Cho Ding ,Xioshi Hu,* ,Hilong Shi ,Weimin Gn ,Kun Wu ,Xiojun Wng,*

a National Key Laboratory for Precision Hot Processing of Metals,Harbin Institute of Technology,92 West Dazhi Street,Harbin,Heilongjiang 150001,PR China

b GEMS at Heinz Maier-Leibnitz Zentrum (MLZ),Helmholtz-Zentrum Geesthacht,Lichtenbergstr.1,D-85748 Garching,Germany

Abstract The hybrid addition of CNTs was used to improve both the strengths and ductility of SiCp reinforced Mg matrix composites.A novel method was developed to simultaneously disperse SiCp and CNTs in Mg melt.Firstly,new CNTs@SiCp hybrid reinforcements were synthesized by CVD.Thus,CNTs were well pre-dispersed on the SiCp surfaces before they were added to Mg melt.Therefore,the following semisolid stirring and ultrasonic vibration dispersed the new hybrid reinforcements well in Mg-6Zn melt.The hybrid composite exhibits some unique features in microstructures.Although the distribution of SiCp was very uniform in the Mg-6Zn matrix,most CNTs distributed along the strips in the state of micro-clusters,in which CNTs were bonded very well with Mg matrix.Most of the CNTs kept their structure integrity during fabrication process.All these factors ensure that the hybrid composite have much higher strength and elongation than the mono SiC/Mg-6Zn composites.The dominant strengthening mechanism is the load transfer effect of CNTs.Apart from grain refinement the CNTs toughen the composites by impeding the microcrack propagation inside the material.Thus,the hybrid CNTs@SiCp successfully realizes the reinforcing advantage of“1+1> 2”.

Keywords: Hybrid;CVD;Mg matrix composites;Strengthening mechanism;Ultrasonic vibration.

1.Introduction

Last decades,micro-particles reinforced magnesium (Mg)matrix composites have made significan progress,especially for silicon carbide particles(SiCp)reinforced Mg matrix composites [1-6].For example,the large size ingots (hundreds of kilograms) and various profile of SiCp/Mg composites have been successfully produced.However,their applications are still limited though they have many advantages such as high specifi strength and stiffness,low coefficien of thermal expansion and low cost [7,8].One of the key reasons is that the poor ductility significantl restricts their engineering applications.Nano-reinforcements,such as nanoparticles,carbon nanotubes(CNTs)and graphene,have exhibited excellent toughening effect to Mg matrix,but it is very difficul to disperse high-content nano-reinforcements in Mg matrix [9-14].As a result,the strength and modulus of Mg matrix composites reinforced by nanomaterials are not as high as the composites reinforced by high-content micro-particles.In view of this dilemma,hybrid multi-scaled reinforcements have been employed in Mg,Al and Cu matrix composites.Fortunately,some researchers have achieved good results [15-18].For example,both strengths and elongation of (CNTs+SiCp)reinforced AZ61 matrix composites are better than mono SiCp/AZ61 composites [19].(CNTs+graphene)/AZ31 composites [20] and (nano-Al2O3+Ti)/Mg composites [21]realized the simultaneously improvement in strengths and ductility.So far,CNTs are considered as one of the ideal nano-reinforcements to toughen micro-particles reinforced Mg matrix composites because of its ultrahigh strengthening and excellent toughening efficiencie attributed by its unique onedimensional hollow nanostructure.In addition,CNTs do not react with Mg at high temperatures,which is an effortless superiority for CNTs/Mg composites to keep the tube structure integrity of CNTs.Thus,CNTs and SiCp hybrid working as reinforcements is an effective way to improve the ductility of micro-particles reinforced Mg matrix composites.

Fig.1.Illustration of the fabrication procedure of CNTs@SiCp/Mg-6Zn composites.(a) Raw SiCp,(b) CNTs@SiCp hybrid reinforcement,(c) As-cast composite,(d) As-extruded composite.

Powder metallurgy has become the most popular approach to disperse CNTs in metal matrix composites such as Al and Cu matrix [22-24],but it is very dangerous for Mg matrix composites due to the explosion risk of Mg powders.It is also difficul to fabricate large ingots of Mg matrix composites by powder metallurgy.As a result,the liquid methods such as stir casting have become the mainstream technology to produce Mg matrix composites [6].However,it is still a challenge to disperse CNTs in Mg melts though large size ingots of micro-SiCp/Mg composites which produced by stir casting due to strong Van der Waals forces [25].Thus,it is urgent to develop a novel method to realize the simultaneous dispersion of CNTs and SiCp in Mg melts.

Accordingly,a novel method was developed to produce(CNTs+SiCp) reinforced Mg matrix composites in this paper.Firstly,chemical vapor deposition (CVD) was used to synthesize hybrid CNTs@SiCp reinforcements,in which a SiC particle was covered with a very thin layer of the in-situ synthesized CNTs [15,26].This realized the pre-dispersion of CNTs before adding them to Mg matrix.The CNTs@SiCp reinforcements were dispersed in Mg melts via semisolid stirring assisted by ultrasonic vibration.Finally,hot extrusion was adopted to further improve the distribution of the reinforcements.The hybrid composites exhibit better strength and ductility than mono SiCp reinforced Mg matrix composites.The hybrid CNTs@SiCp successfully realizes the reinforcing advantage of“1+1>2”for Mg matrix.

2.Experiments

2.1.Fabrication of CNTs@SiCp/Mg-6Zn composite

Fig.1 shows the fabrication process of the CNTs@SiCp/Mg-6Zn composites.It includes 3 procedures:preparation of CNTs@SiCp by CVD,semisolid stirring assisted ultrasonic vibration and hot extrusion.

2.1.1.CVD

The CVD was carried out in a vacuum tube furnace (CD-1200G) by CVD method.Acetylene (C2H2) was used as carbon source and 20μm SiCp acted as CNTs carrier.Ni atoms were adopted as catalyst on SiCp surface.Firstly,100g SiC powder and 1.3g Ni(NO3)2·6H2O were mixed in alcohol.The mixture was heated at 80 °C whilst constantly stirring until all liquid evaporate completely.Secondly,the powder was dried and grinded slightly.Thirdly,the mixture powder was calcined at 450 °C for 1 hour and then reduced at 450°C with the presence of hydrogen in the atmosphere for 2 h to convert the Ni(NO3)2·6H2O to Ni totally and form nanoparticles on the SiCp surface,acting as catalyst in the synthesis process of CNTs.Then 5g pretreat powder were used for subsequent CVD process.Three synthesis parameters were as follows:(1),synthesis temperature:700 °C;(2),deposition time:60 min;(3),C2H2gas f ow rate:25 sccm.The whole synthesis process was operated in hydrogen and argon atmosphere.During the CVD process,CNTs were in-situ synthesized on the SiCp surfaces,so hybrid CNTs@SiCp were obtained as reinforcements for the composites.

2.1.2.Semisolid stirring assisted ultrasonic vibration

Fig.2.Morphology of the CNTs synthesized by CVD on SiCp.(a) CNTs@SiCp,(b) CNT morphology on a SiCp surface,(c) TEM image of as-fabricated CNTs.

The CNTs@SiCp/Mg-6Zn composites were fabricated by semisolid stirring assisted ultrasonic vibration method.First,Mg-6Zn alloy was melted at 720 °C under an atmosphere containing a gas mixture of CO2/SF6and then cooled to 600°C at which the matrix alloy was in semi-solid condition;CNTs@SiCp hybrid reinforcements preheated to 250 °C were quickly added into the semi-solid melt.After adequately stirring the melt,it was rapidly reheated to 700 °C and held at this temperature for 5min.Then the ultrasonic probe was dipped into the melt after the stirrer was removed from the melt.The ultrasonic vibration device consists of a transducer with a maximum power of 2kW and frequency of about 20kHz.The ultrasonic vibration was conduct for 20min.Then the melt was cast into a steel mold preheated to 450 °C and allowed to solidify under a 100MPa pressure.After solidification the ingots were cut into samples with size ofφ60mm×h30mm for extrusion.

2.1.3.Extrusion

The as-cast composites were extruded at 300°C with an extrusion ratio of 14:1.Before hot extrusion process,the composite was homogenized for 12h at 350 °C.Pristine Mg-6Zn alloy,SiCp/Mg-6Zn composite with 10vol.% SiCp and CNTs@SiCp/Mg-6Zn composite with 0.95vol.% CNTs and 10vol.% SiCp were fabricated and extruded at same condition,respectively.

2.2.Characterizations

Scanning electron microscopy (SEM) equipped with an EBSD acquisition camera and transmission Electron Microscope (TEM,Talos F200x) were employed to investigate the morphology and structure of the synthesized CNTs and the composites.The EBSD samples of the composites were firs mechanically polished using a SiC paper and then ion polished with Gatan PECS 685.Raman spectra (B&WTEK,BWS435-532SY) with a 532nm wavelength laser (corresponding to 2.34eV) were used to characterize the as-fabricated CNTs.The tensile mechanical properties of composites were tested using Instron-5583 under a speed of 1mm/min.Dog-bone-shaped specimens with a gage length of 18mm and width of 10mm were used for the tensile test.The yield strength (YS),ultimate tensile strength (UTS) and elongation (El) were obtained by averaging the three testing values.

Fig.3.Raman spectra of the synthesized CNTs and the CNTs in the asextruded composite.

3.Results

3.1.Morphology of the CNTs synthesized by CVD

Fig.2 shows the morphology of hybrid CNTs@SiCp fabricated by CVD.A SiC particle was covered with a very thin layer of the in-situ synthesized CNTs.The seamless,hollow tubular structure was observed,which is the unique feature for CNTs.As shown in Fig.3,the ID/IGintensity of synthesized CNTs is 0.85,which means high graphitization degree of the synthesized CNTs [27-30].All these indicate that the in-situ synthesized CNTs have high quality.The average diameter of the CNTs is about 30nm,and their average length about is about 3.2μm.The CNT wall surface was clean and smooth and without obvious twining.There was no large CNT agglomeration observed on the SiCp surfaces,which is helpful for the CNT dispersion and bonding in Mg melts during the subsequent fabrication process.Thus,the CVD process realized not only the pre-hybrid for CNTs and SiCp but also the pre-dispersion of CNTs on SiCp surface before they were added to Mg matrix.This can significantl reduce the diffi culty of dispersing CNTs into the Mg melts,so that ultrasonic could disperse micro-SiCp particles and bond CNTs with Mg in the melt.

Fig.4.SEM Microstructure of the CNTs@SiCp/Mg-6Zn composites.(a) as-cast composite,(b-d) as-extruded composite,(b) low magnification (c) CNTs on a SiCp surface in the as-extruded,(d) CNTs in a strip.

3.2.Microstructure of the CNTs@SiCp/Mg-6Zn composites

As show in Fig.4(a),CNTs were successfully introduced into Mg-6Zn matrix with the help of the SiCp carrier,and CNTs were mainly located around SiCp in the as-cast composite.SiCp were wrapped by CNT layers.The CNT layer bonded with Mg due to ultrasonic vibration [31].After hot extrusion,SiCp distributed very uniformly in the composites.The CNT layers were peeled off from SiCp surfaces by extrusion,and only a few CNTs were retained,as shown in Fig.4(c).The peeled CNT layers changed to CNT strips,which aligned along the extrusion direction in the Mg matrix,as shown in Fig.4(b) and (d).The strip contained highcontent CNTs,which is further confirme by TEM-EDS mapping,as shown in Fig.5.

The area marked by the yellow dotted circle in Fig.5(a)and (d) is CNTs clusters,which contains high carbon content.Thus,the strips are composed by the CNT clusters.It should be noted that pores were not observed in the CNT clusters,which indicates that CNTs bond well with Mg matrix in the clusters.

Detailed TEM investigations further confirme that both SiCp and CNTs bond very well with Mg matrix.As shown in Fig 6,in the CNT clusters,the CNT/Mg interfaces are very clean,and neither pores nor the interfacial reaction products were observed,as shown in Fig.6(a).This further confirm that CNTs bond well with Mg matrix in the clusters.It should be noted that the multi-walls and the graphitic sheets were very evident,which indicates that the CNTs kept their morphology integrity during the fabrication process.The Raman spectrum in Fig.3 also confirme this.The ID/IGvalue only slightly increased from 0.85 to 0.87 after the fabrication process,which means most of the CNTs were not damaged during dispersion and hot extrusion process.The integrity of CNTs is crucial to their strengthening effect.In addition,the individually dispersed CNTs inside Mg grains were observed beside the CNTs clusters,as shown in Fig.6(a).Additionally,the SiCp and Mg also bond very well,as shown in Fig.6(b).In the CNTs@SiCp/Mg-6Zn composites,the SiCp is stiff phase and the Mg-6Zn matrix is the soft phase.The hot deformation of the matrix was obstructed due to the SiCp existence,leading to distorted strain dislocations at the interface between the SiCp and the matrix.Moreover,the thermal coefficien mismatch between the SiCp and Mg also contributed to the generation of the dislocations at the SiC/Mg interfaces during hot extrusion.Thus,high-density dislocations were observed.

Fig.5.TEM EDS mapping for the CNTs strips in the extruded composites.(a) HAADF image,(b) EDS mapping of related elements,(c) Mg,(d) C,(e) Zn.

Fig.6.TEM micrographs of as-extruded CNTs@SiCp/Mg-6Zn composites.(a)HRTEM image for CNT/Mg interfaces in a CNT cluster,(b)a SiC/Mg interface,(c) individually dispersed CNTs inside Mg grains.

As shown in Fig.7,the CNTs@SiCp addition significantl affected the microstructure of Mg matrix.The grain sizes were refine from 4.5μm to 2.7μm,as shown in Fig.7(a)-(d).This is beneficia for the ductility of the composite.Although the CNTs@SiCp addition did not change the texture type,the maximum texture intensity is reduced from 18.2 to 8.1,as shown in Fig.7(e) and (f).As a result,the average Schmid factor of the basal slip{0002}〈11ˉ20〉for the CNTs@SiCp/Mg-6Zn composite (0.24) is higher than Mg-6Zn alloy (0.21).

3.3.Enhanced mechanical properties of the hybrid composites

The typical stress-strain curves of the as-extruded Mg-6Zn alloy and the composites are shown in Fig.8 the mechanical properties of the as-extruded materials are given in Table 1.Both SiCp and CNTs@SiCp significantl improvedthe YS and UTS of the Mg matrix.The CNTs@SiCp/Mg-6Zn composite exhibited the best mechanical properties with YS,UTS and elongation values of 218MPa 315MPa and 6.1%,respectively.Especially,the UTS was improved from 286 to 315MPa due to the CNT addition compared to mono SiCp reinforced composite,which indicates the superiority of hybrid CNTs@SiCp.It should be noted that the hybrid addition also improved the elongation from 4.6% to 6.1%,which is extremely significan for Mg matrix composites because their poor ductility seriously restricts their applications.All these indicate that the hybrid CNTs@SiCp successfully realize the reinforcing advantage of“1+1>2”[13].

Fig.7.EBSD results for Mg-6Zn and CNTs@SiCp/Mg-6Zn composite.(a) and (b) grain size measured by EBSD,(c) and (d) grain orientation distribution,(e) and (f) pole figures (g) and (h) Schmid factor comparison.

In summary,a hybrid CNTs@SiCp/Mg-6Zn composite was successfully fabricated by a novel method.The as-extruded composite exhibits some unique features in microstructures.Although the distribution of SiCp was very uniform in Mg matrix,most CNTs distributed in strips in the state of microclusters,in which CNTs bonded very well with Mg matrix.Most of the CNTs kept their structure integrity during the fabrication process.All these ensure the reinforcing superiority of hybrid CNTs@SiCp over mono SiCp.

4.Discussions

As stated above,the composite reinforced by the hybrid CNTs@SiCp exhibits much better strengths and elongation than the composite reinforced by mono SiCp.The hybrid composite has some unique features in microstructure,which can evidently affect its mechanical behaviors.Therefore,it is necessary to analyze the strengthening mechanisms of the hybrid composite.For the CNTs@SiCp/Mg-6Zn composite,the YS increment cause by SiCp and CNTs can be expressed as Eqs.(1) and (2):

Fig.8.The typical engineering stress-strain curves of as-extruded Mg-6Zn alloy and composites.

WhereσcomandσMgare YS of hybrid composite and Mg-6Zn alloy,respectively;Δσis YS increment,ΔσSiCandΔσCNTare YS increment contributed by SiC and CNT respectively.Thus,we can calculate and analyze the CNTs and SiCp contributions to the YS increment.According to previous studies,for the mono CNTs reinforced Mg matrix composites,YS increment is mainly attributed to Hall-Petch strengthening,load transfer and Orowan looping strengthening mechanisms;for mono SiCp reinforced Mg matrix composites,YS increment is mainly caused by Hall-Petch strengthening,load transfer mechanism and dislocation strengthening mechanisms which is caused by the difference coefficien of thermal expansion (CTE) between the reinforcement and the matrix.Thus,ΔσSiCandΔσCNTcan be calculated by Eqs.(3) and (4):

In the hybrid composite,the refine grains were jointly caused by CNTs and SiCp.Therefore,Hall-Petch strengthening mechanism can be summarized as Eq.(5):

In addition,most CNTs distributed in strips in the state of micro-clusters in the CNTs@SiCp/Mg-6Zn composite,a small amount of CNTs are located inside Mg grains.Thus,the Orowan looping strengthening mechanism of CNTs can be ignored.Accordingly,YS increment of the hybrid composite can be expressed as Eq.(6):

(1) Hall-Petch strengthening mechanism

The theoretical YS increment caused by grain refinemen can be describe by Eq.(7) [32]:

WheredcomanddMgare the average grain size of CNTs@SiCp/Mg-6Zn and Mg-6Zn matrix.Proportional constant K is given as 0.13MPa m1/2for Mg alloys.The results in Fig.7 show that the average grain size was refine from 4.5μm (Mg-6Zn matrix) to 2.7μm due to the CNTs@SiCp addition.TheΔσHall-Petchhere is 17.8MPa.

(1) Load transfer mechanism from CNTs and SiCp

In this hybrid Mg matrix composites,both SiCp and CNTs bonded very well with Mg matrix,as shown in Fig.6.This ensures that load transfer to SiCp and CNTs can take effect under load.YS increment contributed by CNT load transfer depends on CNT length,because there exists a critical length for CNTs which affects the way to carry load.If the CNT length is larger than the critical length,CNTs are snapped during load;otherwise,CNTs will be pulled out from the matrix.The critical length of CNTs (lCNT) can be define as Eq.(8) [33-35]:

whereσCNTis the strength of CNTs (30GPa) [32],dCNTis the average diameter of CNTs.lCNTin this work is calculated to be 6.7μm,which is much larger than the average length of the synthesized CNTs (3.2μm).Therefore,CNT is pulled-out from Mg matrix under applied load.In this situation,according to Shear-Lag model [36],the theoretical YS increment contributed by CNTs load transfer can be calculated by Eq.(9) [31]:

Where,fCNTis the volume fraction of CNT,Sis the aspect ratio of the CNTs.TheΔσCNT-Loadhere is 39.4MPa.

If we assume that the SiCp are equiaxed in this work,YS increment contributed by SiCp load transfer can be simply expressed as [37]:

(1) Dislocation strengthening mechanism of SiCp

The CTEs and elastic modulus between Mg matrix and SiC are different,which leads to generation of dislocations associated with work hardening or mismatch during the hot deformation.The theoretical YS increment of the composites caused by the thermal mismatch between SiC and Mg can be expressed as [38,39]:

Where the value ofβis 1.25,Δαis the CTE difference between matrix and SiCp reinforcement.ΔTis the temperature difference between the material preparation temperature and mechanical measurement temperature,is the average diameter of SiCp.TheΔσSiC-CTEhere is 12.3MPa.

Fig.9.Theoretic YS increments contributed by different strengthening mechanisms.

Base on the above analysis,the total YS increment of CNTs@SiCp/Mg-6Zn composite can be predicted by the following combined Eq.(12):

Table 2 shows the values of parameters for calculating the theoretical YS increment.The calculated YS in-crement of CNTs@SiCp/Mg-6Zn composite is 77.2MPa.Fig.9 compares the strengthening effects contributed by different strengthening mechanisms.The load transfer of CNTs is the top contributor for the YS increment,which is larger than the sum of the rest.This further confir the necessity and superiority of the CNT hybrid.The significan contribution of CNT load transfer mechanism is attributed to the good interfacial bonding and the good structure integrity of the CNTs,as presented in Figs.4 and 6.

Table 2 The values of parameters used in Eq.(12).

Fig.10.SEM fractography of SiCp/Mg-6Zn and CNTs@SiCp/Mg-6Zn composites.(a)for SiCp/Mg-6Zn composite,(b-d)for CNTs@SiCp/Mg-6Zn composites.(c) microcracks in the lateral surface of the fractured tensile sample,(d) high magnificatio of the microcrack in (c).

Fig.11.Illustration of the fracture models for the two composites.(a) SiCp/Mg-6Zn composite,(b) CNTs@SiCp/Mg-6Zn composite.

The experimental YS increment of the hybrid composite is 64MPa,which is 13.2MPa lower than the calculated value.The deviation between the calculated and experimental values is caused by several reasons.Firstly,the addition of CNTs@SiCp significantl weakened the extrusion texture of Mg matrix.As shown in Fig.7(e) and (f),the texture intensity was reduced from 18.2 to 8.1 due to the addition of the CNTs@SiCp.As a result,the Schmid factor of the basal slip system {0002}〈11ˉ20〉 increased from 0.21 to 0.24.This means that the hybrid composite is easier to yield during tensile testing than the M-6Zn alloy.Secondly,SiCp and CNTs were more or less damaged,which generally reduce their strengthening effects.All these can reduce the YS of the composites,so the experimental value of YS increment is lower than the calculated one.

4.2.Fracture behaviors

Fig.10 shows the tensile fractography of the SiCp/Mg-6Zn and CNTs@SiCp/Mg-6Zn composites.The fracture features were similar for the two composites,as shown in Fig.10(a)and(b).The SiC/Mg interface debonding was very evident for them,which indicates that microcracks were mainly caused by SiC/Mg interface debonding during the tensile test.Thus,the extra addition of CNTs did not change crack initiation mode of the composite.However,the CNTs affected microcrack propagation,as shown in Fig.10(c) and (d).As stated above,CNTs were pulled out from Mg matrix during tensile tests,which is further proved by bridged CNTs in the lateral surface of the fractured tensile sample.When a microcrack tip encounters CNTs during its propagation process,the pulling-out of CNTs can retard the microcrack to further propagate via increasing resistance of its propagation or changing propagation paths.This delayed the fina fracture of the hybrid composite,so the addition of CNTs further improved the elongation of mono SiCp/Mg-6Zn composite by impeding the microcrack propagation except for refinin Mg grains.

According to the above analysis,the illustration of feature models for the two composites was founded in Fig.11.For SiCp/Mg-6Zn composite,cracks initiate at SiCp/Mg interfaces by debonding and then propagate vertical to extrusion direction.In comparison,CNT strips were aligned along the extrusion direction (applied load direction),which restrains crack propagating via bridged mechanisms.

5.Conclusions

A new hybrid CNTs@SiCp/Mg-6Zn composite was successfully fabricated by a novel method.During the fabrication process,a thin CNT fil without evident clusters were in-situ synthesized on the SiCp surface by CVD technique,which well pre-dispersed the CNTs.The semisolid stirring assisted ultrasonic vibration and hot extrusion well dispersed the hybrid reinforcement in Mg matrix.Both SiCp and CNTs bonded well with Mg matrix at the interfaces,and most of the CNTs kept their structure integrity.Both the strengths and elongation were evidently improved due to the hybrid addition of CNTs.Thus,the hybrid CNTs@SiCp successfully realizes the reinforcing advantage of“1+1>2′′.This work further confirm that the CNT hybrid working as reinforcement is an effective way to improve the ductility of micro-particles reinforced Mg matrix composites.In addition,the CVD method is versatile to synthesize CNTs on ceramic reinforcements,so the novel fabrication strategy can be used in other composite systems.

Declaration of Competing Interest

None.

Acknowledgements

This work was supported by ‘‘National Natural Science Foundation of China’’ (Grant Nos.51871074,51971078 and 51671066) and ‘‘The Project National United Engineering Laboratory for Advanced Bearing Tribology,Henan University of Science and Technology’’ (Grant No.201911).