Composites Strengthened with Graphene Platelets and Formed in Semisolid State Based on α and α/β MgLiAl Alloys
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MgLiAl base composites strengthened with graphene platelets were prepared by semisolid processing of ball-milled alloy chips with 2% of graphene platelets. Composites strengthened with graphene platelets show higher hardness and yield stress than the cast alloys, i.e., 160 MPa as compared to 90 MPa for as-cast alloy MgLi9Al1.5. Mechanical properties for MgLiAl-based composites were similar or higher than for composites based on conventional AZ91 or WE43 alloys. The strengthening however was not only due to the presence of graphene, but also phases resulting from the reaction between carbon and lithium, i.e., Li2C2 carbide. Graphene platelets were located at globules boundaries resulting from semisolid processing for all investigated composites. Graphene platelets were in agglomerates forming continuous layers at grain boundaries in the composite based on the alloy MgLi4.5Al1.5. The shape of agglomerates was more complex and wavy in the composite based on MgLi9Al1.5 alloy most probably due to lithium–graphene reaction. Electron diffraction from the two-phase region α + β in MgLi9Al1.5 indicated that α and β directions are rotated about 4° from the ideal relationship  hex ||  bcc phases. It showed higher lattice rotation than in earlier studies what is most probably caused by lattice slip and rotation during semisolid pressing causing substantial deformation particularly within the β phase. Raman spectroscopy studies confirmed the presence of graphene platelets within agglomerates and in addition the presence mainly of Li2C2 carbides in composites based on MgLi4.5Al1.5 and Mg9Li1.5Al alloys. From the character of Raman spectra refinement of graphene platelets was found in comparison with their initial size. The graphene areas without carbides contain graphene nanoplatelets with lateral dimension close to initial graphene sample. Electron diffraction allowed to confirm the presence of Li2C2 carbide at the surface of agglomerates found from Raman spectroscopy results.
KeywordsGNP-strengthened Mg alloy composite metallic matrix composites MgLiAl alloys electron microscopy semisolid processing
Composites based on light alloys strengthened with graphene nanoplatelets (GNPs) or carbon nanotubes (CNTs) have been recently investigated in several research articles (Ref 1-10). The graphene and graphene oxide show promise as reinforcements in high-performance nanocomposites. They have a high level of stiffness and strength, and this means that the nanocomposites ought to have outstanding mechanical properties. There are problems, however, in obtaining a good dispersion, and there are challenges in obtaining the full exfoliation of graphene into single- or few-layer material with reasonable lateral dimensions (Ref 1). To improve dispersion of graphene and obtain enhanced mechanical properties of Al matrix graphene-reinforced composites, the magnesium ions were utilized as an “anchor” that made graphene nanosheets attracted to the surface of Al powder (Ref 2). About 20% increase in yield strength with preserved good plasticity was obtained in the AZ31 magnesium alloy strengthened with 1.5-3% of graphene nanoplatelets (GNPs) fabricated by stir-casting method (Ref 3). It was found that, like CNTs, GNPs also have the potential to sustain tensile strength at high temperatures. In another work (Ref 4) graphene nanoplatelets (few-layer graphene) and carbon nanotubes were used as reinforcement fillers to enhance the mechanical properties of AZ31 magnesium alloy through high-energy ball milling, sintering and hot extrusion techniques. The tensile strength of AZ31 magnesium alloy with the graphene nanoplatelet addition decreased by about 11%, while the carbon nanotubes addition brought about 8% increase in the composite tensile strength. The GNPs addition to the AZ61 alloy had also a significant effect on the grain size refinement and change in basal texture due to their uniform distribution throughout the composite matrix, which resulted in considerable improvement of room temperature micro-hardness, tensile and compression strengths (Ref 5). Due to substantial effect of uniform graphene distribution on mechanical properties, novel nanoprocessing methods that combined liquid-state ultrasonic processing and solid-state stirring were applied for the fabrication of bulk metal–graphene nanoplatelet nanocomposites (Ref 6). The obtained Mg-based metal matrix nanocomposite reinforced with graphene nanoplatelets showed a uniform dispersion of graphene nanoplatelets and enhanced properties by 80% as compared with the basic alloy. The good bonding between the graphene nanoplatelets and the Mg matrix facilitates the stress transfer from the Mg matrix to the high-strength graphene nanoplatelets (Ref 7). The studies of GNPs-strengthened composites based on Mg6Zn alloy (Ref 8) showed an increased strength of synthesized composites that could be attributed to the grain refinement, uniform dispersion of GNPs, changes in basal textures and basic strengthening mechanisms. Moreover, the comparison of synthesized composites with Mg6Zn–CNTs composites revealed that GNPs have high potential to replace CNTs because GNPs are 4-6 times cheaper than CNTs (Ref 8). The increase in Al content in the magnesium matrix strengthened with 0.18% GNPs led to increase in YS, UTS and failure strain. The best improvement was achieved at 1 wt.% Al (Mg–1.0Al–0.18GNPs). The mechanical strength of synthesized composites proved to be better than Mg-Al-CNTs and Mg–ceramic composites (Ref 9) which disagrees with the statements of earlier cited works (Ref 4, 5). Nevertheless, in all papers it was reported that the addition of GNPs or CNTs increases the strength and often ductility of composites. Usually MgZnAl alloys were subjected to strengthening using GNPs and CNTs (Ref 3-10) except for the amorphous MgNiLa one (Ref 11), in which the electrochemical characteristics of amorphous Mg65Ni27La8 electrode were improved by the addition of graphene modified with silver nanoparticles. The interfacial reactions were observed in composites based on AZ61 alloy strengthened with carbon nanotubes (Ref 12). The carbide phase Al2MgC2 was identified at the interface resulting from such reactions. The composite was effectively strengthened by the production of Al2MgC2 compounds at the interface between magnesium matrix and carbon nanotubes.
In the present paper the composites based on MgLiAl alloys consisting either of hexagonal α phase or of hexagonal α + β bcc phase were investigated. Two-phase alloy is of particular interest since it shows very good plasticity attaining superplastic deformation at slightly elevated temperatures (Ref 13, 14). It would be therefore of interest to improve its strength by the addition of GNPs while preserving its good ductility. New procedure, i.e., formation of composite using semisolid processing of milled powders was used to obtain a good dispersion of GNPs in MgLiAl alloys. The presence of lithium in liquid phase in semisolid state of MgLiAl alloys during final formation of composite can favor the reactive wetting of GNPs. Hence, detailed analysis of GNPs, carbides and other compounds at interface will be performed with particular attention to GNP/matrix alloy, where formation of carbides can be expected as reported in (Ref 12).
The Raman spectroscopy is one of the most sensitive techniques to characterize sp2-hybridized carbon materials (graphite, graphene, carbon nanotubes (CNTs), etc.) by the analysis of G and 2D band parameters. The characterization of disorder is also possible by the analysis of the D band which is caused by the structure disorder of sp2-hybridized carbon materials. Hence, Raman spectroscopy can be also very helpful in the analysis of composites containing carbon materials. The micro-Raman spectra were obtained using a Renishaw InVia Raman spectrometer equipped with a Leica optical microscope. The Raman spectra were excited by the 633-nm NeHe laser (1 s exposure time, 14 mW output power) in backscattering geometry using Leica objective 100x (lateral resolution < 1 μm, depth resolution < 1.8 μm). Spectral maps of composites were acquired in point-by-point 2D mode within the scan area of about 20 μm2 (XY) at 1 μm step size using Renishaw Wire ver. 3.4 software (about 400 spectra).
The spectral maps were collected in three ranges of Raman shifts (fast mode at fixed position of grating for limited wavenumber range): approx. (1) 50-1000 cm−1, (2) 1000-2000 cm−1, (3) 2600-3250 cm−1. The peak parameters: position, i.e., Raman shift (wavenumber), integral intensity and full width at half maximum (FWHM), were determined by fitting each fragment of spectrum after background correction with pseudo-Voigt profile (linear combination of the Gaussian and Lorentzian curves) using Renishaw Wire ver. 3.4 software. The sets of peak parameters were used to create image maps and histograms of the distribution of peak parameters which can characterize heterogeneity of graphene and the remaining compounds in micro-areas (crystallite size, average distance between defects, etc.). The La dimension of graphene nanoplatelets (i.e., average lateral dimension of nanoplatelets) was estimated from the average FWHM values of graphene G band on the basis of Eq 20 presented by Mallet-Ladeira et al. (Ref 16).
Results and Discussion
Results of hardness measurement and compression test experiment of as-cast alloys and composites strengthened with graphene platelets
Compressive yield strength, MPa
59 ± 1
58 ± 2
61 ± 1
Mg9Li1.5Al + 2%Gr
85 ± 1
The range of bands for metallic magnesium phase [E2g band for hcp Mg (Ref 19)], salt-like carbides, which contain anion C4− (methanides) aluminum and aluminum-magnesium carbides (Al4C3, Al2MgC2) or anion C34− (allylenides) lithium and magnesium carbides (Li4C3, Mg2C3) (Ref 18) and metal oxides (approx. 50-1000 cm−1),
The range of graphene bands D, D’ and G, amorphous carbon band (Ref 20), main bands of salt-like carbides, which contain anion C22− (acetylides with triple C≡C bonds: Li2C2, MgC2 or ternary (Li,Mg)C2) and allylenides (double C=C bonds of allylenides) (Ref 21) and also main band of carbonates (Li2CO3, MgCO3) (approx. 1000-2000 cm−1),
The range of the 2D band of graphene (Ref 19) (2600-3250 cm−1).
Composites based on MgLiAl alloys strengthened with graphene platelets were prepared from ball-milled alloy chips and graphene platelets using pressure casting from semisolid range at about 30% of liquid phase. Composites strengthened with 2% of graphene platelets showed higher hardness and compression yield stress of 160 MPa compared with 90 MPa for Mg9Li1.5Al as-cast alloy. Mechanical properties for MgLiAl-based composites were similar to those of conventional magnesium alloys. The increase in strength of MgLiAl/GNP composites was limited due to the formation of a Li2C2 brittle layer at graphene platelets interfaces.
Graphene platelets were located at boundaries of globules resulting from semisolid processing of all investigated composites. Graphene platelets appeared in agglomerates forming continuous layers at grain boundaries in the composite based on Mg4.5Li1.5Al alloy. Shape of agglomerates was more round in the composite based on Mg9Li1.5Al alloy. Electron diffraction from two-phase region in Mg9Li1.5Al indicated that α and β directions rotated about 4° from the ideal relationship  hex ||  bcc phases. It confirmed the earlier study of (Ref 17), and slightly larger rotation might be caused by the increased transformation stresses due to the addition of aluminum.
Raman spectroscopy studies confirmed the presence of graphene platelets within agglomerates and the presence of Li2C2 carbides in composites based on Mg4.5Li1.5Al and Mg9Li1.5Al alloys. Refinement of graphene platelets in graphene-carbides areas was observed from the character of Raman spectra in comparison with the initial graphene platelets. Graphene areas without carbides contained graphene nanoplatelets with average lateral dimension (La) close to initial graphene sample. Electron diffraction confirmed the presence of Li2C2 carbide at surface of agglomerates in accordance with Raman spectroscopy results. Composites with lower lithium content (based on Mg4.5Li1.5Al) also contained Li2C2 carbide but embedded in alloy particles at the grain boundary.
Based on results for MgLiAl-based composites strengthened with graphene platelets prepared by semisolid casting, a moderate increase in strength was attained accompanied by improved plasticity for the two-phase α + β alloys. Strengthening however was not only due to the presence of graphene, but also to the occurrence of phases originated from the reaction between carbon and lithium in the form of Li2C2 carbide.
The financial support of the National Science Center (NCN) under Project Number 2014/15/B/ST8/03184 and the Polish National Centre for Research and Development, Grant No.: LIDER/007/151/L-5/13/NCBR/2014, is gratefully acknowledged.
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