Structure,mechanical characteristics,biodegradation,and in vitro cytotoxicity of magnesium alloy ZX11 processed by rotary swaging

Ntli Mrtynenko,Ntli Anisimov,Mikhil Kiselevskiy,Ntli Tchkov,Din Temrliev,Dmitry Prosvirnin,Vldimir Terentiev,Andrey Koltygin,Vldimir Belov,Mikhil Morosov,Vldimir Yusupov,Sergey Dotkin,Yuri Estrin

aNational University of Science and Technology"MISIS",Leninsky prospect,4,119049 Moscow,Russia

b A.A.Baikov Institute of Metallurgy and Materials Science of the RAS,Leninsky prospect,49,119334 Moscow,Russia

c?N.N.Blokhin National Medical Research Center of Oncology?of the Ministry of Health of the Russian Federation,Kashirskoye Highway,23,115478 Moscow,Russia

d A.M.Prokhorov General Physics Institute of the RAS,Vavilova st.,38,Moscow 119991,Russia

eDepartment of Materials Science and Engineering,Monash University,Clayton,Melbourne VIC 3800,Australia

fDepartment of Mechanical Engineering,The University of Western Australia,Crawley,Perth WA 6009,Australia

Received 11 May 2020;received in revised form 2 August 2020;accepted 30 August 2020 Available online 25 September 2020

Abstract Rotary swaging(RS)of alloy Mg–1.03Zn–0.66Ca(ZX11)was shown to refine the average grain size to 4.5±1.2μm in a longitudinal section and 4.8±0.9μm in a transverse section.In addition,a small amount of Mg2Ca particles about 300nm in size and Mg6Zn3Ca2 particles with a size of about 100nm was detected.This resulted in pronounced strengthening:the yield strength and the ultimate tensile strength rose to 210±8MPa and 276±6MPa,respectively,while the elongation hardly decreased(22.0±1.8% and 18.3±2.9% before and after RS).Furthermore,RS led to an increase in the fatigue limit of the alloy from 120MPa to 135MPa and did not impair its resistance to chemical corrosion.The studies in vitro showed that ZX11 induces hemolysis without inhibiting the viability of peripheral blood mononuclear cells and has a more pronounced cytotoxic effect on tumor cells in comparison with non-transformed cells.No significant difference of the latter effect between the initial and the deformed states was observed.

Keywords:Magnesium alloy;Rotary swaging;Mechanical characteristics;Hemolysis;Cell viability;Cytotoxicity.

1.Introduction

Low-alloyed magnesium alloys of the Mg–Zn–Ca system are among the most popular groups of magnesium alloys earmarked for applications in bioresorbable medical devices[1–6].They are suitable for the production of bioresorbable orthopedic implants owing to their good biocompatibility and biodegradability[7,8].However,the strength characteristics of these alloys are usually rather poor,mainly because of a low content of the alloying elements.Therefore,possibilities for strength enhancement by plastic deformation need to be explored.To date,the properties of Mg–Zn–Ca alloys after deformation by conventional methods(extrusion[9,10],rolling[11,12],etc.)and after severe plastic deformation(SPD)by high pressure torsion(HPT)[13,14]and equal channel angular pressing(ECAP)[15,16])have been studied.These methods,especially SPD,were found to be efficient for increasing both the strength characteristics and the ductility of these alloys.However,there is a flip side to both groups of methods.While the conventional deformation techniques do not deliver a significant microstructure refinement of Mg–Zn–Ca alloys,the SPD-based ones are not readily applicable at industry scale–at least currently.As was shown by previous studies,it is possible to increase the ultimate tensile strength of the Mg–1.0Zn–0.5Ca alloy to～250MPa with a ductility of～17% by hot extrusion[10].A good combination of strength(～220MPa)and ductility(～10%)was also obtained by hot rolling of the alloy Mg–2.0Zn–1.2Ca[12].However,these works did not concern such an important aspect of the alloys as their biodegradation rate.This property was considered in the works[14,15]along with studying the mechanical properties of alloy Mg–2.0Zn–0.24Ca processed by HPT.This treatment produced an ultrafine-grained(UFG)structure with uniformly distributed particles of the Mg2Zn3phase.This microstructure led to a decrease in the current density in electrocorrosion tests and reduced the corrosion non-uniformity in the alloy[14].It was also shown that ECAP of alloy Mg–1.0Zn–0.3Ca increases both the strength and the ductility without affecting its corrosion resistance[15].However,a heterogeneous structure caused by ECAP may lead to localization of corrosion and non-uniformity of mechanical characteristics in the future.In addition,traditional deformation methods often lead to the formation of an inhomogeneous microstructure,often bimodal or elongated along the deformation axis[9,17,18].This structure is effective for improving the strength characteristics,but it usually negatively affects the corrosion resistance of the material,accelerating degradation and causing pitting.Corrosion resistance is a highly structure-sensitive property.The dependence of the corrosion resistance of magnesium alloys on their phase composition and microstructure,notably the grain size,was addressed in numerous publications[6,19–27].It was shown previously that phase inhomogeneity(for example,precipitation of a more corrosion-resistant phase on grain and dendrite boundaries)leads to a deterioration of the corrosion resistance.An acceleration of the degradation process in alloys after aging was often observed when the precipitated particles created many micro-galvanic couples.A more uniform phase distribution in the case of aging is more advantageous for corrosion resistance.Despite the progress made with the investigations of the microstructure effect on biocorrosion resistance of Mg alloys,the mentioned studies did not give an unequivocal answer to the question about the effect of the grain size on the degradation rate of magnesium alloys.In most publications on the subject,an improvement in the corrosion resistance with grain refinement down to the UFG state was asserted,cf.,e.g.[6,27].The effect was associated with the acceleration of the formation of a protective film on the surface of the alloy and its greater stability for smaller grain size.However,in some cases the formation of an UFG structure led to anincreasein the degradation rate due to the accumulation of crystal lattice defects and phase transformations occurring during the deformation[28].This controversy shows the significance of further studies into the corrosion properties of magnesium alloys(especially bioresorbable ones)after deformation.Not only would such studies provide much-needed information on one of the most important properties of the magnesium alloys,but they would also help to develop theoretical understanding of the effect of grain refinement on their corrosion rate.As increased degradation rate heightens the risk of implant failure during using,but it also often compromises the biocompatibility,causing cell necrosis due to increase of pH.Therefore,it is important not only to increase the strength of the material,but also to obtain a homogeneous microstructure,which will not worsen its corrosion resistance.Rotary swaging(RS)is an attractive alternative option owing to ease of upscaling and the efficacy of microstructure refinement of metallic materials,including magnesium alloys[29–35].This deformation method enables microstructure refinement of magnesium alloys down to a UFG state and can do that in an industrially viable and economical way.In addition,this method is widely used for the manufacture of hollow metal tubes[36–38],which,in the future,can be used to create hybrid biodegradable implants with a bone-like structure comprising a strengthened skeleton and porous filler material.In this work,we investigated a Mg-1.03Zn-0.66Ca alloy(designated ZX11)focusing on the effect of RS on its microstructure,mechanical characteristics,corrosion resistance,and biocompatibility.In addition,we compared cytotoxicity of the alloy to multipotent mesenchymal stromal cells and tumor cells of theК562 line in vitro.The latter aspect is important in view of the potential use of the alloy in orthopedic implants,especially for replacement of osteosarcoma-affected bone tissue after resection.

Table 1The designed and the actual chemical composition of alloy ZX11.

2.Materials and methods

High purity magnesium(99.98wt% Mg),zinc(99.98wt%Zn),and a master alloy Mg-30wt% Ca of own production were used to prepare the alloy ZX11 for this study.The melt was prepared using a resistance furnace in a steel crucible under a protection gas mixture(N2+1% vol.SF 6).The furnace degassing and refining of the melt was carried out by argon blowing in crucible.Cylindrical ingots were cast in a permanent steel mold.A pouring box was used to prevent the centerline shrinkage of the ingot.The designed and the actual chemical composition of the alloy is shown in Table 1.The concentration of the impurity elements(Fe,Cu)in the alloy did not exceed 0.01wt.%.

Rods for RS with a diameter of 20mm and a length of 200mm were machined from these ingots.They were homogenized at 450°C for 5 h and water quenched.RS was performed on an RCM 2129.02 rotary swaging machine(see[36]for detail)in multiple passes,with a stepwise decrease of the processing temperature from 400°C to 300°C(in 50°Сsteps).The final cumulative strain(ε=ln(A0/Af)was 2.77,whereA0andAfdenote the initial and the final cross-section area of the billets,respectively).The microstructure was examined using an Axio Observer D1m in a Carl Zeiss optical microscope in longitudinal section of samples.The microstructure of the alloy after RS was studied by transmission electron microscopy(TEM)using a JEOL JEM 2010 microscope(Jeol,Tokyo,Japan)equipped with an Energy-Dispersive X-Ray Spectroscopy(EDS)analyzer.The operating voltage was 200kV.The foils for TEM studies were prepared by ion-milling in a precision ion polishing system(Gatan,PIPS II,Gatan Inc.Pleasanton,United States).The average grain size was estimated by the method of random sections with the aid of the Image ExpertPro 3 software.The uniaxial tensile properties were evaluated using an Instron 3382 testing machine with an extension rate of 1mm/min on flat samples with a gage cross-section of 2mm×1mm and length of 5.75mm.The fatigue tests were carried out under cyclic tension using an ElectroPulsTME3000 machine(testing frequency of 30Hz,stress ratioR=0.1)on flat samples with a gage cross-section of 1mm×1mm and length of 5.75mm.

The degradation rate(DR...…),biocompatibility and cytoxicity were studied on samples～6mm×5mm×1.5mm in size(length×width×thickness).The samples were immersed in 70% ethanol and incubated overnight for sterilization.For the evaluation of biodegradation,the samples were incubated at 37°C in the fetal bovine serum(FBS)(Hy Clon,Thermo Fisher,UK)for 1,3 and 14 days.The incubation medium was completely replaced daily,in order to avoid an increase of the pH value above 8.0.Removal of the corrosion products and the calculation of DR...…were conducted according to the ASTM_G1–03-E code.To assess hemocompatibility,the cell viability and the hemolysis ratio were evaluated(see[39]for detail).In short,the peripheral blood mononuclear cells(PBMC)and red blood cells(RBC)were isolated from pooled blood of C57Bl/6 mice(n=3)stabilized with sodium heparin(200 IU/ml).Then PBMCs were washed twice and suspended in the growth medium RPMI-1649,containing 4mM L-glutamine,1% penicillin/streptomycin(all PanEco,Russia)and 10% FBS.The PBMCs concentration in the suspension was 6.2×104cells/ml.RBCs were suspended in phosphate-buffered saline in concentration 7.8×108cells/ml.PBMCs viability was evaluated after 24 h incubation with samples of the alloy while the hemolysis ratio was assessed after 2,4,and 6 h of incubation.In controls,cells were treated only with the growth medium.

To study specific cytotoxicity,the samples were incubated with tumor cells(4×104cells/ml)and non-transformed cells(4.6×104cells/ml)in the growth medium in 24-well culture plates(Costar,USA)for 2 days under 5% CO2at 37°C.As a model of tumor cells,K562 cells(from the collection of NN Blokhin NMRC of Oncology,Russia)were used.The multipotent mesenchymal stromal cells(MMSCs)of the third passage,generated from dog bone marrow,were used as model non-transformed cells(see[39]for detail).Cells incubated on the bottom of alloy-free wells were used as a control.The concentration of dead and apoptotic cells was evaluated with MuseRAnnexin V & Dead Cell Kit(EMD Millipore Corp.,USA)using Muse Cell Analyzer(EMD Millipore Corp.,USA).These experiments were evaluated in triplicate at each of the designated times.The data were presented as a mean value±standard deviation.One-way ANOVA andt-test analysis were used to calculate the p-value with StatisticaR6.0 software(StatSoft,USA).A p-value below 0.05 was considered to represent a statistically meaningful difference.

All studies and manipulations with animals were performed according to the regulations of the Local Ethics Committee of the NN Blokhin National Medical Research centre of Oncology of the Health Ministry of Russia.

3.Results

Fig.1 shows the microstructure of the alloy in the initial state and after RS at 300°С(cumulative strainε=2.77).After homogenization of the cast alloy,an equiaxed structure with an average grain size of 54.1±2.5μm was formed(Fig.1(a)).After RS,a mostly recrystallized microstructure without any signs of the presence of intermetallic particles was observed in longitudinal section of the processed rod.The microstructure was uniform over the cross section of the rod both in the center and on the edge of a specimen.No elongated grains were detected.The average grain size was 4.5±1.2μm in a longitudinal section(Fig.1(b))and 4.8±0.9μm in a transverse section(Fig.1(c)).

Although light microscopy did not reveal any second-phase particles,the study of the microstructure of the alloy by TEM showed the presence of two types of particles in the alloy after RS.Particles of the first type were composed of two elements:magnesium and calcium.They were formed predominantly on grain boundaries.The average size of these particles was about 300nm(Fig.2(a)and(b)).Particles of the second type were located mainly in the bulk of the grains and comprised three elements:magnesium,zinc and calcium.The size of these particles was approx.100nm(Fig.2(c),(d)).Based on the data obtained and on previous studies,we identified these compounds as Mg2Ca and Mg6Zn3Ca2,respectively[40–43].However,it should be noted that the volume fraction of both particles was so small that they could not be detected by Xray analysis.

The grain refinement after RS led to a sizeable strengthening of the alloy(Fig.3(a),(b)).Thus,the yield stress(YS)after RS increased from 121±8MPa to 210±8MPa,and the ultimate tensile strength(UTS)rose from 217±3MPa to 276±6MPa.However,the formation of a homogeneous recrystallized structure,despite a grain refinement by more than a factor of 10,did not diminish the tensile ductility of the alloy.This enhancement of strength was accompanied with a very moderate decrease of tensile elongation(El):from 22.0±1.8% in the initial state to 18.3±2.9% after RS.In addition,the grain refinement led to an increase in the fatigue limit based on 107loading cycles,which rose from 120MPa in the initial state to 135MPa after RS(Fig.3(c)).It is also interesting to note that fatigue failure of samples in both microstructural states occurred mainly in the low-cycle fatigue region(up to 105loading cycles).This observation suggests that the main part of the fatigue life to failure is associated with the stage of initiation of a fatigue crack[44].

Fig.1.The structure of the alloy in the initial state(a)and after RS at 300 °C in a longitudinal(b)and a transverse(c)section.

Fig.4(a)shows that the formation of a recrystallized structure after RS did not impair the corrosion resistance of the alloy after 1,3 and 14 days of incubation in FBS.Neither did an increase in the incubation time affect the degradation rate.The DR...…measurements returned the following values:0.22±0.01 and 0.28±0.01mm/year after 1 day of incubation,0.22±0.07mm/year and 0.22±0.01mm/year after 3 days of incubation and 0.29±0.09mm/year and 0.44±0.09mm/year after 14 days of incubation,for the initial and swaged alloy,respectively.At the same time,the study of the sample surface after 14 days in the growth medium showed that the degradation of the alloy in both microstructural states proceeded uniformly,without a pronounced pitting.Minor signs of corrosion localization were observed only on the edges of both samples(Fig.4(b)).

The hemocompatibility study showed that coincubation of the alloy in the initial state and after RS with RBCs induced hemolysis,whose level corresponded to 8±2% and 8±4% after 2h,12±2% and 13±3% after 4h,and 46±1%and 43±10% after 24h,respectively.It should be borne in mind that according to the ISO 10,993–4(Hemocompatibility)standard,the time of co-incubation of cells to study the level of hemolysis to assess biocompatibility does not exceed 4 h,while the American Society for Testing and Materials(ASTM)standard recommends just 3 h.Usually,in the body the material of the implant does not come into contact with the same RBCs for a long time,since those,being within the blood vessels and capillaries,constantly move along a closed system that provides blood flow.However,we decided to increase the incubation period to 24 h in order to further investigate possible differences in the effect of different alloy states.That is,the result of RBC hemolysis,assessed after 24h of incubation,is not a characteristic of biocompatibility,but is rather intended to show the differences in the cell response to the two states of the alloy over a longer time span.A cytotoxicity assay showed that incubation with the alloy in both states did not lead to a significant decrease in viability of PBMCs in comparison with the control(98±4%and 91±3%for initial and swaged state,respectively).A comparative statistical analysis of the results did not reveal any significant difference between the levels of the cytotoxic activity of the alloy in the two microstructural states.

Fig.2.TEM images of the microstructure of the alloy after RS:(a)a Mg2Ca particle with the corresponding electron diffraction pattern in the insert;(b)EDS pattern for the Mg2Ca particle;(c)a Mg6Zn3Ca2 particle(c);and(d)EDS pattern for the Mg6Zn3Ca2 particle.

Incubation of the alloy in the initial state and after RS with tumor cells for 3 days increased the concentration of dead cells in comparison with the control,whereas a change in this parameter for a MMSC culture could not be established with a sufficient statistical reliability.Co-incubation with the alloy also resulted in an increase in the concentration of cells showing signs of apoptosis,which was appreciably more pronounced in the culture of tumor cells as compared to the effect of exposure to the alloy on MMSCs.No significant differences in cell response to the alloy between the initial state and the post-RS state were found(Fig.5).

The study showed that the alloy in both states induced hemolysis without inhibiting viability of PBMCs.The alloy demonstrated specific cytotoxic activity against tumor cells in vitro.Incubation of tumor cells with the alloy brought about a noticeable increase in the number of dead and apoptotic tumor cells of K562 line compared to the non-transformed MMSCs.There was no appreciable difference between cytotoxicity of the samples in the homogenized initial state and the RS-processed samples.

4.Discussion

The conducted studies have shown that RS is an effective way to increase the strength of the alloy Mg–1.03Zn–0.66Ca,both in terms of its uniaxial tensile and cyclic characteristic,due to significant grain refinement the process effected.By contrast,the swaging process at elevated temperatures(400–300°C)led to the formation of a uniform,generally recrystallized structure and reduced the number of crystal lattice defects,which enable retention of the ductility at the initial level.In addition,it was previously shown that at 300°C and below,the formation of two types of particles was to be expected in this alloy:those of a(Mg,Zn)2Ca Laves phase and a Ca3MgxZn15?xphase(4.6≤x≤12)[18].In the present study the Mg2Ca and Mg6Zn3Ca2particles were detected[40–43],but their volume fraction was very small.A possible reason for that may lie in the high RS speed,high processing temperature,and further exposure to heat during subsequent slow air cooling of the rods.Under such conditions,no time is provided for particles to precipitate,while those particles that precipitated during the slow cooling process at intermediate swaging steps tend to dissolve due to a sufficiently high temperature(400–300°C).The ensuing microstructure of the alloy after the RS thus consists of grains of a solid solution supersaturated with Ca and Zn atoms and small amount of Mg2Ca and Mg6Zn3Ca2particles.It should be noted,however,that further aging may potentially lead to additional precipitation of particles causing extra strengthening of the alloy.An in-depth study of the decomposition of a supersaturated solid solution after RS and additional heat treatment is undoubtedly of interest and will be carried out in our forthcoming work.

Fig.3.Mechanical properties of the alloy in the initial state(IS)and after RS:(a)Engineering stress vs.engineering strain curve,(b)Histograms summarizing the tensile characteristics,and(c)The Woehler(S-N)curves for cyclic deformation.

The small volume fraction of second-phase particles(especially Mg2Ca)in the structure can be one of the reasons why the degradation rate did not worsen after RS.It was shown by earlier investigations that the formation of corrosion-resistant particles can accelerate the degradation of the less noble metal matrix due to formation of numerous microcorrosive cells on the surface of the material[45–46].In our case,the high swaging temperature helped to avoid(or minimize)this effect.Besides,a high concentration of Zn and Ca in solid solution further improves the corrosion resistance of magnesium.The low DR...…values observed can also be associated with the high purity of the alloy and a good quality of the sample surface preparation.In addition,the uniformity of corrosion can be associated with the homogeneity of the alloy structure,after both homogenization and RS.This is important for the application of the material in medical implants,since pitting formation after a surgical operation can lead to a loss of loadbearing capability of an implant and its premature failure.The issue of uniformity of biocorrosion of Mg–Zn–Ca alloys after severe plastic deformation was also considered in[47].A study of the mechanical properties,corrosion resistance and biocompatibility of the alloy Mg–1.0Zn–0.1Ca carried out by ECAP Merson et al.[6]provides a useful dataset against which our results can be gauged.Our results are consistent with theirs.Specifically,the reduction of the average grain size to about 2.1μm attained by using ECAP processing led to an increase in the UTS to 285MPa,which is in the same ballpark as the value of 276±6MPa we obtained.However,the formation of an inhomogeneous structure with the occurrence of elongated grains resulted in a rather low tensile ductility(～13.5%)[6].In addition,inhomogeneous structure and a pretty high content of harmful impurities(mainly Fe)caused a high degradation rate(3.2mm/year)in Ringer’s solution and a strong localization of corrosion[6].In our case,the high purity of the alloy and a uniform structure enabled us to avoid such undesired effects.A comparison of the degradation rate measured in the present study with the data obtained earlier for alloys of the Mg–Zn–Ca system is given in Table 2.

Fig.4.Degradation rate of the alloy in the initial state(IS)and after RS(a)and sample surface after 14 days in the growth medium(b).

Fig.5.Antitumor cytotoxic activity of the alloy before(IS)and after RS compared with the effect on non-transformed MMSCs.The diagrams present the change of the amount of apoptotic cells(Annexin V(+)cells)and dead cells in cultures of K562 tumor cells and MMSCs,?p<0.05 difference with Control.

As can be seen from the data presented in the table,the values of the degradation rate we obtained are substantially lower than those found by previous workers for alloys of the Mg–Zn–Ca system.The results of earlier work on cast alloys showed that the formation of a two-phase structure was detrimental in that it increased the degradation rate[41–43].The case of Mg–1.2Zn–0.5Ca alloy with a more uniform phase distribution,formed after quenching and aging,exemplifies a decrease in the degradation rate almost by a factor of onehalf.That is,if the precipitation of particles during deformation processing cannot be suppressed,one should aim at establishing a more uniform distribution of precipitates in the microstructure.It should also be noted that,in general,the extruded Mg–Zn–Ca alloys exhibited slower degradation than cast alloys,although the DR...…value remained quite high[6,48].In the case of the alloy considered in this work,as noted earlier,the low DR...…value can be associated with its high purity and single-phase structure in the initial state and only a minor volume fraction of second-phase particles in the RS-treated alloy.However,one cannot rule out the possibility that with an increase in the incubation time,the degradation rate may increase,as after 14 days of incubation,the formation of localized corrosion sites was noticed(Fig.4(b)).

Table 2Degradation rate of bioresorbable Mg–Zn–Ca alloys.

It can be conjectured that the absence of a difference in the degradation rate between the two microstructural states of alloy ZX11 is also one of the main reasons for their effect on the cells being similar,which is the case with both normal and tumor cells.Indeed,the alkalization of the growth medium that occurs during degradation of magnesium alloys due to an increase in the number of OH?ions may lead to cell necrosis.In our case,this effect was not prominent due to a low degradation rate.The release of Mg2+,Zn2+and Ca2+ions did not have a significant negative effect on normal cells either,which is consistent with previous studies[6,49–50].The authors of the mentioned publications demonstrated that the binary Mg–6wt%Zn alloy could still support earlier adhesion of pre-osteoblastic cells of the MC3T3-E1 cell line[49].Furthermore,it was shown that alloy Mg-2.0Zn-1.0Ca had no toxic effect on the viability and proliferation of adiposederived mesenchymal stem cells[50].Merson et al.[6],who studied the properties of Mg alloys of the Mg–Zn–X(X=Ca,Zr,Y or none)systems with various microstructures(ZK60(I),Z4,ZX10,WZ31,WZ62),established that none of the alloys tested had a serious cytotoxic impact on the immortalized human fibroblasts in vitro.Notably,alloy ZX10(similar in composition to alloy ZX11 we studied)exhibited cell viability values(90–100%)close to those we obtained.These results,together with the evaluation of the mechanical and corrosion properties of the alloys investigated,led the authors to the conclusion that these alloys are fit for biomedical applications[6].By contrast,the presence of Mg2+,Zn2+and Ca2+ions in the growth medium with alloy ZX11 did have a distinct effect on tumor cells.A similar effect was also found for alloys of the Mg–Ag system[47].A possible reason for this selectivity in the cytotoxic activity of alloy ZX11 is a higher vulnerability of the tumor cell DNA to the biodegradation products due to an increased occurrence of mitosis in tumor cells.

5.Conclusions

(1)As a result of RS processing,the average grain size of alloy ZX11 was reduced from 54.1±2.5μm to 4.5±1.2μm in a longitudinal section and 4.8±0.9μm in a transverse section.Besides,submicron-sized second-phase Mg2Ca and Mg6Zn3Ca2particles,whose number density and volume fraction were small,were detected after RS.

(2)The grain refinement produced a favorable combination of strength and ductility.It raised the UTS level from 217±3 to 276±6MPa,at almost no cost in terms of tensile elongation.The fatigue limit of the alloy rose from 120 to 135MPa.

(3)The microstructure produced by RS did not impair the corrosion resistance of the alloy.

(4)It was shown that alloy ZX11 in both states induced hemolysis without inhibiting the viability of white blood cells.

(5)In the in vitro assays,the ZX11 alloy exerted a cytotoxic effect on the K562 tumor cells line stimulating apoptosis and cell death.A striking result is the selectivity of this effect:it was more pronounced for the tumor cells than for the non-transformed multipotent mesenchymal stromal cells.

(6)Owing to the complex of properties of alloy ZX11 studied,particularly the RS-induced ones,the alloy may be considered as a predilect material for bone replacement in cancer patients.

Acknowledgments

Funding support of investigations of microstructure,mechanical properties,corrosion resistance,biocompatibility and cytotoxicity was provided by the Russian Science Foundation(project #18-45-06010).Part of this work relating to studies of fatigue behavior was carried out within the governmental task #075-00947-20-00.