electron microscope of (FEI) at an accelerating voltage of 30 kV. The scanning step was 0.5 μm.
Mechanical tensile tests were performed on an Instron 5982 tensile testing machine on small specimens with working gauge dimensions of 0.6 × 1 × 4 mm3 at room temperature at a strain rate of 10–3 s–1. The test specimens were cut from deformed billets in longitudinal section.
RESULTS
Structural studies
In initial state, a complex dendritic structure is observed. The LiZn4 phase (Fig1a.) forms the base in the form of dendritic branches, the thickness of which can reach 100 μm. Between these branches, a eutectic of Zn+LiZn4 composition is observed, apparently crystallising a little later when the temperature reaches ~403 °C during cooling [14].
EBSD studies indicate formation of a large number of subboundaries in the branches of LiZn4 phase dendrites. The fraction of low-angle boundaries (LABs) reaches 87% (Fig.1c). In the eutectic mixture of LABs phases are practically not observed.
After hot rolling, the structure acquires a banded appearance and becomes refined. The average transverse size of bands of primary dendrites of LiZn4 phase is 35±16 μm, with grain shape elongation ratio ~1:3. The eutectic also undergoes transformation into a grain structure with grain size of Zn and ZnLi4 ~2 μm. EBSD analysis indicates preservation of sub-grain structure in the body of deformed grains. The fravtion of LABs is preserved and amounts to 86%. Low-angle boundaries are observed both in the bands of primary dendrites and in the eutectic grains.
The structure after rolling and ECAP retains a banded appearance, but the width of bands of primary dendrites of LiZn4 phase decreases and reaches an average value of 14±7 μm. The bands becomes elongated and the grain shape ratio is 1:6. EBSD analysis shows an increase in the fraction of high-angle boundaries (HABs), 54%. Grains of 2–6 µm in size are observed both in bands and in the fragmented eutectic phase. The increase in the fraction of HABs can be associated with the completion of the building of low-angle boundaries observed at the rolling stage.
Mechanical tests
Fig.4 and Table 1 show tensile curves and summary data on mechanical properties of Zn-Li-Mn alloy after various deformation treatments. In initial (after homogenisation) state, the curves were not obtained, as the samples are not ductile and fail before reaching the yield strength. It was shown in [14] that the fine-grained structure effectively prevents crack propagation and significantly increases ductility. Accordingly, the alloy was deformed by the severe plastic deformation method of ECAP. According to the results, it was found that this method did not lead to the ductility changes, the samples, just like in initial state, demonstrated brittle fracture. As a result of deformation by hot rolling method, the specimens acquired not only ductility, but also a high yield strength and a high ultimate tensile. However, the ductility values (8±2%) still do not reach the values required for implant fabrication. The best combination of strength characteristics (ultimate tensile strength 511±12 MPa) and ductility (47±7%) was shown by samples after combined deformation by rolling and ECAP methods. The highest strength in the alloy is observed after rolling and is is 530±17. Conducting ECAP after rolling leads to a slight decrease in strength characteristics to 511±12.
DISCUSSION
The constituent constituent phases of Zn-0.8Li-0.1Mn alloy have a weakly plastic hexagonal close-packed lattice (HCP lattice), and also the alloy was obtained by casting and even after homogenisation annealing retains a dendritic structure, which additionally embrittles the alloy. To increase the technological plasticity it is necessary to incorporate new slip systems, which is possible by increasing the deformation temperature. Carrying out rolling at high homologous temperature with a small deformation in 1 cycle, contributed to an increase in ductility up to 8%. It is most likely that rolling produces mainly normal stresses [15] rather than shear stresses, which lead to the crushing of the dendritic structure. Also, formation of a fine-grained structure in the rolling process allows realizing of shear deformation due to a larger number of grains favourably oriented to shear stresses during subsequent ECAP. After rolling, an increase in the alloy strength up to 530 MPa was observed, which is most likely due to form the fine-grained structure in the eutectic.
The analysis of the structure obtained after the additional operation of ECAP indicates a significant refinement of structural state, a significant increase in the
fraction of HABs, which can be explained by the completion of the building of LABs observed after rolling. However, there is a slight decrease in the ultimate tensile strength and yield strength by ~4% and an increase in ductility. The observed behaviour of the alloy can be explained by formation of a special texture during ECAP, which leads to some softening of the alloy and simultaneous increase in ductility due to orientation of the basal plane in the direction of the action of shear stresses [16]. Similar behaviour is demonstrated by samples of magnesium alloys subjected to ECAP [17, 18]. In addition, a more homogeneous structural state should exhibit higher corrosion resistance as it reduces the likelihood of galvanic corrosion. Homogeneity in grain size eliminates galvanic couple formation between large grains and adjacent small grains [19]. Reduction of the grain size due to decrease in the value of the free path of dislocations reduces inhomogeneity in the grain body by dislocation pileups, which can also cause galvanic corrosion [20]. The formed structural state in the course of combined processing provides the necessary characteristics required for the application of this alloy as a material for the fabrication of bioresorbable implants.
CONCLUSIONS
It is possible to improve the strength characteristics by plastic deformation methods, but the ductility to realise deformation by severe plastic deformation methods remains unsatisfactory. It is shown that pre-deformation by rolling method allows to increase technological plasticity of the material for subsequent realisation of severe plastic deformation. As a result of combined processing, namely rolling+ECAP, the ductility of the samples of the studied alloy was increased, the value reached from 0 (in initial state) to 47%, which is very important for the manufacture of bioresorbable implants. The strength characteristics as a result of the developed combined processing exceeded the values required for the fabrication of bioresorbable metallic implants.
ACKNOWLEDGEMENTS
This work was financially supported by the Russian Science Foundation (project No. 24-43-00154). The research part of the work was carried out using the equipment of the Nanotech Collective Use Centre (Ufa University of Science and Technology).
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
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.
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