Kiki Guo,Mengyo Liu,Jinfeng Wng,b,Yufeng Sun,b,Weiqing Li,Shijie Zhu,b,c,?,Liguo Wng,b,c,Shokng Gun,b,c,?

aSchool of Materials Science and Engineering,Zhengzhou University,Zhengzhou 450001,Chinab Henan Key Laboratory of Advanced Magnesium Alloys,Zhengzhou 450002,Chinac Key Laboratory of Advanced Materials Processing & Mold Ministry of Education,Zhengzhou 450002,ChinaReceived 21 May 2019;received in revised form 17 July 2019;accepted 17 September 2019 Available online 6 August 2020

Abstract Magnesium alloys have narrow available slip result from close-packed hexagonal structure that limit their processing properties.In the recent work,the Mg-2Zn-0.46Y-0.5Nd,as materials for degradable stents,was applied to produce as-extruded micro-tube with an outer diameter of 3.0mm and a wall thickness of 0.35mm by hot extrusion with an extrusion ratio of 105:1 at 653K and rapid cooling.The fin microstructure of the dynamic recrystallization of as-extruded micro-tube could be preserved by rapid cooling such as water-cooled,resulting in more excellent mechanical properties relative to air-cooled micro-tube.The addition of rare earth elements Y and Nd results in continuous dynamic recrystallization dominated the dynamic recrystallization mechanism.During the hot extrusion process,the activation of the non-basal slip system,especially the pyramidal〈c+a〉slip,could significantl weaken the texture strength,and the as-extruded micro-tube exhibits weak“RE”texture components〈01ˉ11〉‖ED and〈ˉ12ˉ11〉‖ED.Hence,the magnesium alloy micro-tube prepared by the rapid cooling has f ne microstructure and weak texture,which is favorable for further process and governance.? 2020 Published by Elsevier B.V.on behalf of Chongqing University.This is an open access article under the CC BY-NC-ND license.(http://creativecommons.org/licenses/by-nc-nd/4.0/)Peer review under responsibility of Chongqing University

Keywords:Magnesium alloy;Micro-tube;Microstructure;Rapid cooling;Texture.

1.Introduction

In general,the traditional permanent vascular stent is made of 316L stainless steel,Ni-Ti and Co-Cr alloy materials,which have excellent corrosion resistance and high mechanical properties[1].However,this permanent stent requires secondary removal surgery to avoid intimal hyperplasia and thrombosis[1,2].

In last decade,degradable stents have received much attention[3-5].Degradable stents are generally divided into two categories:(1)polymeric degradable stents[6,7];(2)metallic degradable stents,such as Mg-based[8-11],Fe-based[12-14]and Zn-based[15-17].Among these biodegradable metallic materials,magnesium and its alloys have attracted much attention due to their excellent degradability,biocompatibility and suitable bearing capacity[18-24].However,several limitations,like poor corrosion resistance and insufficien formability,restrict the development of Mg stents[25,26].Recent years,a number of published works investigated the corrosion behavior of Mg[27-29]and introduced many effective coating methods to improve the corrosion resistance of Mg[30-32].Besides the poor corrosion resistance,the insufficien formability of Mg,owing to its close-packed hexagonal structure,is another problem that have to be settled urgently.In order to solve this problem,several high performance alloys has been designed[33-35].In addition,a number of proper deformation methods have been applied for the Mg processing.This work will focus on the microstructure and texture evolution of Mg during the common used fabrication method for the vascular stents.

In recent years,a series of major breakthroughs have been made in the processing methods of magnesium alloy microtubes,including dieless drawing[36,37],severe plastic deformation(SPD)combined with cold rolling or cold drawing[37-41],and fluid-mandre drawing[42],and so on[43,44].Furushima et al.[37]obtained fine-graine AZ31 alloy rod by equal-channel angular extrusion(ECAE),and then produced fine-graine magnesium alloy micro-tube by hot extrusion combined with dieless drawing.Ge et al.[38]also successfully fabricated ZM21 alloy micro-tube with outer diameter 4mm and inner diameter 2mm by equal-channel angular pressing(ECAP)and hot extrusion.Faraji et al.[39]successfully produced ultra-fin grained AZ91 alloy tube with a mean grain size of?500nm by multi-pass tubular channel angular pressing(TCAP)from as-cast AZ91 alloy.Amani et al.[41]obtained high-strength and high-ductility WE43 alloy micro-tube by a new method combined with cyclic expansion extrusion(CEE),direct extrusion and micro-tube extrusion.These ultrafine-graine alloy billets exhibit excellent ductility and formability at low-temperature due to the activation of the grain boundary slip,and then the ultrafine-graine microstructure was reserved during the production of micro-tube.

Secondary phase plays an important role in the deformation.The fin and distributed uniform secondary phase can improve the formability of magnesium alloys.Minárik et al.[45]suggested that precipitates in Mg-4Y-3RE alloy have positive effect on the microstructure refinement And the dissolution of secondary phase at high temperature result in rapid increase of the grain size[46].Li et al.[47]observed that nanoscale secondary phase exists in ultrafine-graine Mg-2Zn-2Gd alloy with high strength and high ductility.Similarly,the texture component and texture intensity have a significan impact on the mechanical properties of the magnesium alloy micro-tubes as well.Hanada et al.[43]controlled the crystallographic orientation of the tube by changing the deformation amount in the cold drawing.Zhang et al.[44]reported that the type and proportion of twins have a significan effect on the properties of AZ31 alloy micro-tube.Liu et al.[48]successfully fabricated JDBM,AZ31 and WE43 alloy micro-tube with outer diameter of 3mm and wall thickness of 0.18mm by hot extrusion,cold rolling and cold drawing.And they show different texture component,texture intensity and fracture mechanism.Guo et al.[49]found a big difference in microstructure and texture between the fla and the oval regions of extruded magnesium alloy flats-val tube.In our recent work,hot extrusion and rapid cooling were employed to fabricate the fine-graine Mg-Zn-Y-Nd alloy micro-tube with excellent mechanical properties and corrosion resistance[50].However,among these published papers,the texture evolution and dynamic recrystallization mechanism from the billet to the as-extruded micro-tube have rarely been reported.

In this study,the microstructure and texture evolution of fine-graine Mg-2Zn-0.46Y-0.5Nd alloy micro-tubes were evaluated.The effect of microstructure and texture on properties was discussed in detail.Besides,the dynamic recrystallization mechanism during the deformation process was also proposed.We believe that the obtained results will helpful for the optimization of the fabrication process of Mg micro-tubes.

2.Experimental

2.1.Sample preparation

As-cast Mg-2Zn-0.46Y-0.5Nd(wt.%)alloy billet was preextruded.The extruded bar was machined into cylindrical billets with a diameter of 20mm and a height of 10mm by wire cutting.Then,solution treatment was implemented at 723K for 3 days in order to weaken the second particles distributed along the extrusion direction like continuous line.The center of the solid solution billet was drilled into a small hole of 3mm in diameter.The treated samples were extruded into micro-tube(outer diameter of 3.0mm,wall thickness of 0.35mm)at 653K-673K with the extrusion ratio of 105:1.During the extrude process,the extruded tube was directly cooled into the water tank.In addition,the remaining billet after pre-extruded tube was also cooled by water.Samples were cut from the extruded micro-tube or pre-extruded tube for the further analysis.

2.2.Microstructure analysis

For microstructure analysis,the samples of as-extruded,assolution treatment and as-extruded micro-tubes were cut from the longitudinal section and cross section.These specimens were ground with #200,#400,#600 and #800 SiC papers,polished with 0.25μm diamond polishing agent.And then chemically etched by a solution of 2.1g picric acid,10mL acetic acid,70mL ethanol,and 20mL distilled water.The microstructure was observed by an optical microscope(OM;Leica DM 4000M).

Electron backscatter diffraction(EBSD)tests were performed on the remaining billet of pre-extruded 10cm microtube and as-extruded micro-tube by a Focused ion beam fiel emission scanning double beam electron microscopy(FIB;Zeiss Auriga).Specimens for EBSD test were electrolytic polished by a solution of 8.5ml perchloric acid(70%,99.999%metals basis)and 191.5ml ethanol,with a voltage of 15V and an electrolysis time of 12s.EBSD data analysis uses HKL technology.

2.3.Mechanical property test

The tensile test was carried out by a universal testing machine(Shimadzu AG-IC)at a displacement rate of 1mm/min at room temperature.The Specimens for tensile test with a total length of 80mm and a gauge length of 25mm were prepared according to the national standard(GB/T 228.1-2010).Magnesium alloy rods with a diameter 2.28mm were inserted into both end of the micro-tube to prevent the tensile chuck from damaging the tensile specimen.The tensile fracture morphology was observed by a scanning electron microscope(SEM;FEI Quanta 200).

Fig.1.Microstructure of cross section and longitudinal section of different billets:(a)and(b)pre-extruded billet;(c)and(d)solution treated billet.

3.Results

3.1.Microstructure observation

Fig.1(a)-(d)shows the microstructure of the as-extrusion billet and as-solution billet from cross and longitudinal section.It can be observed that grains of as-extrusion are equiaxed but not uniform enough.The grain size distributes in the range of 2.2-16.1μm with the average grain size of 6.7μm.The grain size is significantl decreased in the region where the second particles are concentrated,indicating that the second particles can reduce the mobility of the grain boundary(GB)and hinder the recrystallization grain growth,as shown in Fig.1(b).The second phases of the as-extrusion billet exhibit a continuous streamline distribution along the extrusion direction,mainly distributed in the GB and the crystal.After solid solution at 723K for 3 days,part of the second phases were dissolved into the matrix,but some big second phases with semi-continuous streamline distribution still could be found,as shown in Fig.1(d).

Fig.2 depicts the microstructure of the air-cooled and water-cooled as-extruded micro-tubes from cross and longitudinal section.The completely dynamic recrystallized grains with uniformly distributed second particles were observed instead of the continuous and semi-continuous second par-ticles.The average grain size of the micro-tube increases with the extrusion temperature promoted,as shown in Fig.3.Water-cooled micro-tube has an average grain size reduced from 6.7-11.8μm to 3.2-5.6μm relative to air-cooled microtube,indicated that grain gradually grows in air-cooled.Fig.4 shows a columnar statistical diagram of the grain distribution of extruded micro-tubes.The grain size difference of air-cooled and water-cooled micro-tube is 4.7μm and 15.6μm,respectively.Water-cooled micro-tube displays a more uniform and refine grain distribution.

Table 1Tensile mechanical properties of micro-tubes.

3.2.Tensile mechanical properties

Fig.5 shows the stress-strain curves of air-cooled and water-cooled as-extruded micro-tubes.For comparison,the yield strength(YS)at 0.2% residual strain,ultimate tensile strength(UTS),and breaking elongation(ε)are summarized in Table 1.The values of YS,UTS andεfor air-cooled micro-tubes were 129.3±5.0MPa,193.6±4.1MPa and 9.6±1.2%,respectively.However,the mechanical properties of water-cooled micro-tubes were significantl improved,which were increased to 139.4±8.5MPa,249.4±9.2MPa and 21.1±0.5%,respectively.

Fig.2.Microstructure of different micro-tubes along the longitudinal section:(a)air-cooling at 653K;(b)air-cooling at 673K;(c)water-cooling at 653K;(d)water-cooling at 673K.

Fig.3.Histogram of average grain size of extruded micro-tubes.

3.3.Tensile fracture surface

Fig.6(a)-(d)shows the SEM images of the tensile fracture surface of extruded micro-tube.It can be found that there are significan differences in the way the two pipes are broken.Air-cooled micro-tube exhibits the mixed fracture characteristics,as shown in Fig.6(a)and(c).A large number of cleavage steps,cleavage planes and a small number of shallow dimples could be observed.However,for the water-cooled micro-tube,it shows the ductile fracture characteristics.A large number of dimples are distributed throughout the fracture surface,and the deep dimples respond to high breaking elongation for water-cooled micro-tube.

3.4.EBSD analysis for extruded micro-tubes

Fig.7(a)-(d)shows the inverse pole figur maps(IPF maps)and inverse pole figur(IPFs)on the longitudinal section of the air-cooled and water-cooled extruded micro-tubes in the extrusion direction.The maximum texture intensities are 2.90 and 2.66,respectively,which are much lower than the extruded micro-tube with the texture intensity of 8.97 in our previous study[50].On the one hand,the change of the dynamic recrystallization(DRX)mechanism caused by the addition of rare earth(RE)elements weaken the sharp texture in the non-RE magnesium alloy.On the other hand,elevated temperature and a large extrusion ratio result in the activation of non-basal slip.In general,extruded magnesium alloy containing non-RE elements tend to form two stable texture components ofIn this work,the water-cooled micro-tube retains the fin uniform DRX microstructure,as shown in Fig.2.Fig.7(d)is the IPF of the water-cooled micro-tube,shown a texture component of〈01ˉ11〉‖ED,and it also can be seen a slightly weaker texture component at〈ˉ12ˉ11〉.The dispersed texture peaks are caused by the activation of pyramidal〈c+a〉with similar Schmid factors at the same time.These special texture components,dubbed“RE texture,”associate with texture weakening[53-56].For air-cooled micro-tube,the texture component of〈ˉ12ˉ11〉‖ED disappears,while the texture component of〈01ˉ11〉‖ED is relatively enhanced,and a weak one appears atas shown in Fig.7(c).It is probably due to the slight difference of grain with different orientations in growth rate.

Fig.4.Histogram of grain size distribution of hot extruded micro-tubes at 653K(a)air-cooling;(b)water-cooling.

Fig.5.Stress-strain curve of hot extruded micro-tubes.

4.Discussion

4.1.Microstructure and mechanical properties

The microstructure evolution from billet to as-extruded micro-tube,shown in Figs.1,2 and 8,revealing the uniform and fin regularity of microstructure in micro-tube processing.The grain size of the micro-tube grows up by air-cooled,while the completely DRX microstructure with fin uniform grains was retained by water-cooled.Generally,the mechanical properties of the fine-graine micro-tube processed by water-cooled are better,exhibiting higher strength and ductility as shown in Fig.5.In addition,the texture type and texture intensity also have effect on the mechanical properties of micro-tube.According to the EBSD analysis,the watercooled extruded micro-tube has a weaker texture intensity of 2.66,showing“RE”texture components,which are consistent with the excellent ductility.The Schmid factor of the watercooled micro-tube is 2.7,which is very close to the Schmid factor of the air-cooled micro-tube,as shown in Fig.9.Here,the high strength for the water-cooled extruded micro-tube is considered to be mainly due to the influenc of grain refinement The estimation of YS of a polycrystalline material based on grain size reduction is usually predicted according to Hall-Petch equation[57]:

Whereσy,σo,kanddis the YS,the YS of single crystal,the material constant and the average grain size,respectively.The water-cooled micro-tube has an average grain size reduced from 6.7-11.8μm to 3.2-5.6μm relative to air-cooled micro-tube.The yield strength of the corresponding microtube increased from 129.3±5.0MPa to 139.4±8.5MPa,and the tensile strength increased from 193.6±4.1MPa to 249.4±9.2MPa.Therefore,it can be concluded that the reduction in grain size plays a dominant role in the strength increase.

4.2.Dynamic recrystallization mechanism

Generally,dynamic recrystallization is mainly classifie into discontinuous dynamic recrystallization(DDRX)and continuous dynamic recrystallization(CDRX).For DDRX,the original grain boundary frst bulge,and sub-grain take place once the radius reaches the critical nucleation radius,transformed into high-angle grain boundaries(HAGBs)by absorbing dislocations[58].CDRX is the formation of the progressive accumulation of dislocations into low-angle grain boundaries(LAGBs),and then these LAGBs are gradually transformed into HAGBs by absorbing dislocations and rotation.Magnesium alloy typically undergo DDRX at temperatures above 523K.However,Beer et al.[59]observed the evidence of the existence of CDRX when the AZ31 alloy was hot pressed at 623K with a strain of 0.6.Similarly,Wu et al.[60]reported that CDRX also exists in the hot compression study of GW83K alloy at 673K.

Fig.6.SEM images of the tensile fracture surfaces of(a)air-cooling micro-tube;(b)water-cooling micro-tube;(c)is magnifie image of(a);(d)is magnifie image of(b).

Fig.7.EBSD analysis along the extrusion direction of different hot extruded micro-tubes:IPF maps of(a)air-cooling tube and(b)water-cooling tube;IPFs of(c)air-cooling tube and(d)water-cooling tube.

Fig.8.IPF maps along the extrusion direction of the remaining billet after pre-extruded 10cm at different position:(a)position A;(b)position B;(c)position C;(d)position D.

Fig.9.Schmid Factor of extruded microtubes:(a)water-cooling;(b)air-cooling.

Fig.10.EBSD analysis revealing evidence of CDRX.(a)the cumulative misorientation along the dotted line from A to the grain boundary in Fig.8(a);(b)the SEM image;(c)the counts of Y and Nd along line1;(d)the counts of Y and Nd along line2.

In this work,the sample of the remaining billet after preextruded 10cm tube at 653K was observed by EBSD analysis.Fig.8(a)-(d)shows the IPF maps of the position A,B,C and D,respectively.A dotted line was drawn from the center to the grain boundary of a parent grain,as shown in Fig.8(a).The cumulative misorientation increasing along the dotted line reveals that CDRX dominates dynamic recrystallization mechanism during hot extrusion micro-tube,as shown in Fig.10.This is consistent with the results of previous works[59-62].It may be due to the addition of Nd,which are segregated at the grain boundary and retard the mobility of the grain boundary[63].This is supported by corresponding SEM image,as shown in Fig.10(b).Along the line1,Nd is segregated at the grain boundary.The grain boundary bulging mechanism of DDRX is retarded.However,the Y has no obviously change,indicating that Y plays a different role in DRX.The addition of Y leads to the activation of〈c+a〉slip,which may be another reason for the changing of DRX mechanism.The weak“RE”texture components of as-extruded micro-tubes could prove this point,as shown in Fig.7(c)and(d).Hence,the CDRX mechanism plays a dominate role in the hot extrusion process.

4.3.Texture evolution

Fig.11.IPFs along the extrusion direction of the remaining billet after pre-extruded 10cm tube:(a)position C;(b)position D in Fig.8.

The IPF maps of each position on the remaining billet after pre-extruded 10cm tube are indicated in Fig.8(a)-(d).It can be clearly seen that fin dynamic recrystallized grains have begun to appear at the parent grain boundary at position A.The fraction of recrystallization is rising with the increasing strain.When the billet is extruded into the micro-tube,the original deformed microstructure has been completely dynamic recrystallized,as shown in Fig.8(c)and(d),which is consistent with the microstructure of water-cooled micro-tube.Interestingly,the color of the recrystallized grains is different from that of the original deformed grains,and exhibits more colorful,indicating that the orientation of the dynamic recrystallized grains is significantl different.It is proposed that the change in the dynamic recrystallization mechanism and the addition of the RE elements Y and Nd are responsible for the significan effect.Several papers[53-56,61,63-65]have reported that Y,Ce,La,Nd,Zr and Gd as texture modifier for magnesium alloy significantl affect the texture component and intensity.Sandl?bes et al.[66]reported that the addition of Y can increase the activity of compression twinning,secondary twinning and pyramidal〈c+a〉slip.These additional deformation modes result in more uniform deformation with weaker basal texture.Cottam et al.[61]and Agnew et al.[67]also found that various deformation modes were activated in Mg-Y alloys.Hantzsche et al.[68]reported that the texture weakening with the increasing content of RE elements in magnesium alloy is connected to the solid solubility of respective element,existing a critical value for the significan texture weakening.

Fig.11(a)and(b)is the IPF of the position C and position D,respectively.In Fig.11(a),it can be seen that the strongest texture peak is concentrated at〈0001〉,and a diffused texture peak between at〈01ˉ10〉and〈ˉ12ˉ10〉also can be observed,indicating that basal slip,prismatic slips and pyramidal slips are all activated during the hot extrusion process,which greatly improves the formability of magnesium alloy.Besides the texture distributed between〈01ˉ10〉and〈ˉ12ˉ10〉,the texture component of〈01ˉ11〉‖ED can also be found in the IPF of position D,indicating that the pyramidal slip〈c+a〉is also activated during the extrusion process.With the disappearing of〈0001〉‖ED texture,multiple deformation modes dominate the deformation with a weak texture.

5.Conclusions

The fine-graine Mg-2Zn-0.46Y-0.5Nd alloy micro-tube with an outer diameter of 3.0mm and a wall thickness of 0.35mm was successfully fabricated by hot extrusion and rapid cooling at 653K with an extrusion ratio of 105:1.Further investigation was carried out on the properties of the extrude micro-tube and dynamic recrystallization mechanism during hot extrusion process with the main conclusions as follows:

(1)The fin and uniform microstructure of completely dynamic recrystallization could be achieved by rapid cooling.The average grain size of the micro-tube is decreased from 6.7-11.8μm to 3.2-5.6μm,exhibiting excellent mechanical properties and weak texture components.

(2)The additions of Y and Nd retard the mobility of the grain boundary and activate the non-basal slip,resulting in a change on the dynamic recrystallization mechanism during hot extrusion micro-tube.CDRX dominates the recrystallization mechanism.

(3)The as-extruded micro-tube mainly shows two weak“RE”texture components of〈01ˉ11〉‖ED and〈ˉ12ˉ11〉‖ED.Compared with the air-cooled micro-tube,the texture intensity of the water cooled micro-tube is further decreased.

Declaration of Competing Interest

The authors declare that they have no known competing financia interests or personal relationships that could have appeared to influenc the work reported in this paper.

Acknowledgments

The authors are grateful for the financia support of Key Projects of the Joint Fund of the National Natural Science Foundation of China(U1804251),the National Key Research and Development Program of China(2018YFC1106703,2017YFB0702504 and 2016YFC1102403).