IOP Conference Series: Materials Science and Engineering Studies of the AZ91 magnesium alloy / SiO 2 - coated carbon fibres composite microstructure To cite this article: A Olszówka-Myalska and A Botor-Probierz 2010 IOP Conf. Ser.: Mater. Sci. Eng. 7 012022 View the article online for updates and enhancements. Related content - Effect of TiN nano-coating on the interface microstructure of carbon fibres-az91 alloy composite A Olszówka-Myalska and A Botor-Probierz - Application of EDS microanalysis in the identification of inhomogeneities in surface protective layers on ductile cast iron parts Boro and A Tchórz - Electron microscopy and microanalysis of steel weld joints after long time exposures at high temperatures D Jandová, J Kasl and A Rek Recent citations - Effect of TiN nano-coating on the interface microstructure of carbon fibres-az91 alloy composite A Olszówka-Myalska and A Botor-Probierz This content was downloaded from IP address 37.44.204.200 on 06/12/2017 at 02:40
Studies of the AZ91 magnesium alloy / SiO 2 -coated carbon fibres composite microstructure A Olszówka-Myalska 1, and A Botor-Probierz Silesian University of Technology, Faculty of Materials Science and Metallurgy, ul. Krasińskiego 8b, PL-40019 Katowice, Poland E-mail: anita.olszowka-myalska@polsl.pl Abstract. The microstructure of magnesium matrix composite reinforced with SiO 2 nano-layer coated carbon fibres, deposited by sol-gel method was characterized. The composite was obtained by infiltration method and the effect of SiO 2 on the composite microstructure was analyzed by scanning electron microscopy combined with energydispersive X-ray spectroscopy (SEM+EDS) and transmission electron microscopy combined with energy-dispersive X-ray spectroscopy (TEM+EDS) methods. Good wettability of fibres by the magnesium alloy AZ91 ( 9 wt%, Zn 0.3 wt%) was confirmed since fibres were closely surrounded with alloy and pulling-out effect was not visible. The interface region was evidently with aluminium enriched. Near carbon fibre surface a regular layer of SiO X oxide enriched with was detected by high angle annular dark field image (HAADF) combined with energy-dispersive X-ray spectroscopy (EDS). The plate or needle shaped very fine particles of 12 Mg 17 were identified near the AZ91 matrix zone by bright field (BF) and selected area electron diffraction (SADP). 1. Introduction Magnesium alloys containing aluminium are a very popular group of commercial alloys because of their good technological properties and relatively low price. The literature data on their application as a matrix of composites reinforced with carbon materials unfortunately showed that the composite properties are lower than one may expect [1-3]. The reason of the insufficient properties is the fibre-alloy interface microstructure because the presence of aluminium in matrix alloy results in the 4 C 3 aluminium carbide formation. That phase is not only brittle but also hydrophilic [4-6]. The problem of limitation of aluminium carbide formation was analyzed in the literature mainly for aluminium matrix composite both reinforced with carbon fibres and silicon carbide particles [7, 8]. Generally, either the surface of reinforcement should be modified with a very thin layer or chemical composition of the matrix should be modified. However, each technological solution generates the microstructure adequate to the temperature, time and pressure conditions used during the manufacturing. For fibres reinforced composites the temperature of consolidation process is relatively high because the liquid phase processes are necessary for obtaining the respective microstructure and properties. Additionally the reactivity of carbon fibres depends on their origin. Therefore, the 1 To whom any correspondence should be addressed. c 2010 IOP Publishing Ltd 1
microstructure examination for each matrix-reinforcement system and for various parameters of their consolidation is indispensable. In our earlier studies on the magnesium alloy AZ91 (%, % Zn) reinforced with the carbon fibres TB 300B composite without any fibre surface modification it was revealed that the hydrophilic phase is formed at the interface and composite destruction in air atmosphere occurs [9, 10]. The prevention of aluminium carbide formation at the carbon fibre-az91 magnesium alloy interface by the TiN- or TiC-nano-coatings deposited on fibres surface before composite processing has been proposed [1, 11]. The aim of the presented work was to verify the structural effects of application of a new technological concept of AZ91 magnesium matrix composite reinforced with the carbon fibres processing. Fibre surface modification by SiO 2 nano-layer, obtained by sol-gel method, was proposed. In the current experiment the same conditions of components consolidation were used as in our earlier experiments for fibres without surface modification and matrices of pure magnesium and AZ91 magnesium alloy presented in [9, 10]. Preservation of those conditions can help to analyze the effect of carbon surface modification with SiO 2 on the composite microstructure formation. 2. Experiment 2.1. Composite processing 2.1.1. Reinforcement preparation. The sol-gel method was used for the surface modification of carbon fibres (PAN-type FT300B with circular cross-section). As the preform the 3-D granules from cut carbon fibres were used. The thin layer of SiO 2 on the fibres was formed during immersion of carbon material in metallo-organic solution. Tetramethoxysilane (TMOS) and phenylotrietoxysilane (PhTEOS) as siliceous precursors were used to the synthesis [12]. Nano-layer of amorphous SiO 2 with thickness of approximately 100 nm, uniformly surrounding fibre core and good bonded with carbon fibre surface was obtained (figures 1 and 2). Figure 1. Silicon oxide coating on T300B carbon fibre produced by sol-gel method: single fibre with uniform SiO 2 coating. Figure 2. Silicon oxide coating produced by sol-gel method on T300B carbon fibre: interface of fibre-sio 2 coating. 2
2.1.2. Components consolidation. Carbon fibres coated with SiO 2 were applied as a reinforcement of magnesium alloy AZ91 (7 wt%, 0.3 wt% Zn). The samples were prepared from components configured in the metal/fibres preform/metal system by heating in vacuum at a temperature of 670 C, time of 20 min, then pressed under the pressure of 15 MPa in the time of 5 min and cooled passively. The uniform samples with a diameter of 20 mm were obtained. 2.2. Examination of composite microstructure 2.2.1. Methods. The microstructure was preliminarily investigated using a scanning electron microscope (Hitachi FE-SEM 4200S) equipped with an energy-dispersive X-ray spectrometer (EDS) (Noran 3500). The samples for SEM observations were prepared in the form of non-etched metallographic micro-sections and fractured cross-sections. More detailed structural investigations concerning fibre-matrix interface were performed in a FEI Tecnai G2 FEG high resolution electron microscope equipped with EDS and HAADF detectors. Thin foils of composite were prepared by dimpling and ion milling using Gatan equipment (DuoMill 600 with the following milling conditions: gun voltage 5 kv, specimen current 40 µa, gun angle 15, room temperature). 2.2.2. Cross-sections examination. The distribution of carbon fibres in AZ91 matrix was analyzed on polished and fractured cross-sections (figures 3 and 4). The fibres were uniformly distributed in the matrix and the reinforcement was closely surrounded with a metal. Pores in the matrix and at the fibre-matrix interface were not detected by the SEM method. The de-cohesion through the carbon fibres was typical for the investigated material independently of the fibres orientation in the matrix and the effect of carbon fibres pulling-out was not observed (figure 4). The SEM micrographs taken from both polished and fractured cross-section demonstrate the zone of very thin grains between fibres and matrix (figures 3 and 4). Figure 3. Microstructure of (Cf) SiO2 - AZ91 composite, polished cross- section. For the comparison the SEM images of the Cf-AZ91 composite without fibres surface modification were presented in figure 5. The destruction of interface as a result of 4 C 3 reaction with water is evident in that material. 3
Figure 4. Microstructure of (Cf) SiO2 - AZ91 composite, fractured cross-section. Figure 5. Microstructure of Cf - AZ91 composite without fibre surface modification, polished cross-section. On the basis of X-ray microanalysis results (figures 6 and 7) it was found that in (Cf) SiO2 - AZ91 composite the zone around carbon fibres was strongly enriched with aluminium and oxygen in comparison to the matrix chemical composition. Any places with evident silicon higher concentration were not detected. 2.2.3. Interface examination. The observation of fibre-matrix interface by TEM-BF technique (figure 8) revealed the presence of irregular zone between fibre and matrix with thickness up to 800 nm. It consists of regular oxide layer (20-200 nm) around the fibre and a region of irregular sharp shaped crystals directed to the magnesium alloy matrix (figures 9 and 10). In the oxide layer presence of silicon and aluminium was detected at the X-ray scans and maps. The irregular 4
Figure 6. Interface of (Cf) SiO2 - AZ91: a) SEI; X-ray mapping of Mg (b), (c), Si (d), O (e). O Si Mg C O Figure 7. Interface of (Cf) SiO2 - AZ91 and X-ray line scans (SEM+EDS). plate-shaped crystals with thickness of approx. 30 nm were identified as cubic body-centred 12 Mg 17 by the selected area diffraction pattern (SADP) (figure 11). No carbides were detected at the interfaces. 5
Figure 8. Interface of (Cf) SiO2 - AZ91, TEM BF. C K O K Mg K K Si K Zn K Figure 9. Interface of (Cf) SiO2 - AZ91 HAADF (z-contrast and X-ray maps). 6
HAADF 300 O-K 2000 Mg-K matrix C f 250 200 150 100 1500 1000 500 50 0.00 0.05 0.10 0.15 Position (um) 0.20 0.00 0.05 0.10 0.15 Position (um) 0.20 600 -K 80 Si-K 1000 800 C-K 400 60 40 600 400 200 20 200 0.00 0.05 0.10 0.15 Position (um) 0.20 0.00 0.05 0.10 0.15 Position (um) 0.20 0 0.00 0.05 0.10 0.15 Position (um) 0.20 Figure 10. Interface of (Cf) SiO2 - AZ91 and X-ray line scans (HAADF+EDS). 3. Discussion Results of composite cross-sections investigation by SEM suggest that the parameters of components consolidation used in the process ensured good infiltration of carbon fibres with SiO 2 nano-layer by liquid magnesium matrix AZ91 and that the bonding between carbon and matrix is strong (figures 3 and 4). The effects of interface microstructure degradation visible for composite without SiO 2 layer (figure 5) were not observed despite of existence of zone with a microstructure different from that of matrix (figure 3). While the conditions of proper infiltration of fibres modified by SiO 2 were ensured the question is the effect of SiO 2 nano-layer on the final interface microstructure formation. On the polished cross-section not even a local growth of Si concentration was found by SEM either on X-ray maps or X-ray line scans (figure 6 and 7). The probable reason is the very low resolution. The second probable reason is reaction of SiO with aluminium and silicon diffusion into the matrix [13]. 2 More accurate data about interface microstructure was obtained by the TEM. The results of investigations confirmed both the presence of a zone with microstructure different than that of the matrix (figure 8) and the increase of aluminium content at the interface (figure 9 and 10). That zone was built of oxide layer near fibres surface and a region of nano-sized plates of 12 Mg 17 -phase. This observation explains the growth of aluminium concentration noticed by SEM-EDS. It also suggests that the SiO 2 coating interacted with aluminium during components consolidation and additionally it was a substrate for nucleation and growth of intermetallic phase on the fibres surface. The presence of 4 C 3 -carbide at the C- and C-(Mg+) systems interface well known from literature was not revealed in examined material. For the some orientations, the plates of 12 Mg 17 -phase can look however like needle-shaped C crystals. 4 3 The microstructure of interface enriched with aluminium observed in examined composite does not exclude the carbide formation during composite processing or operation at high temperature. That problem should be analyzed in further studies. 7
HAADF BF 12 Mg 17 [111] 12 Mg 17 [111] 1 2-101 011 SADP SADP 400 nm 1 μm 4000 3000 Mg Mg EDX HAADF Detector Area 1 1 2000 1000 0 0.0 C O Zn Zn Zn Zn Zn 1.0 SiSi 2.0 Energy (kev) 3.0 4.0 5.0 Element Weight % Atomic % Mg(K) 60.4 64.2 (K) 35.8 34.3 Zn(K) 3.8 1.5 4000 Mg EDX HAADF Detector Area 2 Mg 2 3000 2000 1000 0 0.0 C O Cu Cu Cu Cu 1.0 SiSi 2.0 Energy (kev) 3.0 4.0 5.0 Element Weight % Atomic % Mg(K) 93.0 94.0 (K) 6.4 5.8 Zn(K) 0.6 0.2 Figure 11. Interface of (Cf) SiO2 - AZ91 HAADF and BF with SADP of 12 Mg 17 -phase and results of microanalysis. 4. Summary In summary the modification of carbon fibres surface with SiO 2 nano-layer caused: - good infiltration of fibre reinforcement by AZ91 magnesium alloy, - de-cohesion of composite both through the matrix and fibres without their pulling-out, - interface microstructure of oxide-type near fibres surface and nano-sized grains of 12 Mg 17 -phase near AZ91 matrix. 8
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