Xiaoshi Hu,Zhenming Sun,Chunlei Zhang,Xiaojun Wang,Kun Wu

School of Materials Science and Engineering,Harbin Institute of Technology,Harbin,China

Abstract Inspired by an old f sh skin structure,the Cf/Ti/Mg laminated composites were fabricated by squeeze casting technology.No porous or voids were found in fina composite,and carbon fibe was uniformly dispersed in Mg matrix.Furthermore,the addition of net-shaped Ti adsorbed Al element and facilitated the nucleation of Mg17Al12 nearby Ti.The reaction product Al4C3 was found at the Cf and AZ91 interface.Mechanical tests indicate that the introduction of Ti could greatly improve the toughness of Cf/Mg composites.

Keywords:Magnesium matrix composites;Carbon fiber Laminated composites.

1.Introduction

The requirement for energy-efficien technology in modern society necessitates the development of lightweight and highperformance structural materials[1].Carbon fibe reinforced magnesium composites as ultrahigh specifi strength material are widely fabricated and investigated during the past several decades.It was found that the addition of carbon fibe into Mg could not only greatly improve the strength and stiffness but also enhance the high thermal and electrical conductivity of magnesium alloys.However,the toughness is significantl declined due to the addition of carbon fibe,which limits the application of Cf/Mg composites[2].

To overcome the decrease of toughness under the preservation of composites,the natural structures of animals were mimicked to fabricate new structural composites,such as wood,turtle and nacre[3–5].Research results indicated that these highly sophisticated natural structures with hierarchical design enable good combination of strength and toughness of natural materials[6,7].Many scientists have made efforts to replicate the hierarchical structures of natural materials[8–11],aiming to synthesize materials with good mechanical properties.Bruet et al.[12]investigated an old f sh skin structure and found that this fis skin has evolved as a natural armour with high strength and stiffness,as well as toughness.Study shows that the structure of this f sh skin consists of four graded layers.The outest stiff layer called ganoine can sustain the outside attack.The following two layers named dentine and isopedineare are relatively “soft”.The innerside bone layer(300μm in thickness)releases the load from environment.As all these layers are well bonded with each other;the force is well dissipated during the load transfer from the outer layer to the inners[12].Finally,the internal organs of the f sh are well protected[13].

Inspired by this fis skin structure,we incorporated netshape Ti between carbon fiber and fabricated laminated Cf/Ti/Mg composites by squeeze casting method in this study.The laminated Cf/Ti/Mg composites inspired by this f sh skin structure are shown in Fig.1.The relationship between the microstructure and mechanical properties was well studied and discussed.

Fig.1.Schematic diagram for the laminated Cf/Ti/Mg composites inspired by this fis skin structure.

Table 1 Chemical compositions of AZ91 alloys.

2.Materials and method

2.1.Raw materials

T700 and M40 carbon fibe purchased from Toray Industries,Inc.(Japan)was adopted as reinforcement.Commercial AZ91 alloy was employed as the matrix.The chemical composition of AZ91 is listed in Table 1.In addition,commercial net-shape pure titanium with 0.1mm in thickness and 1×1mm in size was incorporated between carbon fibe layers to strengthen composites.

2.2.Preparation of Cf/Ti preform

In this study,the unidirectional carbon fibe was twined on a graphite bulk by automobile textile machine.The graphite bulk is 150mm in length,75mm in width and 50mm in height.The twinning parameter is 89.5°of twinning angle and 3N of twinning force.During the carbon fibe is twinned on the bulk,the net-Ti should be tiled on the bulk so that the net-Ti could be implanted in the perform.One layer net-Ti was implanted by every two layers of carbon fibe.As shown in Fig.2(a),the perform is a multilayered structure composed of stacked Cf and net-Ti layers.The diagram of Cf/Ti perform and macrograph can be found in Fig.2(b)and(c).

2.3.Fabrication of Cf/Ti/Mg composites

Cf/Ti/Mg composites are fabricated by squeeze casting,as shown in Fig.3.Firstly,the Cf/Ti perform was put in a steel die(Ф200×280mm)and preheated at 450°C–550°C for 30min.Simultaneously,the AZ91 magnesium alloy was melted in a furnace at 750–820 °C.Then molten magnesium was poured into the die and subsequently pressed into carbon fiber/i pre-form by a punch at a constant pressure of 5MPa for 20min.After thesolidificatio of the molten AZ91,the ascast Cf/Ti/Mg composites were obtained.It is worth emphasizing that the whole process was under the protection of SF6gas to avoid oxidation of magnesium alloy.Besides,carbon fibe reinforced AZ91 matrix composites(without net-shape Ti)and Ti/AZ91 were also fabricated by the same method to make comparison with Cf/Ti/Mg laminated composites.

2.4.Microstructure characterization

Before mechanical property measurement,carbon fibe volume fraction Vfof each composites was determined by measuring their density using drainage method.[14].As the porosity is negligible,Vfcan be computed using the rule of mixtures based on the average value of 3 specimens.After mechanical testing,the measurement was further confirme by the subsequent dissolution of the matrix in an acid solution,followed by f ltering and weighing of the fibers

Scanning electron microscopy (SEM) (Quanta 200FEG)was used to analyze cross sections of each type of samples.The test samples were etched with nitroxanthic acid (picnic acid (5g)+glacial acetic acid(5ml)+ethanol(100ml)+distilled water(10ml))to examine the microstructure of the as-cast composites.Fracture surfaces were examined using SEM.The fiber/matri interfacial bonding was analyzed using transmission electron microscopy(TEM)(Tecnai F30).

2.5.Mechanical properties test

The tensile mechanical properties of Cf/Ti/Mg composites were measured by using an Instron 5569 universal testing machine at a constant cross-head speed of 0.5mm/min.The tensile tests were in accordance with ASTM:E8/E8M-13a standards.Three point bending tests were according to HB-7617-1998 standards.Tensile tests were based on the average of three sample tests.

3.Results and discussion

3.1.Microstructure of Cf/Ti/Mg composites

The SEM observation in Fig.4 illustrates that Cf,Ti and Mg matrix was well bonded with each other,no porous or void was observed.This indicates that the squeeze casting technology was feasible to fabricate Cf/Ti/Mg composites.The carbon fibe layer was about 400μm in thickness,and Ti/Mg layer was about 300μm in thickness.In addition,some precipitates at both AZ91/Ti and AZ91/Cf interface were found,as shown in Fig.4.However,the precipitates tended to dissolve out near the AZ91/Ti interface because more precipitates were found around Ti while not near Carbon fibe.The element distribution in Fig.5 illustrates that Al element diffused and segregated at the AZ91/Ti and AZ91/Cf interface.Particularly,we found that the segregation level of aluminum at AZ91/Ti interfaces is much higher than that of aluminum at Cf/AZ91 interfaces.In addition,the precipitates were confirme as Mg17Al12via the observation of TEM(in Fig.6).Wu and Zhu[15]have investigated the morphology of Mg17Al12in Cf/AZ91 composites.They found that the surface energy of carbon fibe and the solidificatio rate have a great influenc on the diffusion of aluminum element and the distribution of second-phase precipitates.The high-surface energy of carbon fibe can adsorb aluminum element and provide nucleation sites for precipitation.Besides,as the diffusion of aluminum is a time-dependent process,solidificatio rate absolutely affected the segregation of Al.The coefficien of thermal conductivity of Ti(15.24W/(m·K))is less than that of T300 carbon fibe(615W/(m·K)).Therefore,the part of matrix near Carbon fibe side solidifie much faster than that near Ti side.Subsequently,the aluminum element had less time to diffuse towards the carbon fibe interfacial area than the AZ91/Ti interface.In addition,as Ti had higher surface energy than carbon fibe,Ti could adsorb more Al element and provide nucleation sites for precipitates.Therefore,more Mg17Al12precipitates in the matrix were found near Ti side,rather than carbon fibe.

Fig.2.Schematic diagram for Cf/Ti preform preparation.(a)The schematic image of carbon fibe textile;(b)schematic diagram of Cf/Ti perform;(c)image of Cf/Ti perform.

Fig.3.The images of Cf/Ti/Mg composites,(a)macrograph of Cf/Ti/Mg;(b)cross-section graph of Cf/Ti/Mg.

Fig.4.SEM images of the Cf/Ti/AZ91 composites(a)X50,(b)X200.

Fig.5.element distribution of Cf/Ti/Mg(a)all elements distribution;(b)Al distribution.

Fig.6.TEM images of Cf/Ti/Mg composites,(a)TEM image;(b)and(c)HRTEM image.

Fig.7.TEM images of Cf/AZ91 composites,(a)morphology of Al4C3 precipitates;(b)high-resolution image of Al4C3 precipitates.

Concerning about Cf/AZ91 composites,many needle-like product were found with about 200–400nm in length.These precipitates nucleated heterogeneously from the carbon fibe surface and grew towards the AZ91 matrix.These randomly distributed precipitates were confirme as Al4C3via TEM observation(in Fig.7).However,for Cf/Ti/Mg composites,only little Al4C3reaction product was found at the Cf/AZ91 interface.This is caused by the introduction of Ti,which adsorbed a large amount of Al.Finally,less Al was left and reacted with C to form Al4C3.

Differently,clean and smooth AZ91/Ti interface was found.Wang et al.[16]observed TiAl3reaction product when they investigated the interface of TC4p/AZ91 composites(580°C).Given the composition of Ti and Al element in the composite and the experimental condition,chemical reaction would not occur according to the Ti–Al system phase diagram.

3.2.Mechanical properties

Fig.8 shows the tensile stress–strain curve of Ti/Cf/Mg composites.The tensile strain was improved to 6%,which is very high for continuous carbon fibe composites.Due to the intrinsic mechanical properties,carbon fibe reinforced metal matrix composites are pretty brittle.A large number studies have shown that the tensile strain is generally lower than 1%.In this study,the tensile strain was improved by 6 times due to the introduction of Ti layer.

Fig.9 shows the tensile fracture of Cf/Ti/Mg composites.The tensile fracture includes the fibe pull-out,crack deflec tion at Mg/Cf interface,interfacial debonding between net-Ti and Mg matrix as shown in Fig.10,when the crack met a carbon fibe,the interfacial stress concentrated.When the component force along?direction reached to the one certain value,the composite would be failed.As for Cf/Mg composites,the crack propagated along 0°or 90°.When the stress concentration reached to the strength of carbon fibe,carbon fibe was destroyed and the crack passed the fibe manifesting brittle failure at low-stress level.If the stress along 90°reached to the interfacial strength,the crack propagated along Cf/Mg interface.This would manifest carbon fibe pull-out.It is well established that pull-out of fiber could dissipate more power during tensile test and the toughness should be improved.

Fig.8.Tensile stress–strain curve of Cf/Ti/AZ91 composites and AZ91.

Besides,according to the work of on the fracture of laminated composites[17,18],the laminated structure improved the toughness and strength of composites.Due to the different physical parameters such as elastic modulus and Poisson’s ratio between different layers,the deformation will differ from each other.As each layer is well bonded with others,the different components in composite perform coordinate deformation other than free-form deformation.The brittle layer transfers its deformation to adjacent plastic layer.Thus,the toughness of the brittle composites be improved[17,18].In this study,the plastic net-Ti absorbed much deformation from Cf/Mg layer.This avoided the premature failure of Cf/Mg composite and improved its toughness.In addition,some cracks were found around net-Ti in Ti/Mg layer(in Fig.9(e)–(h)).This is attributed to the weak bonding between Ti and Mg alloy.Based on the fabrication temperature of Cf/Ti/Mg composites,Ti would not react with Mg or Al.Thus,they were mechanically bonded with others.As the debond of Ti from Mg dissipated energy,this also contributed to the toughness improvement of Cf/Ti/Mg laminated composites.

Fig.9.SEM photograph at tensile fracture(a)–(d)lateral view of the tensile fracture;(e)–(h)partial magnifie images.

Fig.10.diagram of Cf/Mg composites at crack tip.

4.Conclusion

In this study,the fish-ski structure was mimicked to fabricate the bio-inspired Cf/Ti/Mg composite.Microstructure characterization indicated that the addition of Ti influence the diffusion of Al in AZ91D layer.Besides,the toughness of the composites was improved due to the contribution of Ti.The main conclusions are drawn as follows:

1 Cf/Ti/Mg composites were successfully fabricated using pressure infiltratio method.Interfacial reaction happened and the product was detected as Al4C3.

2 The addition of Ti(net)improved the tensile strain to 6%.The laminated structure strongly improved the toughness of Cf/Mg composites.

Acknowledgments

This work was supported from National Key Research and Development Program of China(No.2017YFB0703102),National Natural Science Foundation of China(Nos.51671066 and 51471059),and the Key Laboratory of Superlight Materials&Surface Technology(Harbin Engineering University),Ministry of Education.