Continuous observation of twinning and dynamic recrystallization in ZM21 magnesium alloy tubes during locally heated dieless drawing

Peihua Du,Shusaku Furusawa,Tsuyoshi Furushima

Department of Mechanical and Biofunctional Systems,Institute of Industrial Science,The University of Tokyo,4-6-1,Komaba,Meguro,Tokyo 153-8505,Japan

Abstract Compared to cold drawing,dieless drawing has shown great potential for manufacturing biodegradable Mg alloy microtubes due to the large reduction in area acquired in a single pass.However,owing to the local heating and local deformation,the deformation mechanism during dieless drawing is not clear,and thus causing difficultie in controlling the microstructure of dieless drawn tubes.For the purpose of acquiring a desired microstructure,in this study the deformation mechanism of ZM21 Mg alloy tube was clarifie by conducting continuous observation of the microstructural evolution during dieless drawing.The results show that both SRX and DRX occurred during dieless drawing.SRX occurred before the plastic deformation to soften dieless drawn tubes.With increase of feeding speed,the deformation mechanism changed accordingly:(1) At the low-speed of 0.02 mm/s,the deformation mechanism was dominated by twin-slip sliding,during which {10-12} tension twins were generated inside grains to accommodate the plastic deformation by changing the crystal orientation.(2) At the intermediate-speed of 2 mm/s,a twin-DRX process related to {10-12} tension twin was observed,which was characterized by the generation of abundant {10-12} tension twins and the evolution of misorientation angle of {10-12} tension twins.Moreover,the transformation from twin-DRX to CDRX can be observed at the late stage of plastic deformation,which was attributed to the inhomogeneous conditions of dieless drawing.(3) At the high-speed of 5 mm/s,a CDRX process was observed,during which grain boundary sliding and grain tilting were observed,in addition to the gradual rotation of subgrains.These results show that during dieless drawing,DRX is not only a temperature-dependent phenomenon,but also influence by the variation of feeding speed.

Keywords: Dieless drawing;Inhomogeneous condition;Twining;Dynamic recrystallization;Microstructure evolution;Magnesium alloy;Twinning.

1.Introduction

Magnesium (Mg) alloy stents have attracted worldwide attention since they can be completely absorbed into the human body and avoid the problems of in-stent restenosis and thrombosis after using bare-metal or drug-eluting stents[1-6].However,manufacture of the biodegradable Mg alloy microtubes for cutting stents is difficul by cold drawing,owing to the low plasticity of Mg alloys [7,8].On the basis that a large reduction in area can be acquired in a single pass,dieless drawing was proposed to manufacture biodegradable Mg alloy microtubes [9-12].For example,Furushima et al.acquired reduction in area up to 50% in a single pass by dieless drawing,which proved the feasibility and the efficien y of dieless drawing for manufacturing biodegradable Mg alloy microtubes [10].

Fig.1.(a) Schematic illustration of dieless drawing;(b) schematic illustration of high-temperature uniaxial tensile test;(c) the variation of temperature,strain rate,and strain as a function of distance during dieless drawing;(d) the variation of temperature,strain rate,and strain as a function of time during tensile test.

In addition to the large reduction in area,the second advantage of dieless drawing is the grain refinemen of dieless drawn tubes.Furushima et al.observed the microstructures of dieless drawn tubes and found that the grain size was greatly refine because of dynamic recrystallization (DRX) [13].Du et al.further clarifie the relationship between the microstructure and the performance of dieless drawn tubes,findin that the corrosion performance of dieless drawn tubes was seriously damaged when the microstructure was characterized by a mixed {10-12} tension twin and necklace microstructure[14].At present,it is still unclear why {10-12} tension twins and necklace microstructure are maintained in dieless drawn tubes.The difficult is that dieless drawing was achieved under the condition of local heating and local deformation (variable temperature and strain rate,Fig.1a) [15],but most of the studies on the deformation mechanism of Mg and Mg alloys were conducted under the homogeneous condition (constant temperature and strain rate,Fig.1b) [16-18].At present it is still unclear how the influenc of variable temperature and strain rate affect the deformation mechanism of dieless drawn tubes.To have better control of the microstructure and to achieve a higher performance of dieless drawn tubes,it is necessary to study the deformation mechanism of Mg alloy tubes during dieless drawing.

The deformation mechanism of Mg alloys has been investigated based on the activation of twins and slip systems under the condition of constant strain rate and temperature.A high strain rate often causes stress concentration near grain boundaries,and further causes the generation of twins [19-21].Twins accommodate the plastic deformation by changing the crystal orientation.Temperature is the second factor that affects the activation of slip systems and twins [17,23].At room temperature,the plastic deformation of Mg and Mg alloys is mostly related to basal 〈a〉 slip [24,25].When temperature increases,the activation of twins is suppressed because the stress concentration near grain boundaries is alleviated when non-basal slip systems(prismatic〈a〉slip and pyramidal〈c+a〉 slip) are activated [26-29].Grain size is the third factor that affects slip systems and twins.C.M.Cepeda-Jiménez et al.observed that the basal slip system became more active when grain size decreased from 36 to 5 μm,and twinning was suppressed in fin grains [22].

In Ma’s work,the nucleation and subsequent grain growth during DRX were considered to be correlated with defect activities[30].Based on the activation of twins and slip systems,different types of DRX mechanisms are clarified For example,Continuous DRX (CDRX) is characterized by the activation of cross-slip of a dislocation on non-basal planes,with the continuous absorption of dislocations in subgrain boundaries [27].Discontinuous DRX (DDRX) is characterized by dislocation climb controlled by self-diffusion,with the local migration of pre-existing high angle grain boundaries [27,31].Twin-DRX is often related to the generation of {10-11} compression twins,which is characterized by a CDRX process inside {10-11} compression twins [17,29].However,under the inhomogeneous conditions of dieless drawing,it is extremely difficul to understand the activation of slip systems and twins with varied temperature,strain rate and grain size,which in turn causes the difficult in controlling the microstructures of dieless drawn tubes.

Considering a direct relationship between the microstructure and the performance,controlling microstructural evolution is a key step to acquire the desired performance of dieless drawn tubes.In this study,the deformation mechanism of ZM21 Mg alloy tubes is studied under the inhomogeneous conditions of dieless drawing,to achieve a better understanding about the microstructural evolution of dieless drawn tubes.The reason of choosing ZM21 mg alloy is based on its good biocompatibility[33],good mechanical performance[34],and good corrosion resistance[35],which shows great potential in biomedical applications.In this study,we want to clarify (1)twinning behavior;(2) DRX mechanism;and (3) transformation of DRX mechanism under the inhomogeneous conditions of dieless drawing.The microstructures of dieless drawn tubes were observed along the drawing direction by electron back scattering diffraction (EBSD).

2.Principle of local-heated dieless drawing

Dieless drawing is often considered as a processing technology,which has shown great potential for manufacturing brittle materials and micro-sized products.For example,by dieless drawing,Furushima et al.manufactured noncircular multicore microtubes [36],Zn-22%Al alloy microtubes [37],Mg alloy microtubes [38],ceramic tubes [39],and so on.As shown in Fig.1a,drawing dies are not necessary for dieless drawing,but reduction in arearis determined by drawing speedV1and feeding speedV2[40]:

whereA1andA2are cross areas before and after dieless drawing,respectively.In particular,the gradual decrease of cross area is related to the change of fl w stress based on equilibrium equation,

where,σaandσbare fl w stresses at position A and B,AaandAbare cross areas at position A and B (Fig.1a).

Owning to the gradual proceed of plastic deformation during dieless drawing,continuous observation of microstructural evolution is proposed in this study to clarify the deformation mechanism of ZM21 Mg alloy tubes.Generally,the deformation mechanism of Mg and Mg alloys are studied by high-temperature uniaxial tensile test who provides a certain strain rate and temperature,as shown in Fig.1b.Fig.1d is the schematic illustration showing the according variation of strain rate,strain,and temperature of position A(in Fig.1b) during high-temperature uniaxial tensile test.Because temperature and strain rate are kept unchanged,with increase of testing time,temperature and strain rate are constant,and strain is linearly increased in Fig.1d.Accordingly,the deformation mechanism is investigated by observing the microstructure when Mg and Mg alloys are deformed to a certain strain,in turn causing the microstructural information at the initial and middle stage is missed out,such as the interaction and evolution of grains,and the transformation of deformation mechanism.Compared to uniaxial tensile test,samples during dieless drawing are deformed gradually to the preset strain.Fig.1c is the schematic illustration showing the variable strain,strain,and temperature during dieless drawing.Due to the local heating and local deformation of dieless drawing,the peak-distributed temperature and strain rate can be seen.Based on the gradual decrease of outer radius,strain is increased gradually.In fact,these results are referred from the published work about basic principles of dieless drawing[15],which quantitatively analyzed the plastic deformation during dieless drawing.Meanwhile,these results also imply the feasibility of conducting continuous observation of microstructural evolution in Mg alloys,just by changing the observation position.

3.Experiments

3.1.Material and experimental methods

Fig.2.Schematic illustration of dieless drawing machine.

Cold-drawn Mg alloy ZM21 tubes (outer diameter,6 mm;wall thickness,1.1 mm) were used as mother tubes.Fig.2 shows the schematic illustration of dieless drawing machine.Two servo motors driven two movable plates,to control feeding speedV2and drawing speedV1,separately.Feeding speedV2had three values of 0.02 mm/s (low-speed),2 mm/s(intermediate-speed),and 5 mm/s (high-speed).Reduction in area was set as 44% by changing drawing speedsV1based on Eq.(1).High-frequency induction heating device,with a power of 10 kW and a frequency ranged 150-400 kHz,was used to heat Mg alloy tubes,accompanied by a threeturn heating coils.Water-cooling coil was used to acquire a concentrated temperature distribution.During dieless drawing,the maximum temperature was set as 350 °C,at which DRX could be observed under various loading direction and strain rate [41-43].The distribution of temperature was measured by thermography,with an accuracy of 0.1 °C.To have a clear discussion about the influenc of inhomogeneous condition,strain rate and strain were also calculated based on outer radius of dieless drawn tubes.Outer radius was measured by laser diameter measurement device,with an accuracy of 0.001 mm.Based on outer radius,strainε,and strain ratewere expressed as [15]:

where,V2is feeding speed,R0is outer radius of mother tube,andRis outer radius along dieless drawn tubes.

3.2.Microstructural observation

Fig.3.Positions for microstructural observation (position 1,2,and 3 are before the plastic deformation,position 4,5,and 6 are during the plastic deformation,and position 7 is after the plastic deformation).

Fig.4.(a) IPF map showing the microstructure of mother tube;(b) IQ map showing the compression twins (highlighted in green) and the tension twins(highlighted in red) inside grains;(3) pole figur showing grains are oriented with basal plane parallel to DD.

Microstructural evolution of dieless drawn tubes were observed as a function of distance,as shown in Fig 3.A total of seven positions were selected:position 1 ?3 were before the plastic deformation);position 4 ?6 were during the plastic deformation;position 7 was after the plastic deformation.Specimens were cold-mounted and grounded with sandpaper,then polished with 0.1 μm Al2O3suspension.EBSD was conducted by field-emissio scanning electron microscopy (FESEM,JEOL 7100F) at accelerating voltage of 15 kV with step size of 0.5 μm.Three different directions were define as thickness direction (TD),circumferential direction (CD),and drawing direction (DD).The specimens prepared for EBSD were finishe by chemical polishing at room temperature for 90 s (with 180 mL of ethanol,50 mL of glycerin,and 80 mL of phosphoric acid).Fig.4 shows the microstructure of mother tube.Fig.4a shows that a large number of twins generated inside grains during cold drawing.Fig.4b shows twins are characterized by the {10-12} tension twins (highlighted in red) and {10-11} compression twins (highlighted in green),and grains were oriented with the basal plane parallel to DD(Fig.4c).

4.Results

4.1.Drawing conditions

Fig.5 shows the temperature distribution,the photos of dieless drawn tubes,and the variation of physical values at different feeding speeds.The temperature distribution was measured by thermography.Based on the change of outer radius,strain rate and strain are calculated Eq.(3) and (4).The point to start plastic deformation is considered to be coincident with the point of maximum temperature,and the plastic deformation is proceeded with decrease of temperature.Fig.5a shows the temperature distribution at different feeding speeds,a more concentrated temperature distribution was acquired with increase of feeding speed.Fig.5b,5c,and 5d are the photos of dieless drawn tubes and the corresponding physical values during dieless drawing.With increase of feeding speed,the length of deformation zone decreases,25 mm for 0.02 mm/s,15 mm for 2 and 5 mm/s.Meanwhile,the maximum strain rate increases from 0.0009 to 0.3s-1when feeding speed increased from 0.02 to 5 mm/s.

Fig.5.(a) photos showing the temperature distribution at three different feeding speeds;(b) -(d) photos showing the deformation zone and the physical values at three different feeding speeds:(b) 0.02 mm/s;(c) 2 mm/s;(d) 5 mm/s.

The length of deformation zone is thought to be mainly determined by the temperature distribution.Based on Eq.(1)and 2,reduction in arearcan be expressed by fl w stress at heating partσh(point to start plastic deformation) and cooling partσc(point to stop plastic deformation),asr=1 -σh/σc.The fl w stress at cooling part can be expressed as:σc=(1 -r)/σh.Since the maximum temperature (main factor that determines fl w stress at heating partσh) and reduction in area are define in this study,the similar fl w stress at cooling partσccan be expected for three different feeding speeds.That is,the plastic deformation stops at the similar temperature (around 200 °C from Fig.5b,5c,and 5d).When feeding speed is increased from 0.02 to 5 mm/s,temperature distribution becomes more concentrated(Fig.5a),and the length of deformation zone is decreased correspondingly.

4.2.Twining-slip sliding

Fig.6 shows the microstructural evolution at feeding speed of 0.02 mm/s.It can be observed that both static recrystallization (SRX) and DRX occur during dieless drawing.SRX occurs before the plastic deformation,as shown in Fig.6b,6c and 6d.The generation of SRXed grains near grain boundaries(Fig.6b),and the gradual growth of SRXed grains (Fig.6c and 6d) can be observed.Fig.7 is the pole figure showing the crystal orientation at seven different positions.Similar as the work of Zeng et al.,SRX during dieless drawing also has three stages based on the information in Fig.6 and 7 [44]:(1) the generation of refine grains with randomly orientation (poles are randomly deviated from CD in Fig.7b);(2)the preferential growth of SRXed grains that have the similar orientation to the parent grains;and (3) the formation of texture similar to mother tube (poles are gathered to CD in Fig.7b and Fig.7c).Fig.8 shows the change of grain size at feeding speed of 0.02 mm/s,the decrease of grain size can be observed during SRX,which is suspected to be caused by the generation of {10-12} tension twins that split grains.

After SRX,most grains have the similar orientation as mother tube.It should be pointed out under such crystal orientation,the generation of compression twins can be expected from room temperature tensile tests [45].But during dieless drawing,twins are mostly related to {10-12} tension twins,but the compression twins seem to be inhabited.It is suspected to be caused by the inhomogeneous conditions of dieless drawing since the deformation mechanism of Mg alloys is influence by crystal orientation [46],temperature [47],grain size [48],and strain rate [41].

Fig.6.(a) Photo showing the positions for microstructural observation;(b)?(h) microstructural evolution at feeding speed of 0.02 mm/s,as a function of distance:(b) 345 °C;(c) 350 °C;(d) 350 °C;(e) 300 °C,0.004 s -1,0.10;(f) 250 °C,0.008 s -1,0.31;(g) 200 °C,0.002 s -1,0.56;(h) after dieless drawing.(Strain,strain rate and temperature are acquired from the curves in Fig 5(b).).

Fig.7.Pole figure showing the crystal orientation at different positions at feeding speed of 0.02 mm/s:(a) photo showing the position from 1 -7;(b) -(h)crystal orientation from position 1 -7.

{10-12} tension twins inside grains are parallel to each other or go across to each other,as shown in Fig 6e,6f and 6g.With the proceeding of plastic deformation,the number and volume of {10-12} tension twins also change,as more{10-12} tension twins are generated inside grains at high strain rate (Fig.6f).After dieless drawing,{10-12} tension twins are maintained inside grains (Fig.6h).From Fig.7e,7f and 7g,it can also be observed that most grains are oriented with basal plane parallel to DD.Meanwhile,due to the generation of {10-12} tension twins,the decrease of grain size can be observed at position 5 where a large number of twins are generated inside grains (Fig.8).A comparison between position 3 (after SRX) and position 7 (after the plastic deformation) shows that the grain size does not obviously change after the plastic deformation (Fig.8).Thus,the plastic deformation at feeding speed of 0.02 mm/s is thought to be mainly dominated by twining-slip sliding.

4.3.Twin-CDRX

When feeding speed increases to 2 mm/s,the temperature distribution becomes more concentrated (Fig.5a).Fig.9 shows the microstructures at seven different positions at feeding speed of 2 mm/s.It can be observed that SRXed grains do not fully grow up (shown by the arrows in Fig.9c and 9d).Fig.10 is the pole figure showing the crystal orientation.Due to the uncomplete growth of SRXed grains,the poles are observed to be randomly oriented in Fig.10d.

Fig.8.The change of grain size from position 1 -7 at feeding speed of 0.02 mm/s.

When feeding speed increases to 2 mm/s,the maximum strain rate is increased to 0.11s-1(Fig.5b).Under this condition,more {10-12} tension twins are generated inside grains (Fig.9e,9f and 9g),some grains are even totally twined(shown by the arrow in Fig.9f).Due to a large number of {10-12} tension twins generated inside grains,the crystal orientation at position 4,5 and 6 also changes (Poles are tilting to DD in Fig.10e,10f and 10g).Fig.11 shows the change of grain size.Because of the generation of {10-12}tension twins that split grains,the grain size dramatically deceases from 18.63 to 10.68 μm from position 3 to 4,which shows {10-12} tension twins can also contribute to the grain refinement From position 4 to 6,the grain size keeps around 10 μm (Fig.11),which implies DRX related to {10-12} occurs at feeding speed of 2 mm/s.

At the late stage of plastic deformation,not only twin-DRX,but also CDRX occurs.Fig.12 shows the IPF maps of the twin partition and matrix partition at position 4 and 6(Fig.9e and 9g).Fig.12a shows that low angle grain boundaries are linearly distributed inside twin partition,which is caused by the coalescence of {10-12} tension twins based on the work of Hong et al.[49].Fig.12d shows that necklaceshaped low angle grain boundaries are generated near grain boundaries,which is a typical characteristic of CDRX [16].By observing the shape of low angle grain boundaries in Fig.12,the transformation of deformation mechanism from twin-DRX to mixed twin-CDRX is identifie at the late stage of plastic deformation.And finall,dieless drawn tube is characterized by a mixed twin-necklace microstructure(Fig.9h).

4.4.CDRX

From Fig.5a,it can be observed that the temperature distribution become more concentrated when feeding speed is increased to 5 mm/s.The maximum strain rate is increased to 0.3s-1(Fig.5d).Fig.13 shows the microstructural evolution of dieless drawn tube.Fig.14 shows the crystal orientation from position 1 -7.Similar as feeding speed of 2 mm/s,SRX occurs before the plastic deformation (Fig.13b,13c and 13d),and SRXed grains are randomly oriented due to the uncompleted growth (Fig.14b,14c,and 14d).

Normally,the increase of strain rate causes stress concentration near grain boundaries and leads to the generation of more {10-12} tension twins.However,when feeding speed increases to 5 mm/s,twinning does not occur but CDRX occurs during dieless drawing.CDRX is a process that has been reported in many works,which is characterized by(1)the formation of subgrain boundaries;and (2) the gradual change of subgrain boundaries into high angle boundaries [16].Fig 13e-13 h shows that low angle grain boundaries (white lines)are generated insides grains and a typical necklace structure(large grains are surrounded by small grains) is observed.

Meanwhile,Fig.14e,14f,and 14g also show that the tilting of grains during the process of CDRX,as the poles are tilting to the direction of DD with an angle of 30 -60° The tilting of grain is different from the rotation of subgrains.The tilting of grain is suspected to be caused by grain boundary sliding,which is commonly found in ultrafine-grai Mg alloys[50,51].Fig.15 is the change of grain size during dieless drawing,it can be observed that the grain size is refine to 12.61 μm,larger than that of 2 mm/s (Fig.11).From the published work,a higher strain rate and a lower temperature often led to a fine grain size after DRX[52,53].In this study,it is suspected the existence of {10-12} tension twins inside grains leads to the additional grain refinemen for 2 mm/s.

Fig.9.(a) Photo showing the positions for microstructural observation;(b)?(h) microstructural evolution at feeding speed of 2 mm/s,as a function of distance:(b) 345 °C;(c) 350 °C;(d) 350 °C;(e) 320 °C,0.08 s -1,0.12;(f) 300 °C,0.11 s -1,0.31;(g) 270 °C,0.06 s -1,0.48;(h) after dieless drawing.(Strain rate and temperature are acquired from the curves in Fig 5(c).).

Fig.10.Pole figure showing the crystal orientation at different positions at feeding speed of 2 mm/s:(a) photo showing the position from 1 -7;(b) -(h)crystal orientation from position 1 -7.

Fig.11.The change of grain size from position 1 -7 at feeding speed of 2 mm/s.

5.Discussion

5.1.Twin-DRX

In Ma’s opinion,the nucleation and subsequent grain growth during DRX are correlated with defect activities that are temperature dependent[30].Based on the change of deformation temperature,a number of DRX mechanism for Mg and Mg alloys have been proposed,such as CDRX and DDRX[27].However,the deformation mechanism of Mg and Mg alloys are influence not only by temperature,but also by loading direction [54,41],grain size [51],strain rate [41],and even the existence of particles [55].That is,with the change of processing condition,the change of DRX mechanism can be observed for Mg and Mg alloys.For example,Yu et al.found the change of DRX mechanism from CDRX to twin-DRX with increase of strain rate in high-temperature out-plane compression [41].Robason et al.confir the particle stimulated DRX in Mg-Mn alloy [55].During dieless drawing,a large number of {10-12} tension twins are generated inside grains,it is reasonable to suspect the occurrence of twin-DRX related to {10-12} tension twins.

From the previous works,twin-DRX was often observed in related to {10-11} compression twins because {10-11}compression twins can effectively split grains [16,32,56].As a comparison,owing to the rapid growth of {10-12} tension twins that could lead to the generation of total-twined grains,{10-12} tension twins are thought to be ineffective in splitting grains [49].In the work of Talal et al.,twin-DRX was observed from high-temperature C-axis compression,which was characterized by the generation of {10-11} compression twins,followed by CDRX inside {10-11} compression twins[16].In the work of Peng et al.,twin-DRX related to {10-11} compression twins was also reported under hot rolling process [32].Nevertheless,Partridge et al.also revealed the abnormal migration of {10-12} tension twin boundaries and the abnormal evolution of {10-12} tension twin plane [57].Based on the work of Partridge et al.,Ma et al.considered{10-12}tension twins playing an important role in DRX[30].It should be pointed out that even twin-DRX related to {10-12} tension twin was proposed by Ma et al.,but a direct observation was not provided in his study.

In this study,twin-DRX related to {10-12} tension twin is determined based on the change of grain size during dieless drawing.Fig.16b and 16c further prove this process by showing the generation of secondary {10-12}-{10-12} tension twins,which implies that {10-12} tension twin lamella can be reserved inside grains that split grains,even the fast growth and the coalescence of {10-12} tension twins are observed.In addition to the generation of{10-12}tension twins,the evolution of {10-12} tension twins are also proved in this study.Fig.16a is amplifie IQ map of area 1 (Fig.9e) showing the change of misorientation angle of twin boundaries.It can be observed that the misorientation angle of {10-12} tension twin boundaries were ranged from 75 to 87° That is,the generation of {10-12} tension twins are the process of nucleation,and the evolution of twin boundaries are the process of grain growth for twin-DRX.Based on the easy generation of{10-12} tension twins and the transition in the misorientation angle,this work provides evidences to the work of Ma et al.[30],showing twin-DRX related to {10-12} tension twins.

Fig.12.IPF maps showing twin partition and matrix partition:(a)and(b)from Fig.9e;(b)and(d)from Fig.9g(white lines shows low angle grain boundaries,5°-15°).

Fig.13.(a) Photo showing the positions for microstructural observation;(b)?(h) microstructural evolution at feeding speed of 5 mm/s,as a function of distance:(b) 345 °C;(c) 350 °C;(d) 350 °C;(e) 330 °C,0.2 s -1,0.11;(f) 300 °C,0.3 s -1,0.29;(g) 250 °C,0.2 s -1,0.46;(h) after dieless drawing.(White lines show the low angle grain boundaries,5 -15° Strain rate and temperature are acquired from the curves in Fig 5(d).).

Fig.14.Pole figure showing the crystal orientation at different positions at feeding speed of 5 mm/s:(a) photo showing the position from 1 -7;(b) -(h)crystal orientation from position 1 -7.

Fig.15.The change of grain size from position 1 -7 at feeding speed of 5 mm/s.

5.2.CDRX

Generally,more twins will be generated inside grains when strain rate is increased due to the stress concentration near grain boundaries.But in this study,twining cannot be observed when the maximum strain rate is increased to 0.3s-1(5 mm/s).Instead,CDRX is observed.The occurrence of CDRX is suspected to be caused by the grain refinemen after SRX,since twinning could be suppressed in refine grains of Mg alloys [22,58].For example,Lapovok et al.suggested that there existed a critical grain size below which twinning could be suppressed,and this critical grain size was 3-4 μm for ZK60 Mg alloys [58].C.M.Cepeda-Jimenez et al.further confirme that fine-grai microstructure led to a boundary percolation effect,during which basal slip could transfer to the neighboring grains,so that the stress concentration near grain boundary was released.When dieless drawing is conducted at feeding speed of 5 mm/s,the grain size after SRX is refine because of the insufficien time for grain growth.Thus,it is suspected that CDRX at 5 mm/s is caused by the fin grain size after SRX.

Fig.16.(a) Amplifie IQ map of area 1 (Fig.9e) showing the change of misorientation angle of twin boundaries;(b) Amplifie IPF map of area 1 (Fig.7e)showing the generation of secondary {10-12}-{10-12} tension twins;(c) pole figur showing the corresponding crystal orientation of matrix,{10-12} tension twin,and secondary {10-12}-{10-12} tension twins.

Fig.17.Polar figure showing the crystal orientation during CDRX of Fig.8:(a) position 4;(b) position 5;(c) position 6;(d) position 7.

Fig.18.Schematic illustration of DRX mechanism during dieless drawing.

CDRX is characterized by (1) the formation of low angle grain boundaries,and (2) the gradual rotation of subgrains [16].In this study,the tilting of grains is also observed when CDRX occurs,in addition to the rotation of subgrains.Fig.17 shows the polar figure and the corresponding crystal orientation during CDRX.Fig.17a shows at the initial stage of CDRX,most grains are oriented with {0001} basal plane parallel to DD.Fig.17b and 17c show that c-axis of some grains tilt to DD with a range of 30 ?60° at the intermediate and late stage of CDRX.Fig.17d shows that after the plastic deformation,grains are oriented with {0001} basal plane parallel to DD again.The tilting of grains is different from the rotation of subgrains.The tilting of grains is suspected to be caused by grain boundary sliding,which is commonly found in ultrafine-grai Mg alloys.As a comparison,the rotation of subgrains during CDRX is caused by the accumulation of dislocations near grain boundaries.Meanwhile,the tilting of grains could also benefi the activation of slip sliding by changing crystal orientation.That is,during CDRX,grains are firstl tilted to DD with an angle of 30 ?60° to active slip sliding,and then the formation of low angle grain boundaries and the gradual rotation of subgrains are observed because of the accumulation of dislocations near grain boundaries.

5.3.The transformation of DRX mechanism

DRX mechanisms of Mg and Mg alloys are often considered as a temperature-dependent phenomenon.For example,Galiyev et al.suggested DRX at temperature＜200 °C was related to twinning,at 200-250 °C is related to CDRX,and at 300-450 °C is related to DDRX [27].In addition to temperature,the change of loading direction and strain rate could also change the DRX mechanism of Mg and Mg alloys.In the work of Yu et al.,the transformation from CDRX to twin-DRX was observed with increase of strain rate,or by changing the loading direction from out-plane to in-plane compression.These results all imply that the transformation of DRX mechanism can be expected during dieless drawing with variable strain rate,and temperature.In this study,the transformation from twin-DRX to twin-CDRX is observed at feeding speed of 2 mm/s.Even at present it is still unclear the boundary for transformation of deformation mechanism,this results still show that the DRX during dieless drawing is not only a temperature-dependent phenomenon,the other factors,such as strain rate,grain size,also affect DRX mechanism.

Fig.18 is the schematic illustration showing the deformation mechanism at different feeding speeds.At feeding speed of 0.02 mm/s,the grain refinemen of dieless drawn tubes cannot be achieved,twin-slip sliding dominates the deformation mechanism (Fig.18a).When feeding speed is increased to 2 mm/s,a large number of {10-12} tension twins are generated inside grain,causing twin-DRX.At the late stage,CDRX occurs,and the deformation mechanism changes into twin-CDRX (Fig.18b).When feeding speed is increased to 5 mm/s,the generation of twins are totally inhabited,and the deformation mechanism is dominated by CDRX (Fig.18c).

Conclusions

Dieless drawing a novel technology characterized by the local heating and local deformation,which has shown great potential in manufacturing the biodegradable Mg alloy microtubes since a large reduction in area is achieved in a single pass.In the study,ZM21 Mg alloy dieless drawn tubes are firs manufactured at different feeding speeds,the microstructures of dieless drawn tubes are observed by conducting continuous observation,and finall the deformation mechanism of dieless drawn tubes are discussed based on the inhomogeneous conditions of dieless drawing.The following conclusions are made:

(1) During dieless drawing,SRX occurred before the plastic deformation.Three types of deformation mechanisms were observed with the proceed of plastic deformation:twin-slip sliding,twin-CDRX,and CDRX.

(2) At feeding speed of 0.02 mm/s and reduction in area of 44%,{10-12} tension twins were generated inside grains with the proceed of plastic deformation.High strain rate initiated more {10-12} tension twins generated inside grains.The plastic deformation was dominated by twining-slip sliding.

(3) At feeding speed of 2 mm/s and reduction in area of 44%,more{10-12}tension twins were generated inside grains,resulting in twin-DRX.Twin-DRX was characterized by:(1) the generation of abundant {10-12} tension twins;(2) the change of misorientation angle of twin boundaries.At the late stage of plastic deformation,CDRX occurred,and the deformation mechanism was transformed into twin-CDRX.

(4) At feeding speed of 5 mm/s and reduction in area of 44%,CDRX was observed,which was suspected to be caused by grain refinemen after SRX.In addition to the generation of low angle grain boundaries and the rotation of subgrains,CDRX was also characterized by grain tilting.Some grains during CDRX changed their orientation,by tilting to DD with a range of 30 ?60°

Acknowledgment

This study was supported by JSTP KAKENHI Grant No.19H02476 and The Light Meal Education Foundation,Inc.Peihua Du thanks China Scholarship Council for the award of fellowship and funding (No.201707040058).