Control of Al 4 C 3 phase formation in aluminum matrix composites reinforced with carbon nanostructures

. This brief overview provides a systematic organization of the known thermodynamic data to justify approaches for inhibiting the in-situ formation of Al 4 C 3 reaction between carbon nanotubes and aluminum matrix in composite materials. Based on the literature data, the Gibbs free energy value at a temperature of 600 ºC is calculated for aluminum and carbon interactions with various substances. Approaches for inhibiting the interfacial reaction and the formation of the Al 4 C 3 phase in aluminum matrix composites are proposed by controlling their composition. These approaches involve alloying the matrix with different elements and ex-situ modification of carbon nanotubes through the creation of coatings with varying compositions on their surface prior to their incorporation into the matrix. Literature data on the effect of the interfacial layer on the properties of Al/CNT composites are presented. The promising outlook of the interface design strategy by controlling the type and thickness of the interphase layer for the engineering of composites with improved properties is shown. This approach can be useful in the development of aluminum matrix composite materials with balanced properties for a wide range of applications.


Introduction
The production of aluminum matrix composites is an effective method for obtaining structural materials that offer the possibility to control the properties by combining aluminum matrix alloys with different types, shapes, and sizes of reinforcing particles.Reinforcing aluminum with carbon nanotubes (CNTs) holds great potential due to its ability to enhance the material's physical and mechanical properties without increasing its density.During composite deformation, load redistribution between the matrix and nanoparticles is achieved under the condition of good matrix-reinforcement adhesion.However, this condition is not met for aluminum and carbon nanoparticles (fullerenes, nanotubes, graphene, and graphite) [1]- [3].In practice, aluminum particles are almost always covered with a thin layer of native oxide, which hinders achieving maximum adhesion [4].It can be removed through mechanical processing in an inert environment.The surface of real carbon nanotubes after synthesis and mechanical treatment often possesses regions with high chemical activity, such as radially opened defects, exposed ends, and layers of amorphous carbon [5].This allows carbon nanotubes to locally react with the matrix material, leading to the formation of carbide phases.Often, the focus is on the formation of aluminum carbide phases [6], [7], along with this, in-situ formation of carbide phases such as SiC is known when utilizing hypereutectic Al-Si alloys as the matrix [8], as well as the formation of TiC phase through additional alloying of aluminum with titanium particles [9].
Carbide formation reactions can enhance interfacial interaction by increasing the adhesion work with aluminum compared to the initial graphene surface by factors of 1.3, 5.7, and 6.5 for the Al-SiC [10], Al-Al4C3 [11] , and Al-TiC [12] systems, respectively.However, excessive reactions lead to a decrease in the physical and mechanical properties of composite materials [13]- [15].Control over the extent of reacted substance is achieved by ensuring the required temperature, pressure, and material holding time.As the reaction progresses, the proportion of aluminum carbide and the average size of its particles increase.For example, in [13], it was shown that increasing the annealing time of the Al+0.6 vol.%CNT composite from 0.02 to 0.5 hours led to an increase in the average length of carbide rods from 175 to 420 nm.Additionally, in [14], researchers observed a change in the morphology of the carbide phase in the Al+1 wt.% CNT composite with 20 nm particles, transforming into rods with aspect ratios ranging from 5 to 15 as the SPS temperature increased from 577 to 627 ºC.Furthermore, in [15], an increase in the average length of carbide rods in the Al+1% SiC+1% CNT composite from 82 to 175 nm was observed with an increase in annealing temperature from 550 to 630 ºC.Both groups of researchers noted a decrease in the strengthening effect when the sizes of carbide rods entered the micro-scale range.In [14], the highest strength limit of 221 MPa was achieved with an aluminum carbide morphology consisting of nanocrystalline rods with a diameter of 20 nm and an aspect ratio of ~5, surpassing the strength limit of the composite without carbide formation by 30%.Further increase in the length of carbide rods reduced the material's strength by 11 MPa, and the authors claim that this effect is not due to coarsening of the matrix material structure at higher consolidation temperatures but rather to a decrease in the strengthening contribution from nanostructures.In [15], an increase in the strength limit of 10% was achieved, from 291.9 to 323.1 MPa, with an average length of carbide rods at 102 nm.However, further growth of carbide rods resulted in a 16% decrease in the strength limit compared to the reinforcement with pristine nanotubes.This could be attributed to the reduction in the effective volume of CNT in the composite material [16], [17], as well as the hygroscopic nature of this phase in the microstate compared to nanoscale aluminum carbide [5].
Another method of improving interfacial interaction in Al-CNT composites is ex-situ surface modification of carbon nanotubes, which involves the deposition of coatings with various ceramic [18]- [20] or metallic phases [21]- [23] in island-like or continuous morphologies before introducing them into the aluminum matrix.Works utilizing interfaces such as TiC, Ni, and Cu [24]- [28] are most commonly encountered, and one of the most specific materials is non-stoichiometric cubic tungsten carbide [29].Besides enhancing the adhesion between hybrid nanostructures and aluminum, coating materials are also capable of chemically reacting with aluminum, leading to the formation of new bulk phases.For instance, the formation of Al3Ni and AlCu2 particles has been observed in studies using Nicoated [30] and Cu-coated [31] CNTs, respectively.It is known that coatings can also inhibit undesirable chemical reactions, such as carbide formation.In the study [32], for example, annealing at 550 ºC and hot rolling of bulk nanocomposites Al + 1 vol.%CNTs for 2-12 hours resulted in the formation of carbide rods with lengths ranging from 87 to 231 nm.The presence of a copper coating layer on the surface of nanotubes, with a thickness less than 5 nm, inhibits carbide formation: under similar heat treatment conditions, the length of carbide rods varies from 0 to 114 nm.In the case of composites with initial CNTs, increasing the annealing time led to a decrease in the strength limit from 343 to 268 MPa.However, when copper-coated CNTs were used as the reinforcing additive, the strength limit changed from 379 to 418 MPa.A similar trend was observed in the change of microhardness.Increasing the annealing time of composites with pristine CNTs resulted in an 18% reduction in microhardness from 107.1 to 90.9 HV, while using copper-coated CNTs increased it by 11% from 109.4 to 121.9 HV.Researchers demonstrate that the strengthening effect due to the formation of a copper solid solution in aluminum is weak, and the main strengthening mechanism occurs through inhibiting the growth of aluminum carbide rods.
The equilibrium state of a polyatomic system is determined from the maximum entropy condition, the consequence of which is the equality of the chemical potentials of the components.For a system with a given constant pressure and temperature the chemical potential is defined as (1) (1 where G is the Gibbs free energy, Ni is the number of particles of a given variety, and the indices i denote the different components of the system.For an equilibrium system of reacting components, the values of Gibbs energies determine the concentrations of reactants and reaction products, while in the nonequilibrium case, the change in Gibbs energy during the reaction (or phase transition) completely determines its direction.
This paper systematizes the thermodynamic substantiation of the existing methods of inhibiting the in-situ reaction of Al4C3 formation consisting in alloying the matrix with various elements and ex-situ modification of CNT with creation of different intermediate layers on its surface before introducing them into the aluminum matrix.

Calculation of thermodynamic characteristics of interfacial interaction of Al/CNT composites
During this work, characteristic carbide formation reactions (2-5) specific to carbon nanotubes, as well as intermetallic compound formation in the systems Al-TiC (6), Al-Ni (7), Al-Cu (8), Al-W (9), and aluminum oxidation (10) were considered.Calculations were performed at a temperature of 600 ºC, which was chosen from considerations about the limiting temperature of solid-phase consolidation processes.The Gibbs energy values for these reactions at the selected temperature were obtained from literature sources [17], [33]- [39].It is worth noting that during the consolidation processes at a relatively low temperature of 300 ºC the Gibbs energy values differ by no more than 15%.
The values of Gibbs energy for various reactions at 600 ºC are shown in Fig. 1.Negative Gibbs free energy values for the selected reactions at 600 ºC confirm the feasibility of all described reactions.In practice, among several possible reactions, the one with the lower Gibbs free energy value typically occurs more rapidly.The formation reaction of aluminum carbide occurs more actively in localized melting areas of the aluminum matrix, as liquid aluminum atoms possess higher kinetic energy and diffusion coefficient, allowing them to reach the reaction site faster compared to solid aluminum atoms.In-situ formation of silicon carbide is thermodynamically more favorable compared to the formation of titanium carbide and aluminum carbide.The Gibbs free energy of intermetallic compound formation reactions consistently increases for Al-TiC, Al-Ni, Al-Cu, and Al-W interactions.The presence of a native oxide shell on aluminum is practically inevitable without special measures to restrict the oxygen supply to the aluminum surface during mechanical processing in a ball mill, as the energy required for its formation is almost an order of magnitude lower than the energies of the other reactions.

Influence of the interfacial layer on the properties of Al/CNT composites
It is known that using CNTs with minimal defects as reinforcement in aluminum matrix composites leads to improved physico-mechanical properties compared to CNTs with a higher number of defects.However, the defect density of CNTs often increases during the fabrication process of Al/CNT composites.This is primarily observed during the preparation of composite powder mixtures, which involves mechanical damage to CNTs, as well as after consolidation processes due to compressive forces.The application of various deformation post-processing stages only slightly increases the defect density, as the CNTs are protected by the soft matrix.In a study [38], in-situ formation of SiC interfacial layers in Al-23%Si alloy-based composites during gas-thermal coating synthesis is reported, and a thermodynamic rationale is provided for inhibiting the formation of Al4C3.The authors justify this by the higher reaction rate of Al4C3 formation at the CNT/Al-Si alloy/vapor triple point, attributed to the lower Gibbs free energy.
In the study [9], it was observed that in-situ formation of TiC occurred on the surface of CNTs in the form of nanocapsules and nanoblocks during the consolidation of composites based on aluminum powder alloyed with 2% Ti and reinforced with 2 wt.%CNTs through cold pressing, sintering, and subsequent extrusion.Along with the formation of TiC, the composite also contained phases of Al3Ti and Al4C3.Since Ti was present in the composite in a free rather than strictly on the surface of CNTs interface, this simultaneous formation of different phases can be explained.Aluminum, in contact with titanium particles, led to the formation of intermetallic particles.The interaction between aluminum and pure CNTs resulted in the formation of Al4C3.Titanium, in contact with CNTs, resulted in the formation of TiC.The presence of a TiC interface on the surface of CNTs exhibited prominent barrier properties that prevented interfacial carbide formation reactions under elevated temperature conditions, as confirmed by recent work on high-temperature compression testing of AA5049 alloy-based composites reinforced with TiC/CNT nanostructures [40].The presence of a TiC coating, with particle sizes ranging from 10-30 nm, on the surface of CNTs led to a 60% increase in deformation resistance at 500 °C compared to the composite reinforced with a similar weight fraction of untreated CNTs.
There are studies that highlight the prospects of modifying the surface of CNTs with various metals to prevent carbide formation reactions.For instance, in a study [40], composites based on AA2024 alloy were produced with hybrid reinforcement by adding 20% ABOw and 5% CNTs.When untreated CNTs were used, interfacial reactions between them and the aluminum matrix were observed.As a result, the microhardness of the composite material decreased by 20%, from 176.3 to 136 HV, compared to the singly reinforced material.However, the use of CNTs coated with a continuous ring-shaped Ni layer prevented interfacial reactions and increased the microhardness of the composite by 75% to 311 HV.
In another study, pristine CNTs and Ni-coated CNTs with nanoparticles were used as reinforcing nanostructures [27].TEM observations and Raman spectroscopy results demonstrated that the consolidation of composites reinforced with pristine nanotubes led to a partial reaction between CNTs and the matrix alloy.Raman spectroscopy is a fundamental method for characterizing the crystalline structure of CNTs.Intensity, position, and full width at half maximum (FWHM) of the D and G bands are used to assess the quality of CNTs in this technique.The D band, arising from elastic scattering of phonons by defects in the graphene structure, is observed around 1350 cm -1 and increases proportionally with the number of defects in CNTs.The G band is characteristic of ideal hexagonal graphene and appears around 1580 cm -1 .Reduction in the intensity of the G line and broadening of its FWHM indicate a defective structure in CNTs.Thus, the ratio of D and G band intensities can serve as a semi-quantitative measure of the degree of structural damage in CNTs.In the Al-4.4Cu-0.5Mg+ 1.5 wt.% CNT composite [27], the density of structural defects decreased after consolidation, as demonstrated by a decrease in the ID/IG ratio from 1.39 to 1.01.Conversely, consolidation of powder mixtures reinforced with Nicoated CNTs resulted in an increase in the ID/IG ratio from 0.73 to 0.99, indicating an excessive interfacial reaction.The researchers attributed this to the dissolution of Ni atoms from the coating into the matrix during mechanical processing in a ball mill, leaving defects on the surface of CNTs, significantly increasing their density and intensifying the formation of Al4C3.
In several independent studies, it has been observed that the formation of a copper layer on the surface of CNT inhibits the formation of the Al4C3 phase [32], [41]- [43], limiting its growth to the nanometer range.For instance, in [42], it was noted that the fabrication of bulk layered Al-CNT specimens was accompanied by the intense formation of Al4C3 nanoparticles, resulting in a 20% increase in the ID/IG ratio in Raman spectra from 1.26 to 1.49.Fabricating bulk layered composites with the same fraction of CNTs coated with 10-50 nm copper particles led to a slight reduction in the ID/IG ratio from 1.08 to 1.07, indicating minimal interfacial reaction.Researchers attributed this limitation, but not complete cessation of carbide formation, to the inhibitory effect of the copper coating.The ability of the copper coating to inhibit, but not completely halt, carbide formation may be attributed to the fact that at temperatures < 130 ºC, the Gibbs energy for the formation of Al4C3 is higher than that for the formation of AlCu2.Considering the gradual growth of aluminum carbide rods with increasing temperature and/or holding time [13], [44], during the gradual heating of Al-Cu/CNT composites during consolidation, carbide rods may initially form, and then the priority shifts to the formation of intermetallic compounds, halting the carbide formation reaction.
Therefore, despite the fact that decorating CNTs with both Ni and Cu particles can enhance the Al-C reaction intensity due to the formation of solid solutions in aluminum and the creation of defects on the CNT surface, surface modification with a Cu layer on the CNT surface, through the formation of an Al-Cu diffusion layer that impedes aluminum diffusion to carbon, can contribute to the formation of nanocrystalline aluminum carbide, improving interfacial interaction.
Finally, in [46], it was demonstrated that annealing of AA5049 aluminum alloy-based composites reinforced with non-stoichiometric cubic WC-coated CNTs in the temperature range of 500-600 ºC leads to significant structural changes, which were characterized using HRTEM, XRD, and Raman spectroscopy.At an annealing temperature of 500 ºC, diffraction patterns of the composite revealed the appearance of peaks corresponding to the solid solution of tungsten.As the annealing temperature increased to 525 ºC, the formation of the intermetallic phase Al12W was observed.Annealing the composite at 550 ºC resulted in the appearance of Al4C3 peaks on the diffraction patterns.Further increasing the annealing temperature to 600 ºC intensified the interfacial reaction.Raman spectroscopy results showed minimal changes in the intensity ratios of the D and G peaks throughout the entire temperature range, indicating insignificant influence of thermal treatment on the structural integrity of the nanotubes.Concurrently, an increase in the ratio of carbide and D peak intensities indicated an intense carbide formation reaction.Based on the aforementioned observations, it was concluded in the study that the carbon source for the formation of Al4C3 was not the CNTs but the non-stoichiometric WC from the coating.Thus, the overall volume of the nanotubes remained preserved, resulting in a decrease in the microhardness and elastic modulus of the composites after high-temperature annealing (~0.95 Tm) by only 20% and 30%, respectively.

Conclusion
This study provides rationales for the main methods of inhibiting the reaction between carbon nanotubes (CNTs) and the aluminum matrix, which can be divided into two primary groups: alloying the aluminum matrix with Si and Ti, as well as coating the CNT surfaces with TiC, Ni, Cu, and WC.The justifications were based on literature data on the thermodynamics of various reactions.

Acknowledgements
The research was carried out within the state assignment in the field of scientific activity of the Ministry of Science and Higher Education of the Russian Federation (theme FZUN-2020-0015, state assignment of VlSU).The study was carried out using the equipment of

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Web of Conferences 431, 06012 (2023) ITSE-2023 https://doi.org/10.1051/e3sconf/202343106012 the interregional multispecialty and interdisciplinary center for the collective usage of promising and competitive technologies in the areas of development and application in industry/mechanical engineering of domestic achievements in the field of nanotechnology (Agreement No. 075-15-2021-692 of August 5, 2021).