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Interfacial microstructure and tensile properties of carbon fiber reinforced Mg–Al-RE matrix composites
Accepted Manuscript Interfacial microstructure and tensile properties of carbon fiber reinforced Mg-Al-RE matrix composites Shaolin Li, Lehua Qi, Ting Zhang, Jiming Zhou, Hejun Li PII:
S0925-8388(15)31956-3
DOI:
10.1016/j.jallcom.2015.12.165
Reference:
JALCOM 36254
To appear in:
Journal of Alloys and Compounds
Received Date: 19 November 2015 Revised Date:
16 December 2015
Accepted Date: 20 December 2015
Please cite this article as: S. Li, L. Qi, T. Zhang, J. Zhou, H. Li, Interfacial microstructure and tensile properties of carbon fiber reinforced Mg-Al-RE matrix composites, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2015.12.165. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT Interfacial microstructure and tensile properties of carbon fiber reinforced Mg-Al-RE matrix composites Shaolin Lia, Lehua Qib,∗, Ting Zhanga, Jiming Zhoub, Hejun Lia (aState Key Laboratory of Solidification Processing, Carbon/Carbon Composites Research Center, Northwestern
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Polytechnical University, Xi’an, P.R.China, 710072 b
School of Mechatronic Engineering, Northwestern Polytechnical University, Xi’an, P.R.China, 710072)
Abstract: 2D carbon fiber reinforced Mg-Al-RE matrix composites (2D-Cf/AE44 composites)
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were fabricated by liquid-solid extrusion following vacuum pressure infiltration technique (LSEVI). Pyrolytic carbon (PyC) coating was deposited on surface of T700 carbon fiber by
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chemically vapour deposited (CVD) to modify the interface. TEM observation of the composites revealed that particle shaped Al2RE and lamellar shaped Al11RE3 precipitated at the interface. The precipitation of Al-RE phase inhibited the formation of Mg-Al phase and the PyC coating protected the fiber effectively. SEM observation of fracture surface, as well as TEM analysis of the interface, were employed to explain the inherent relation between
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interface behavior and mechanical properties of 2D-Cf/AE44 composites. Keyword: Metal matrix composites; matrix alloying; interface; tensile properties
Introduction
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1.
Recently, carbon fiber reinforced Mg matrix composites (Cf/Mg composites) have been
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attracting much attention in various industrial fields, such as aerospace industries and automobile industry, for their high specific strength and modulus, low coefficient of thermal expansion (CTE), etc [1-4]. It is generally accepted that the performance of metal matrix composites (MMCs) are dominated by the interfacial status between carbon fiber and matrix. However, the wettability of the interface between carbon fiber and Mg matrix is poor, which impairs the final performance of Cf/Mg composites significantly. Many efforts have been made to improve the wettability and modify the interfacial status, such as fiber surface modification and matrix alloying. ∗
Corresponding author. Tel.:+86-29-88460447; fax:+86-29-88491982 E-mail address: [email protected] (Lehua Qi) 1
ACCEPTED MANUSCRIPT Fiber surface coating processing is an effective way to modify the interface and improve the performance of Cf/Mg composites. It was reported that the carbon fibers could modified with 5.0 mol.% yttria stabilized zirconia by sol–gel route, and an about 20 nm thick interfacial reaction layer consisting of nanocrystalline particles was found. The Cf/Mg composite
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exhibited a tensile strength of 1.08 GPa, which reached 90% of the theoretical prediction by means of the rule of mixture [5]. In Cf/Mg composites with a boron nitride (BN) layer on fiber surface, no carbides were found at the interface and a small amount of B element diffused into the Mg matrix, which indicated a good interface bonding [6]. T800H/AZ91
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composites were fabricated with a pyrolytic carbon (PyC) coating on carbon fiber. It was found that coating the fibres with PyC could strongly effect the formation of interfacial phases
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and the micromechanical response of the composites [7]. It have been demonstrated that surface coating modification could significantly improve the wettability of the interface. Except surface coating, matrix alloying is another promising method to modify the interface. Adding Al element into magnesium alloy have proved to be effective to improve the wettability between carbon and magnesium and modify the interface. It was found that the
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interface reactivity of carbon and magnesium can be controlled by Al element [8]. The continuous and discontinuous precipitated Mg17Al12 in Cf/Mg composites affected the interface state of carbon and magnesium [9]. Al element in the Mg matrix could strongly
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influence the mechanical properties of Cf/Mg composites due to the formation of carbide precipitates Al2MgC2 and Al4C3 at the interface [10]. However, Al4C3 is unfavorable for the
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dimensional stability and the mechanical properties of the composites [11, 12]. Moreover, interfacial reaction products contained C element, including Al4C3 and Al2MgC2, would bring damage to the carbon fibers, and then jeopardize the performance of the composites [13]. In order to minimize the above-mentioned disadvantages, some scholars added rare earth element into Mg matrix. It has been known that Mg alloys containing rare earth (RE) element exhibit a significant improvement in room temperature strength and fatigue resistance [14]. Recently, Y or Gd element has been added into Mg matrix to fabricate Cf/Mg composites, which proved to be effective to modify the interface and improve the mechanical properties [1, 15, 16]. However, Al element was not taken into account in these researches. The combined effect of Al and RE on interface and mechanical properties of Cf/Mg composites was 2
ACCEPTED MANUSCRIPT uncertain. The AE44 alloy, a typical Mg-Al-RE alloy, which contains 4% Al and 4% RE (mischmetal), was known for its good mechanical properties in room and high temperature [17, 18]. These features should be attributed to the high content of mischmetal which can
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stabilize the AlxREy phase, and subsequently suppress the formation of Mg17Al12 phase, a common precipitation in Mg-Al alloys. AE44 magnesium alloy based composites reinforced with Saffil fibers have been fabricated recently [19]. The rare-earth alloying elements are distributed at the interface in the form of Al2RE particle phase on the surface of fiber, and
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Al11RE3 lamellar phase in the matrix. Mechanical tensile strength of AE44 alloy was improved by incorporating Saffil fibers. However, no literature reported the interfacial
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behavior of carbon fiber reinforced Mg-Al-RE matrix composites and their mechanical properties.
In the present paper, 2D-Cf/Mg composites were fabricated by liquid-solid extrusion following vacuum pressure infiltration technique (LSEVI), a special MMCs forming technique developed by our team. AE44 were adopted as the matrix to study the combined the mechanical
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effect of Al and RE on interfacial status of Cf/Mg composites. Furthermore properties of the composites were investigated.
Experimental
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2.
2.1 Experimental material
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AE44 magnesium alloy were chosen as matrix material. Chemical component of AE44 alloy are shown in Table 1. Toray T700-12k unidirectional carbon fiber fabrics were used as reinforcement. Properties of T700 carbon fiber are shown in Table 2.
Table 1. Chemical component of AE44 alloy (mass fraction, %)
Elements
Mg
RE(La,Ce)
Al
Zn
Mn
Si
Fe
Cu
Ni
Be/ppm
AE44
Bal.
3.96
4.05
0.02
0.39
0.01
0.002
0.001
0.00006
8
3
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Elongation /%
Bending strength /MPa
Interlaminar shear strength /MPa
4.1
242
1.72
782
47.7
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Tensile strength /GPa
2.2 Fabrication process of 2D-carbon fiber preform
The unidirectional carbon fiber fabrics were stacked orthogonally layer by layer, and then
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punctured by carbon fiber string to obtain a fix preform. PyC coating was deposited on carbon fiber surface by CVD to modify the interface of the composites and protect the fiber.
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Deposition parameters are shown in Table 3.
Table 3. Deposition parameters of PyC coating on carbon fiber
Holding time /h
CH4/ m3/h
C3H8/ m3/h
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Temperature
2.0
0.45
/ 1050
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2.3 LSEVI to fabricate 2D-Cf/Mg composites
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LSEVI integrates melting, pouring, infiltration, and liquid-solid forming under high infiltration pressure. At first, the preform with PyC coating was placed in mold. Then a mechanical pump was used to vacuumize the mold chamber and keep the vacuum level
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0.01-0.05 MPa. The mold and the preform were preheated to 650 °C. At the same time, magnesium alloy was melted under the temperature of 750 °C, and holding for 1h. Then the molten alloy were pressed into the mold by gas pressure (argon, 0.1-0.5 MPa). When the molten alloy was cooled to 650 °C, hydraulic machine was drived to infiltrate the molten alloy to the preform. The pressure was about 30 MPa. 2.4 Characterizations The morphology of the 2D-Cf/Mg composites were analyzed by optical microscope and scanning electron microscope (FE-SEM, Supra-55), equipped with energy dispersive spectroscopy (EDS). The interfacial microstructure observation was implemented by Tecnai 4
ACCEPTED MANUSCRIPT G2 F30 transmission electron microscopy (TEM). The crystalline structure of the fibers was measured with X-ray diffraction (XRD, X’Pert Pro MPD). Tensile tests were conducted on CMT5304-30KN electronic universal testing machine in accordance with HB 7616-1998 standard. The rectangular specimens were 75 mm length, 10 mm width and 2 mm thickness,
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with a constant span length of 20 mms. The tests were conducted at room temperature with a strain rate of 0.5 mm/min. Fracture surface was characterized by SEM. 3.
Results and discussion
3.1 Microstructure and interface status of 2D-Cf/AE44 composites
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Fig. 1 shows typical optical microstructure of the as-squeezed AE44 alloy, and its composites reinforced with carbon fibers (Fig. 2). The lamellar phases (marked in Fig. 1) are
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distributed in grain boundary region in the unreinforced alloy, as shown in Fig. 1. However,
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the second phases in the composites are much fewer, according to Fig. 2.
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Fig. 1. Optical micrograph of as-squeezed AE44 alloy
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Fig. 2. Optical micrograph of 2D-Cf/AE44 composites
Back-scattered electron (BSE) detector was used to explore the infiltration microstructure and element distribution of the composites, and the z-contrast image was shown in Fig. 3. The composites exhibited good fiber dispersion without apparent porosity or significant casting defects. Small interstices between single fibers were filled with alloy, which indicate a good
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infiltration. The interface areas between the Cf and the Mg matrix were very good without discernible debonding or micro-crack. Moreover, bright interfacial zones were observed between carbon fiber and magnesium alloys, as shown in Fig. 4(a).
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To explore the composition of the bright zones, EDS mapping was employed, as shown in Fig. 4. It was demonstrated that the bright interfacial layer was composed of Al and RE(Ce,
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La), which indicated that the Al and RE were prone to segregate around the carbon fiber surface. For 2D-Cf/AE44 composites, a large number of interface were induced by carbon fibers. During fabrication, the fiber surface attracted the alloying element in the matrix. The consumption of the alloying element suppressed the formation of the eutectic phases. That is why the eutectic phases in the composite are much fewer in the alloy (Fig. 1, Fig. 2).
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Fig. 3. SEM micrograph of 2D-Cf/AE44 composites (BSE image)
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Fig. 4. Microstructure of 2D-Cf/AE44 composites(a) and element distribution, Mg
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mapping(b), Al mapping(c), La mapping(d) , Ce mapping(e)
XRD analysis was conducted to identify the crystallographic structure of the phases in the
composite, as shown in Fig. 5. The result revealed that the composites were composed of magnesium, Al2RE and amorphous carbon. This indicated the bright interfacial layer might be Al2RE phase, a common phase in AE44 alloy.
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Fig. 5. XRD pattern of 2D-Cf/AE44 composites
For a further understanding on the interface, and confirming the interface phase, TEM
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observation was conducted, as shown in Fig. 6. An obviously interfacial layer with fine nano-scale particles was observed. The interfacial layer was distributed on fiber surface uniformly in a thickness of about 100 nm. The interface mainly consisted of blocky particle phase (A in Fig. 6(a)). The SAED pattern showed the particle phase were Al2RE (face-centred cubic crystal structure), which was consistent with XRD result. In addition, a few amount of
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lamellar phase were also found at the interface (B in Fig. 6(a)). The SAED pattern showed the lamellar phase were Al11RE3 (orthorhombic crystal structure). The amount of Al11RE3 phase was too small to be detected by XRD.
A uniform PyC coating in a thickness of about 150 nm was wrapped on fiber surface, as
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shown in Fig. 6(b). No diffusion between matrix and fiber was observed. And the coating was intact without any structural damage, which indicated that the PyC coating served as a barrier
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between matrix and fiber, and protected the fiber effectively. During fabrication, the molten alloy infiltrated into the fiber bundle and then solidified.
Due to the poor wettability and incoherent crystal structure between Mg and C, the surface of carbon fiber with PyC coating failed to serve as nucleation base for Mg. Moreover, due to the relatively high preheating temperature of the preform (650°C), the solidification started at locations away from fiber (inter-fiber region), and finished at fiber surface. And the enrichment of alloying elements Al and RE occurred at fiber surface during the final stage of solidification. This was demonstrated by the observation of blocky Al2RE and lamella Al11RE3 at the interface. The Al2RE and Al11RE3 compounds are both typical equilibrium phase in 8
ACCEPTED MANUSCRIPT Mg-Al-RE alloys [20]. The reaction enthalpies of Al2La and Al11 La3 are listed below [21]: ∆H = -50.5 ± 2 kJ/mol
1/3{La(α)+2Al(s)→LaAl2} 1/14{3La(α)+11Al(s)→La3Al11}
∆H = -41.0 ± 2 kJ/mol
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Al2RE, characterized by high-melting-point(1400-1500°C), has been reported as the most stable compound in Al-RE system [22]. According to the phase diagram and enthalpy of formation, Al2RE is with the minimum ∆H, which was lower than that of Al11RE3
However, the reaction enthalpies of Mg17Al12, a common eutectic phase in Mg-Al alloys,
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is much higher than those of Al2La and Al11 La3 [23]: L ↔ (Al) + β
∆H = -8.1 kJ/mol
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During solidification, the RE elements preferentially reacted with Al to form Al-RE phases, which consumed a certain amount of the aluminum atoms. The depletion of Al reduced the precipitation of the Mg17Al12 phase. And the Al-RE phases would enhance the mechanical properties of Mg-Al-RE alloys by replacing the continuous and discontinuous Mg17Al12 precipitation. In this work, no Mg17Al12 precipitation was detected in both XRD and
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TEM observation, which indicated that the formation of Mg17Al12 was inhibited by the
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nucleation of Al-RE phases.
Fig. 6. Transmission electron micrographs of 2D-Cf/AE44 composites (a)interfacial precipitates and their SAED patterns, (b)PyC coating and its SAED pattern
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caused thermal stress. The thermal stress, as well as fabrication pressure, brought a large number of dislocations [24]. Dislocations tended to be piled up at the grain boundary and near the second phase, which lead to stress concentration [25]. For hexagonal close-packed (hcp) crystal structure, the nucleation and growth of twinning were promoted consequently. In the
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Cf/AE44 composites, twinning was easily formed due to various kinds of interface, including
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fiber/matrix, Mg/Al-RE, and grain boundary.
Fig. 7. Transmission electron micrographs of twinning in matrix alloy of 2D-Cf/AE44 composites and the SAED pattern
3.2 Mechanical properties and fracture behavior of 2D-Cf/AE44 composites The mechanical properties of AE44 alloy and its composites are shown in Table 3. The ultimate tensile strength of 2D-Cf/AE44 composites was 412MPa, which improved by 127% compared with the AE44 alloy. And Young's module improved by 74%. Typical tensile curves 10
ACCEPTED MANUSCRIPT of 2D-Cf/AE44 composites and AE44 alloy were shown in Fig. 8. There was no obvious yield point on the tensile curve of 2D-Cf/AE44 composites. The fracture surface of the composites are shown in Fig. 9. The fracture mechanism is fiber bundle pulled out, which indicated a good interface bonding.
UTS (MPa) 185
2D-Cf/AE44
412
E (GPa) 43
75
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AE44
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Table 3. Mechanical properties of 2D-Cf/AE44 composites and AE44 alloy
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Fig. 8. Tensile curves of 2D-Cf/AE44 composites and AE44 alloy
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A moderate interfacial bonding strength is essential for Cf/Mg composites. The performance of composites would be tremendously degraded if the interfacial bonding strength was too high or too low. For Cf/Mg composites, stress discrepancy between fiber and matrix was caused due to the mismatch of Young's module and thermal expansion between the fundamental constituents. For composites with weak interfacial bonding strength, single fiber pulled out from matrix is the main failure mechanism due to the interface debonding and sliding, which can reduce the stress concentration [15]. In the present work, the wettability between fiber and matrix and interface bonding were improved by alloying elements Al and RE, manifested by the formation of Al2RE and Al11RE3 compounds on fiber surface. Poor 11
ACCEPTED MANUSCRIPT infiltration and weak interfacial bonding strength, characterized by long and single fiber pulled out at fracture surface, were avoided. It was reported that in short Saffil fiber reinforced AE44 alloy, the yield strength of composites increased by 66.5% compared with the matrix alloy, which was attributed to the formation of Al2RE at the interface [19].
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On the other hand, brittle interfacial reaction products, such as Al2MgC2 and Al4C3 in carbon fiber reinforced Mg–Al matrix composites, are commonly generated in metal matrix composites [8,26]. Normally, the interfacial bonding is mechanical bonding since the wettability between carbon fiber and Mg matrix are poor. For Cf/Mg composites with carbides
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at the interface, the formation of carbides consumed the C element of carbon fibers. And their interfacial bonding become chemical bonding, which is much stronger than mechanical
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bonding. It has been generally accepted that the interfacial reaction products contained C element have adverse effects on mechanical properties of the composites, by damaging the structure of carbon fibers and causing stronger interfacial bonding [9, 27]. The fracture surface of composites with carbides at the interface usually presents brittle characteristic, a flat surface. In Cf/AE44 system, no carbide was found, according to XRD and TEM results.
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Al-RE intermetallic compounds brought no damage to carbon fibers, which mean the fibers were intact and with full carrying capability. In addition, the infiltration of matrix alloy during fabrication was improved by the formation of Al-RE phases at the fiber surface. The fracture
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surface of Cf/AE44 composites, characterized by fibers pulled out in bundle (Fig. 9),
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indicated the interfacial bonding strength was neither too low nor too high.
Fig. 9. Tensile fracture surface morphology of 2D-Cf/AE44 composites
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cracks in Fig. 10(a)). When cracks reached the surface of longitudinal fibers (parallel to tensile direction), crack would stop and deflect at interface, since the interfacial bonding strength was moderate. With the crack stopped and deflected at the interface, the longitudinal fibers began to bearing tensile load. Most of the tensile load was bearing by longitudinal
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fibers. In the final stage of tensile test, the longitudinal fibers were pulled out in bundle and
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the composites failed (Fig. 10(b)).
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Fig. 10. Failure process schematic of 2D-Cf/AE44 composites
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Recent research showed Cf/Mg composites with 3.2% Gd was provided with a moderate
interfacial bonding strength. The formation of particle shaped Mg7Gd and Gd2O3 interfacial layer enhanced the mechanical properties of Cf/Mg composites [15]. In our work, the Al-RE system optimized the interface by forming particle shaped Al2RE and lamellar shaped Al11RE3, and thus enhanced the mechanical properties of the 2D-Cf/AE44 composites. In Cf/Mg composites made of a pure Mg matrix, the fracture behavior was characterized by a large amount of single fiber pull-out, which indicated a weak interface bonding and showed a poor mechanical performance [8]. Compared with Cf/pure Mg composite, Cf/AE44 composites showed favorable interfacial characteristics by the formation of Al-RE phases at the interface. 13
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4. Conclusion In this work, the interfacial microstructure and mechanical properties of carbon fiber reinforced Mg-Al-RE matrix composites were investigated. The results are summarized as
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follows: 1) 2D-Cf/AE44 composites were well fabricated by liquid-solid extrusion following vacuum pressure infiltration. The Al and RE tend to be segregated at fiber surface. A bright interfacial layer was visible under back-scattered electron observation of SEM.
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2) Particle shaped Al2RE and lamellar shaped Al11RE3 precipitated at the interface. The Al-RE phase precipitated preferentially and inhibited the formation of Mg-Al phase.
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PyC coating was uniformly deposited on fiber surface by CVD and protected the fiber effectively.
3) The ultimate tensile strength of 2D-Cf/AE44 composites was 412 MPa, which improved by 127% compared with the AE44 alloy, while Young's module improved by 74%. The failure mechanism of the composites was fiber bundle pulled out, which
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Acknowledgements
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indicated the interface bonding was moderate under the combined effect of Al and
This work was supported by the National Nature Science Foundation of China (No. 51221001),
the
National
High-tech
R&D
Program
of
China
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51275417,
(No.2015AA8011004B).
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ACCEPTED MANUSCRIPT Highlights Interfacial interaction between carbon fiber and Mg-Al-RE alloy is proposed.
2.
Al-RE phase precipitate at the interface and inhibit the formation of Mg-Al phase.
3.
The tensile properties of the 2D-Cf/AE44 composites are studied.
4.
The failure mechanism of the composites is proposed.
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1.
S0925-8388(15)31956-3
DOI:
10.1016/j.jallcom.2015.12.165
Reference:
JALCOM 36254
To appear in:
Journal of Alloys and Compounds
Received Date: 19 November 2015 Revised Date:
16 December 2015
Accepted Date: 20 December 2015
Please cite this article as: S. Li, L. Qi, T. Zhang, J. Zhou, H. Li, Interfacial microstructure and tensile properties of carbon fiber reinforced Mg-Al-RE matrix composites, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2015.12.165. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT Interfacial microstructure and tensile properties of carbon fiber reinforced Mg-Al-RE matrix composites Shaolin Lia, Lehua Qib,∗, Ting Zhanga, Jiming Zhoub, Hejun Lia (aState Key Laboratory of Solidification Processing, Carbon/Carbon Composites Research Center, Northwestern
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Polytechnical University, Xi’an, P.R.China, 710072 b
School of Mechatronic Engineering, Northwestern Polytechnical University, Xi’an, P.R.China, 710072)
Abstract: 2D carbon fiber reinforced Mg-Al-RE matrix composites (2D-Cf/AE44 composites)
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were fabricated by liquid-solid extrusion following vacuum pressure infiltration technique (LSEVI). Pyrolytic carbon (PyC) coating was deposited on surface of T700 carbon fiber by
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chemically vapour deposited (CVD) to modify the interface. TEM observation of the composites revealed that particle shaped Al2RE and lamellar shaped Al11RE3 precipitated at the interface. The precipitation of Al-RE phase inhibited the formation of Mg-Al phase and the PyC coating protected the fiber effectively. SEM observation of fracture surface, as well as TEM analysis of the interface, were employed to explain the inherent relation between
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interface behavior and mechanical properties of 2D-Cf/AE44 composites. Keyword: Metal matrix composites; matrix alloying; interface; tensile properties
Introduction
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1.
Recently, carbon fiber reinforced Mg matrix composites (Cf/Mg composites) have been
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attracting much attention in various industrial fields, such as aerospace industries and automobile industry, for their high specific strength and modulus, low coefficient of thermal expansion (CTE), etc [1-4]. It is generally accepted that the performance of metal matrix composites (MMCs) are dominated by the interfacial status between carbon fiber and matrix. However, the wettability of the interface between carbon fiber and Mg matrix is poor, which impairs the final performance of Cf/Mg composites significantly. Many efforts have been made to improve the wettability and modify the interfacial status, such as fiber surface modification and matrix alloying. ∗
Corresponding author. Tel.:+86-29-88460447; fax:+86-29-88491982 E-mail address: [email protected] (Lehua Qi) 1
ACCEPTED MANUSCRIPT Fiber surface coating processing is an effective way to modify the interface and improve the performance of Cf/Mg composites. It was reported that the carbon fibers could modified with 5.0 mol.% yttria stabilized zirconia by sol–gel route, and an about 20 nm thick interfacial reaction layer consisting of nanocrystalline particles was found. The Cf/Mg composite
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exhibited a tensile strength of 1.08 GPa, which reached 90% of the theoretical prediction by means of the rule of mixture [5]. In Cf/Mg composites with a boron nitride (BN) layer on fiber surface, no carbides were found at the interface and a small amount of B element diffused into the Mg matrix, which indicated a good interface bonding [6]. T800H/AZ91
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composites were fabricated with a pyrolytic carbon (PyC) coating on carbon fiber. It was found that coating the fibres with PyC could strongly effect the formation of interfacial phases
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and the micromechanical response of the composites [7]. It have been demonstrated that surface coating modification could significantly improve the wettability of the interface. Except surface coating, matrix alloying is another promising method to modify the interface. Adding Al element into magnesium alloy have proved to be effective to improve the wettability between carbon and magnesium and modify the interface. It was found that the
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interface reactivity of carbon and magnesium can be controlled by Al element [8]. The continuous and discontinuous precipitated Mg17Al12 in Cf/Mg composites affected the interface state of carbon and magnesium [9]. Al element in the Mg matrix could strongly
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influence the mechanical properties of Cf/Mg composites due to the formation of carbide precipitates Al2MgC2 and Al4C3 at the interface [10]. However, Al4C3 is unfavorable for the
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dimensional stability and the mechanical properties of the composites [11, 12]. Moreover, interfacial reaction products contained C element, including Al4C3 and Al2MgC2, would bring damage to the carbon fibers, and then jeopardize the performance of the composites [13]. In order to minimize the above-mentioned disadvantages, some scholars added rare earth element into Mg matrix. It has been known that Mg alloys containing rare earth (RE) element exhibit a significant improvement in room temperature strength and fatigue resistance [14]. Recently, Y or Gd element has been added into Mg matrix to fabricate Cf/Mg composites, which proved to be effective to modify the interface and improve the mechanical properties [1, 15, 16]. However, Al element was not taken into account in these researches. The combined effect of Al and RE on interface and mechanical properties of Cf/Mg composites was 2
ACCEPTED MANUSCRIPT uncertain. The AE44 alloy, a typical Mg-Al-RE alloy, which contains 4% Al and 4% RE (mischmetal), was known for its good mechanical properties in room and high temperature [17, 18]. These features should be attributed to the high content of mischmetal which can
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stabilize the AlxREy phase, and subsequently suppress the formation of Mg17Al12 phase, a common precipitation in Mg-Al alloys. AE44 magnesium alloy based composites reinforced with Saffil fibers have been fabricated recently [19]. The rare-earth alloying elements are distributed at the interface in the form of Al2RE particle phase on the surface of fiber, and
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Al11RE3 lamellar phase in the matrix. Mechanical tensile strength of AE44 alloy was improved by incorporating Saffil fibers. However, no literature reported the interfacial
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behavior of carbon fiber reinforced Mg-Al-RE matrix composites and their mechanical properties.
In the present paper, 2D-Cf/Mg composites were fabricated by liquid-solid extrusion following vacuum pressure infiltration technique (LSEVI), a special MMCs forming technique developed by our team. AE44 were adopted as the matrix to study the combined the mechanical
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effect of Al and RE on interfacial status of Cf/Mg composites. Furthermore properties of the composites were investigated.
Experimental
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2.
2.1 Experimental material
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AE44 magnesium alloy were chosen as matrix material. Chemical component of AE44 alloy are shown in Table 1. Toray T700-12k unidirectional carbon fiber fabrics were used as reinforcement. Properties of T700 carbon fiber are shown in Table 2.
Table 1. Chemical component of AE44 alloy (mass fraction, %)
Elements
Mg
RE(La,Ce)
Al
Zn
Mn
Si
Fe
Cu
Ni
Be/ppm
AE44
Bal.
3.96
4.05
0.02
0.39
0.01
0.002
0.001
0.00006
8
3
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Elongation /%
Bending strength /MPa
Interlaminar shear strength /MPa
4.1
242
1.72
782
47.7
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Tensile strength /GPa
2.2 Fabrication process of 2D-carbon fiber preform
The unidirectional carbon fiber fabrics were stacked orthogonally layer by layer, and then
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punctured by carbon fiber string to obtain a fix preform. PyC coating was deposited on carbon fiber surface by CVD to modify the interface of the composites and protect the fiber.
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Deposition parameters are shown in Table 3.
Table 3. Deposition parameters of PyC coating on carbon fiber
Holding time /h
CH4/ m3/h
C3H8/ m3/h
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Temperature
2.0
0.45
/ 1050
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2.3 LSEVI to fabricate 2D-Cf/Mg composites
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LSEVI integrates melting, pouring, infiltration, and liquid-solid forming under high infiltration pressure. At first, the preform with PyC coating was placed in mold. Then a mechanical pump was used to vacuumize the mold chamber and keep the vacuum level
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0.01-0.05 MPa. The mold and the preform were preheated to 650 °C. At the same time, magnesium alloy was melted under the temperature of 750 °C, and holding for 1h. Then the molten alloy were pressed into the mold by gas pressure (argon, 0.1-0.5 MPa). When the molten alloy was cooled to 650 °C, hydraulic machine was drived to infiltrate the molten alloy to the preform. The pressure was about 30 MPa. 2.4 Characterizations The morphology of the 2D-Cf/Mg composites were analyzed by optical microscope and scanning electron microscope (FE-SEM, Supra-55), equipped with energy dispersive spectroscopy (EDS). The interfacial microstructure observation was implemented by Tecnai 4
ACCEPTED MANUSCRIPT G2 F30 transmission electron microscopy (TEM). The crystalline structure of the fibers was measured with X-ray diffraction (XRD, X’Pert Pro MPD). Tensile tests were conducted on CMT5304-30KN electronic universal testing machine in accordance with HB 7616-1998 standard. The rectangular specimens were 75 mm length, 10 mm width and 2 mm thickness,
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with a constant span length of 20 mms. The tests were conducted at room temperature with a strain rate of 0.5 mm/min. Fracture surface was characterized by SEM. 3.
Results and discussion
3.1 Microstructure and interface status of 2D-Cf/AE44 composites
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Fig. 1 shows typical optical microstructure of the as-squeezed AE44 alloy, and its composites reinforced with carbon fibers (Fig. 2). The lamellar phases (marked in Fig. 1) are
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distributed in grain boundary region in the unreinforced alloy, as shown in Fig. 1. However,
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the second phases in the composites are much fewer, according to Fig. 2.
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Fig. 1. Optical micrograph of as-squeezed AE44 alloy
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Fig. 2. Optical micrograph of 2D-Cf/AE44 composites
Back-scattered electron (BSE) detector was used to explore the infiltration microstructure and element distribution of the composites, and the z-contrast image was shown in Fig. 3. The composites exhibited good fiber dispersion without apparent porosity or significant casting defects. Small interstices between single fibers were filled with alloy, which indicate a good
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infiltration. The interface areas between the Cf and the Mg matrix were very good without discernible debonding or micro-crack. Moreover, bright interfacial zones were observed between carbon fiber and magnesium alloys, as shown in Fig. 4(a).
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To explore the composition of the bright zones, EDS mapping was employed, as shown in Fig. 4. It was demonstrated that the bright interfacial layer was composed of Al and RE(Ce,
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La), which indicated that the Al and RE were prone to segregate around the carbon fiber surface. For 2D-Cf/AE44 composites, a large number of interface were induced by carbon fibers. During fabrication, the fiber surface attracted the alloying element in the matrix. The consumption of the alloying element suppressed the formation of the eutectic phases. That is why the eutectic phases in the composite are much fewer in the alloy (Fig. 1, Fig. 2).
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Fig. 3. SEM micrograph of 2D-Cf/AE44 composites (BSE image)
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Fig. 4. Microstructure of 2D-Cf/AE44 composites(a) and element distribution, Mg
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mapping(b), Al mapping(c), La mapping(d) , Ce mapping(e)
XRD analysis was conducted to identify the crystallographic structure of the phases in the
composite, as shown in Fig. 5. The result revealed that the composites were composed of magnesium, Al2RE and amorphous carbon. This indicated the bright interfacial layer might be Al2RE phase, a common phase in AE44 alloy.
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Fig. 5. XRD pattern of 2D-Cf/AE44 composites
For a further understanding on the interface, and confirming the interface phase, TEM
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observation was conducted, as shown in Fig. 6. An obviously interfacial layer with fine nano-scale particles was observed. The interfacial layer was distributed on fiber surface uniformly in a thickness of about 100 nm. The interface mainly consisted of blocky particle phase (A in Fig. 6(a)). The SAED pattern showed the particle phase were Al2RE (face-centred cubic crystal structure), which was consistent with XRD result. In addition, a few amount of
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lamellar phase were also found at the interface (B in Fig. 6(a)). The SAED pattern showed the lamellar phase were Al11RE3 (orthorhombic crystal structure). The amount of Al11RE3 phase was too small to be detected by XRD.
A uniform PyC coating in a thickness of about 150 nm was wrapped on fiber surface, as
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shown in Fig. 6(b). No diffusion between matrix and fiber was observed. And the coating was intact without any structural damage, which indicated that the PyC coating served as a barrier
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between matrix and fiber, and protected the fiber effectively. During fabrication, the molten alloy infiltrated into the fiber bundle and then solidified.
Due to the poor wettability and incoherent crystal structure between Mg and C, the surface of carbon fiber with PyC coating failed to serve as nucleation base for Mg. Moreover, due to the relatively high preheating temperature of the preform (650°C), the solidification started at locations away from fiber (inter-fiber region), and finished at fiber surface. And the enrichment of alloying elements Al and RE occurred at fiber surface during the final stage of solidification. This was demonstrated by the observation of blocky Al2RE and lamella Al11RE3 at the interface. The Al2RE and Al11RE3 compounds are both typical equilibrium phase in 8
ACCEPTED MANUSCRIPT Mg-Al-RE alloys [20]. The reaction enthalpies of Al2La and Al11 La3 are listed below [21]: ∆H = -50.5 ± 2 kJ/mol
1/3{La(α)+2Al(s)→LaAl2} 1/14{3La(α)+11Al(s)→La3Al11}
∆H = -41.0 ± 2 kJ/mol
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Al2RE, characterized by high-melting-point(1400-1500°C), has been reported as the most stable compound in Al-RE system [22]. According to the phase diagram and enthalpy of formation, Al2RE is with the minimum ∆H, which was lower than that of Al11RE3
However, the reaction enthalpies of Mg17Al12, a common eutectic phase in Mg-Al alloys,
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is much higher than those of Al2La and Al11 La3 [23]: L ↔ (Al) + β
∆H = -8.1 kJ/mol
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During solidification, the RE elements preferentially reacted with Al to form Al-RE phases, which consumed a certain amount of the aluminum atoms. The depletion of Al reduced the precipitation of the Mg17Al12 phase. And the Al-RE phases would enhance the mechanical properties of Mg-Al-RE alloys by replacing the continuous and discontinuous Mg17Al12 precipitation. In this work, no Mg17Al12 precipitation was detected in both XRD and
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TEM observation, which indicated that the formation of Mg17Al12 was inhibited by the
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nucleation of Al-RE phases.
Fig. 6. Transmission electron micrographs of 2D-Cf/AE44 composites (a)interfacial precipitates and their SAED patterns, (b)PyC coating and its SAED pattern
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caused thermal stress. The thermal stress, as well as fabrication pressure, brought a large number of dislocations [24]. Dislocations tended to be piled up at the grain boundary and near the second phase, which lead to stress concentration [25]. For hexagonal close-packed (hcp) crystal structure, the nucleation and growth of twinning were promoted consequently. In the
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Cf/AE44 composites, twinning was easily formed due to various kinds of interface, including
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fiber/matrix, Mg/Al-RE, and grain boundary.
Fig. 7. Transmission electron micrographs of twinning in matrix alloy of 2D-Cf/AE44 composites and the SAED pattern
3.2 Mechanical properties and fracture behavior of 2D-Cf/AE44 composites The mechanical properties of AE44 alloy and its composites are shown in Table 3. The ultimate tensile strength of 2D-Cf/AE44 composites was 412MPa, which improved by 127% compared with the AE44 alloy. And Young's module improved by 74%. Typical tensile curves 10
ACCEPTED MANUSCRIPT of 2D-Cf/AE44 composites and AE44 alloy were shown in Fig. 8. There was no obvious yield point on the tensile curve of 2D-Cf/AE44 composites. The fracture surface of the composites are shown in Fig. 9. The fracture mechanism is fiber bundle pulled out, which indicated a good interface bonding.
UTS (MPa) 185
2D-Cf/AE44
412
E (GPa) 43
75
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AE44
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Table 3. Mechanical properties of 2D-Cf/AE44 composites and AE44 alloy
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Fig. 8. Tensile curves of 2D-Cf/AE44 composites and AE44 alloy
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A moderate interfacial bonding strength is essential for Cf/Mg composites. The performance of composites would be tremendously degraded if the interfacial bonding strength was too high or too low. For Cf/Mg composites, stress discrepancy between fiber and matrix was caused due to the mismatch of Young's module and thermal expansion between the fundamental constituents. For composites with weak interfacial bonding strength, single fiber pulled out from matrix is the main failure mechanism due to the interface debonding and sliding, which can reduce the stress concentration [15]. In the present work, the wettability between fiber and matrix and interface bonding were improved by alloying elements Al and RE, manifested by the formation of Al2RE and Al11RE3 compounds on fiber surface. Poor 11
ACCEPTED MANUSCRIPT infiltration and weak interfacial bonding strength, characterized by long and single fiber pulled out at fracture surface, were avoided. It was reported that in short Saffil fiber reinforced AE44 alloy, the yield strength of composites increased by 66.5% compared with the matrix alloy, which was attributed to the formation of Al2RE at the interface [19].
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On the other hand, brittle interfacial reaction products, such as Al2MgC2 and Al4C3 in carbon fiber reinforced Mg–Al matrix composites, are commonly generated in metal matrix composites [8,26]. Normally, the interfacial bonding is mechanical bonding since the wettability between carbon fiber and Mg matrix are poor. For Cf/Mg composites with carbides
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at the interface, the formation of carbides consumed the C element of carbon fibers. And their interfacial bonding become chemical bonding, which is much stronger than mechanical
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bonding. It has been generally accepted that the interfacial reaction products contained C element have adverse effects on mechanical properties of the composites, by damaging the structure of carbon fibers and causing stronger interfacial bonding [9, 27]. The fracture surface of composites with carbides at the interface usually presents brittle characteristic, a flat surface. In Cf/AE44 system, no carbide was found, according to XRD and TEM results.
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Al-RE intermetallic compounds brought no damage to carbon fibers, which mean the fibers were intact and with full carrying capability. In addition, the infiltration of matrix alloy during fabrication was improved by the formation of Al-RE phases at the fiber surface. The fracture
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surface of Cf/AE44 composites, characterized by fibers pulled out in bundle (Fig. 9),
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indicated the interfacial bonding strength was neither too low nor too high.
Fig. 9. Tensile fracture surface morphology of 2D-Cf/AE44 composites
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cracks in Fig. 10(a)). When cracks reached the surface of longitudinal fibers (parallel to tensile direction), crack would stop and deflect at interface, since the interfacial bonding strength was moderate. With the crack stopped and deflected at the interface, the longitudinal fibers began to bearing tensile load. Most of the tensile load was bearing by longitudinal
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fibers. In the final stage of tensile test, the longitudinal fibers were pulled out in bundle and
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the composites failed (Fig. 10(b)).
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Fig. 10. Failure process schematic of 2D-Cf/AE44 composites
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Recent research showed Cf/Mg composites with 3.2% Gd was provided with a moderate
interfacial bonding strength. The formation of particle shaped Mg7Gd and Gd2O3 interfacial layer enhanced the mechanical properties of Cf/Mg composites [15]. In our work, the Al-RE system optimized the interface by forming particle shaped Al2RE and lamellar shaped Al11RE3, and thus enhanced the mechanical properties of the 2D-Cf/AE44 composites. In Cf/Mg composites made of a pure Mg matrix, the fracture behavior was characterized by a large amount of single fiber pull-out, which indicated a weak interface bonding and showed a poor mechanical performance [8]. Compared with Cf/pure Mg composite, Cf/AE44 composites showed favorable interfacial characteristics by the formation of Al-RE phases at the interface. 13
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4. Conclusion In this work, the interfacial microstructure and mechanical properties of carbon fiber reinforced Mg-Al-RE matrix composites were investigated. The results are summarized as
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follows: 1) 2D-Cf/AE44 composites were well fabricated by liquid-solid extrusion following vacuum pressure infiltration. The Al and RE tend to be segregated at fiber surface. A bright interfacial layer was visible under back-scattered electron observation of SEM.
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2) Particle shaped Al2RE and lamellar shaped Al11RE3 precipitated at the interface. The Al-RE phase precipitated preferentially and inhibited the formation of Mg-Al phase.
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PyC coating was uniformly deposited on fiber surface by CVD and protected the fiber effectively.
3) The ultimate tensile strength of 2D-Cf/AE44 composites was 412 MPa, which improved by 127% compared with the AE44 alloy, while Young's module improved by 74%. The failure mechanism of the composites was fiber bundle pulled out, which
RE.
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Acknowledgements
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indicated the interface bonding was moderate under the combined effect of Al and
This work was supported by the National Nature Science Foundation of China (No. 51221001),
the
National
High-tech
R&D
Program
of
China
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51275417,
(No.2015AA8011004B).
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ACCEPTED MANUSCRIPT Highlights Interfacial interaction between carbon fiber and Mg-Al-RE alloy is proposed.
2.
Al-RE phase precipitate at the interface and inhibit the formation of Mg-Al phase.
3.
The tensile properties of the 2D-Cf/AE44 composites are studied.
4.
The failure mechanism of the composites is proposed.
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1.