Al4C3 Particles Research Articles

Superior tribological properties of highly-loaded graphite/A356 surface composites fabricated using friction stir processing

In this work, friction stir processing (FSP) was used to prepare graphite (Gr) / A356 alloy composites with superior tribological properties. The Gr content and number of FSP passes (up to three passes) were used as processing variables. The microstructure, hardness and wear resistance of the composites were studied and compared with a neat sample. The microstructural studies demonstrated that super-fine Gr particles were uniformly distributed and in-situ sub-micron Al4C3 particles were formed after three FSP passes. Hardness measurements proved that all composites exhibited higher hardness than the neat sample. The composite prepared using two Gr loading rounds and three FSP passes achieved the highest hardness value of 331 kg.f/mm2, which is more than 4 times that of the neat sample. Additionally, the wear rate of this composite was approximately 57 % lower than that of the neat sample. The superior hardness and wear resistance of the fabricated composites are attributed to the in-situ exfoliation of graphite and the formation of super-fine Al4C3 particles during FSP.

Degree of Impurity and Carbon Contents in the Grain Size of Mg-Al Magnesium Alloys.

In this study, the tendency of having different grain structures depending on the impurity levels in AZ91 alloys was investigated. Two types of AZ91 alloys were analyzed: commercial-purity AZ91 and high-purity AZ91. The average grain size of the commercial-purity AZ91 alloy and high-purity AZ91 is 320 µm and 90 µm, respectively. Thermal analysis revealed negligible undercooling in the high-purity AZ91 alloy, while undercooling of 1.3 °C was observed in the commercial-purity AZ91 alloy. A CS analyzer was employed to precisely analyze the carbon composition of both alloys. The carbon content of the high-purity AZ91 alloy was found to be 197 ppm, while the commercial-purity AZ91 alloy contained 104 ppm, indicating a difference of approximately 2 times. The higher carbon content in the high-purity AZ91 alloy is believed to be due to the use of high-purity pure Mg in its production (the carbon content of high-purity pure Mg is 251 ppm). To simulate the vacuum distillation process commonly used in the production of high-purity Mg ingots, experiments were conducted to investigate the reaction of carbon with oxygen to produce CO and CO2. XPS analysis and simulation results for activities confirmed the formation of CO and CO2 during the vacuum distillation process. It could be speculated that the carbon sources in the high-purity Mg ingot provide Al-C particles, which act as nucleants for Mg grains in the high-purity AZ91 alloy. Thus, it can be considered the main reason that high-purity AZ91 alloys have a finer grain structure than that of commercial-purity AZ91 alloys.

Laser powder bed fusion of AlSi10Mg-based composites with graphene and nanodiamond additions

Laser powder bed fusion (LPBF) processing of aluminum matrix composites (AMC) with blends of AlSi10Mg powder and nanodiamond and graphene additives was investigated. AMC with 0.5 wt% nanodiamonds addition was prepared by mechanical mixing, while AMC with 0.5 wt% of multi-layered graphene was prepared by electrochemical deposition. Initial powders for LPBF were thoroughly characterized and optimal LPBF parameters were found based on the relative density analysis and optical microscopy. Processing windows of both AMCs moved towards lower energy density possibly due to the increased total absorptivity of the powders. Raman spectroscopy results, SEM and HRTEM studies of samples after LPBF demonstrated the absence of nano-diamond in the structure after processing which is explained by graphitization of the nanodiamond particles during laser melting. In case of AMC with graphene additives, carbon-containing particles remained in the structure with partial in situ formation of Al4C3, along with a noticeable strengthening effect, which increased material’s microhardness by more than 40% compared to the initial AlSi10Mg alloy. The strengthening effect is explained by the presence of a network of graphene and Al4C3 particles across the solidified melt-pool promoting the Orowan strengthening mechanism.

Formation of Interfacial Reaction Products in Al-4Si-1Mg/SiC Composite

In Al/SiC composite, Al and SiC reacts and forms Al4C3 at the interface according to the reaction 4A1 + 3SiC → Al4C3+ 3Si. Hence in the present work, the Al-4Si-1Mg/SiC is fabricated through stir casting process. The fabricated composite is also ultrasonically treated prior to superheating to induce the interfacial reaction. The as cast composite and the ultrasonic treated composite are superheated at 900°C and computer aided cooling curve analysis is performed in the composite for different time to quantify the extent of interfacial reaction and to study the solidification behaviour. The Si released as a by-product can alter the composition and solidification behaviour. The salt flux reclamation technique is employed to extract the interfacial reaction products from the composite. The reclaimed powders are subjected to X-ray diffraction analysis and confirmed the presence of Al4C3 particles, which be used for grain refinement applications.

Strength adjust mechanism of high toughness Al4C3/Al-xMg composites

In this paper, Al4C3 was in-situ synthesized in Al4C3/Al-xMg (x = 0, 8 wt%) composites by liquid-solid reaction, the corresponding microstructure evolution and strength adjust mechanism were systematically studied. The Al4C3 particles in-situ generated in Al4C3/Al-xMg composites are both submicron and nanoscale. By adding 8 wt% Mg to Al4C3/Al composite, high content of Mg atoms dissolve in the matrix, which affects the distribution of Al4C3 and plays an active role in improving the mechanical properties. The ultimate tensile strength of the Al4C3/Al–8Mg composites at room temperature increases from 171 MPa to 435 MPa, and the elongation still holds about 17.8%. The Brinell hardness was 105.8 HBW, 2.04 times higher than Al4C3/Al composite. What's more, the introduction of 8 wt% Mg weakens the fiber texture strength of Al4C3/Al composite, changes the preferred orientation from <111>Al//ED to <100>Al//ED, and increases the recrystallization fraction from 36.5% to 56.4%. Therefore, the work hardening ability of the Al4C3/Al–8Mg composites in the deformation process is significantly improved. This work provides a new perspective for designing high strength and toughness aluminum alloy strengthened by solid solution and in-situ nanoparticles simultaneously through liquid-solid method.

THEORETICAL-EXPERIMENTAL POSSIBILLITIES OF MICROSTRUCTURE QUANTIFICATION OF DISPERSION STRENGTHENED MATERIALS

The present paper is devoted to the possibilities to classify the spatial arrangement of the elements (features) of a stochastic process with geometrical objects. The fundamental quantities describing point processes were introduced. Experimental possibilities of structural objects determination, possibilities of evaluation of the size distribution of the secondary phases, testing of planar point structures (estimation of the process intensity, square method and characteristics of the second order) were estimated. Interparticle distances, namely mean interparticle distance, mean minimum distance, mean visibility and mean path of spherical contact were defined. Selected processes were described and demonstrated from simulated realizations on Al-Al4C3 dispersion strengthened material, prepared by a powder metallurgy method of reaction milling. Interparticle distances of Al4C3 particles were evaluated. Polygonal methods and quadrant counts method were used for characterization of the particle arrangement.

Behavior of Al4C3 Particles During Flotation and Sedimentation in Aluminum Melts

Al4C3 particles form during the primary production of aluminum via molten salt electrolysis due to the carbon solubility and direct contact between bath, metal, and carbon anodes. Additional Al4C3 may form during melt processing through direct contact between the melt and carbonaceous materials. As a result of their small size and similar density to aluminum, removal of aluminum carbide particles can be challenging. If not removed, carbides can produce inclusion defects or poor surface condition in aluminum products. The current work studies the removal and behavior of Al4C3 particles during flotation with different gas mixtures, as well as sedimentation. The interaction between carbide particles and Al2O3 films during the melt treatment processes was also studied and reported. Factsage thermochemical software was used to model the interactions at the interface of inclusions and bubbles covered by films. The highest degree of carbide removal was obtained after flotation with an H2O-containing argon gas mixture, where the carbide concentration dropped below the measured solubility limit of carbon at the corresponding temperature. Strong interaction between Al4C3 particles and Al2O3 films was observed during sedimentation which worked as an efficient removal method for the particles. Oxidation of carbides and formation of oxycarbides were suggested as the mechanisms promoting the attachment of carbides on oxide films.

Synergistic refining mechanism of Mg-3%Al alloy refining by carbon inoculation combining with Ca addition

Abstract Mg-3%Al alloy was refined by carbon inoculation combining with 0.2%Ca addition. High grain refining efficiency was obtained and the synergistic refining mechanism was deeply discussed in the present study. Al–C–O particles, actually Al4C3 particles, were formed in the carbon-inoculated Mg-3%Al alloy acting as nuclei for α-Mg grains. Ca addition had no obvious effect on size distribution of the nucleating particles. Ca segregation was proved on Al4C3 particles, which should reduce the interfacial energy of nuclei/Mg. The constitutional undercooling in front of nucleus/liquid was increased from 0.12 °C to 0.15 °C induced by 0.2%Ca addition. The synergistic grain refining efficiency can be attributed to the higher constitutional undercooling and lower the interface energy of nucleus/Mg induced by Ca addition. More nucleating particles with small size could be activated acting as potent nuclei of α-Mg grains. Consequently, Mg-3%Al alloy could be effectively refined due to the synergistic effect induced by carbon inoculation combining with Ca.

Theoretical study of active Ca element on grain refining of carbon-inoculated Mg-Al alloy

It is crucial to study grain refinement and refinement mechanism for promoting mechanical properties of Mg alloys. In this work, the influence mechanism of Ca on the grain refinement of Al2MgC2 and Al4C3 in Mg alloys was studied. It shows that Ca is easier to adsorb on the surfaces of Al2MgC2 and Al4C3 particles than Mg, which further promotes the adsorption Mg on the surfaces of Al2MgC2 and Al4C3 particles, leading to more crystal nuclei beyond the critical nucleation radius rK. The interface of Ca replaced Mg-2 is found to be more stable, for both Mg/Al2MgC2 interface and Mg/Al4C3 interface. We further revealed that Ca could effectively migrate from the first-layer to the second-layer in both the adsorption slabs and doped interface slabs under ambient condition.

The Effect of Melt Ultrasound Treatment on the Microstructure and Age Hardenability of Al-4 Wt Pct Cu/TiC Composite

In the present work, Al-4 wt pct Cu/TiC composite was synthesized by the molten salt route that contains submicron-sized TiC particles in the Al-4 wt pct Cu matrix. The concentration of the TiC particle in the base alloy is 7.5 wt pct. Melt ultrasound treatment was done by remelting the as-cast composite at 1023 K (750 °C) in a view to refine the size of TiC particles to nanoscale and distribute them evenly in the matrix. The microstructure and age hardenability of the untreated and ultrasound-treated composites were investigated. The TiC particles accelerate the precipitation kinetics of CuAl2 phase in Al-4 wt pct Cu alloy. In the present study, the hardness obtained for untreated Al-4 wt pct Cu/TiC composite is 120 VHN within 5 hours of peak aging time, which is higher than the hardness of the monolithic Al-4 wt pct Cu, which is 104 VHN at 35 hours of peak aging time. Melt ultrasound treatment of Al-4 wt pct Cu/TiC composite shows no significant effect on the distribution and refinement of TiC particles in the matrix. However, it partially disintegrates the TiC into Al3Ti and Al4C3 particles. The ultrasound-treated composite showed an improved hardness of about 132 VHN at 5 hours of peak aging, in comparison to that of the untreated composite, by forming denser and homogeneous CuAl2 precipitates.

Revealing the Nuclei Formation in Carbon-Inoculated Mg-3%Al Alloys Containing Trace Fe.

In this study, Fe-bearing Mg-3%Al alloys were inoculated by combining carbon with or without Ca. Both processes can significantly refine the grain size of Mg-3%Al alloys. The highest refining efficiency can be obtained by carbon combined with Ca. The synergistic grain refining efficiency can be attributed to the constitutional undercooling produced by the addition of Ca. Two kinds of carbon-containing nuclei with duplex-phase particles and cluster particles were observed in the carbon-inoculated alloys. A thermodynamic model was established to disclose the formation mechanisms of the duplex-phase particles and Al4C3 cluster particles. This thermodynamic model is based on the change of Gibbs free energy for the formation of these two kinds of particles. The calculated results show that these two particles can form spontaneously, since the change of Gibbs free energy is negative. However, the Gibbs free change of the duplex-phase particle is more negative than the Al4C3 cluster particle. This indicates that the adsorption process is more spontaneous than the cluster process, and tiny Al4C3 particles are preferred to form duplex-phase particle, rather than gathering to form an Al4C3 cluster particle. In addition, the addition of Ca can reduce the interfacial energy between the Al4C3 phase and the Al–Fe phase and promote the formation of duplex-phase particles.

Influence of Ca adsorption on the heterogeneous nucleation of α-Mg on Al4C3 particles: First-principles calculation and experiment

Abstract The addition of Ca in magnesium melt has a significant influence on the heterogeneous nucleation effect of Al4C3 particles with open question for the mechanism. We have conducted first-principles calculations of the adsorption of Ca and Mg atoms on Al4C3 surfaces to study the initial nucleation and growth of Mg atoms on Al4C3 surfaces. The single Ca or Mg adsorption, as well as Ca Mg co-adsorption on a 28-layer slab (A-type) of Al-terminated Al4C3(0001) surface has been simulated. We demonstrate that both Ca and Mg atoms tend to stabilize at the H2 site of Al-terminated Al4C3(0001) surface. Meanwhile, a comparative experiment is conducted to study the influence of Ca on the grain refining effect, showing that Ca atoms promotes the adsorption and nucleation growth of Mg atoms on Al4C3 surfaces.

Phase-field investigation on the growth orientation angle of aluminum carbide with a needle-like structure at the surface of graphite particles

We adopt a phase-field model for peritecitc phase transition to investigate the critical orientation angle of a needle-like Al4C3 structure on the surface of graphite. The critical orientation angle is defined as the maximum angle for which the Al4C3 particle can grow. A crystalline (facetted) anisotropy is exploited to depict the formation of the needle-like morphology. While the classical nucleation theory addresses the critical radius for crystal growth, it is still challenging to determine the critical orientation angle when strong anisotropy is involved. For a single Al4C3 particle, we explore the effect of the basis radius of the graphite and the particle size on the critical orientation angle. For two adjacent Al4C3 particles, we rationalize the critical orientation angle with the apart distance, the radius and the individual orientation of the particle. Depending on the geometric configuration, we observe three typical competing effects which determine the critical orientations differing from a single Al4C3 particle. We anticipate that the simulation results can be applied to interpret some microstructural evolutions with strong anisotropies.

Microstructure-based modeling on structure-mechanical property relationships in carbon nanotube/aluminum composites

The present work investigates structure-mechanical property correlations of carbon nanotube/aluminum (CNT/Al) composites through characterization and microstructure-based modeling. The nanoscale architecture of these composites, produced by flake powder metallurgy, was established using a transmission electron microscope equipped with an automated crystal orientation mapping system. The architecture mainly consists of elongated Al grains with submicron widths and micron lengths, intergranular CNTs and an interfacial reaction product, namely Al4C3 particles. The stress-strain behavior found with our dislocation-boundary/reinforcement interaction mechanical model, matched our experimental tensile curves. This model was also used to predict several key factors, namely grain dimensions, CNTs volume fraction, intergranular or intragranular distribution and aspect ratio, interfacial reaction rate, impacting tensile strength and ductility. It was found that intergranular CNTs induced strengthening will not result in lower uniform elongations, while intragranular distribution of low aspect ratio CNTs and Al4C3 nanoparticles can increase both strength and uniform elongations by the Orowan mechanism. It is expected that the model in the present work can assist in microstructure design of metal matrix nanocomposites.

Effect of C2H2 as a novel gas inoculant on the microstructure and mechanical properties of as-cast AM60B magnesium alloy

Herein we investigate the effect of C2H2 inoculation on the grain refinement of AM60B magnesium alloy. The sample inoculated by CO2 was also prepared for comparison. Corresponding microstructures and mechanical properties of inoculated alloys at different holding times were analysed to explore the fading behaviours of the inoculants. The grain size of AM60B alloy with C2H2 inoculation decreased from the initial 412 μm to a minimum 101 μm after holding for 30 min, while that inoculated with CO2 reached the minimum of 92 μm after only 10 min A model was proposed to explain the lag of the optimum refinement point for C2H2 inoculated alloy. The heterogeneous nucleation of Al4C3 was considered the main reason for the grain refinement. However, the samples treated with CO2 exhibited poorer tensile strength than those treated with C2H2 due to the formation of MgO and MgAl2O4. With increasing holding time, the size of Al4C3 particles became increased and gradually transformed to the Al-C-Mn phases, resulting in the increased grain size. During the holding procedure, the Al elements sank down in the melt, leading to a slight increase in plasticity after holding for 2 h.

Encapsulation of Reactive Ti2AlC and Nb2AlC Particles via a Boehmite Precipitation Route

Reactive Ti2AlC and Nb2AlC particles are encapsulated in a continuous boehmite shell by hydrolysis of ultrafine AlN powder in aqueous environment. The hydrolysis of AlN leads to the heterogeneous precipitation of nanolamellar boehmite on the surface of the Mn + 1AXn phase particles after 24 h reaction time. The boehmite coatings completely cover the MAX phase particles (Nb2AlC d50 = 7.9 μm; Ti2AlC d50 = 9.6 μm) with a thickness of 200 nm (Nb2AlC) and 135 nm (Ti2AlC). The lamellar structure of the boehmite coating is maintained after calcination at 400 °C in air, resulting in coatings with a maximum surface area of 41 m2 g−1 (Nb2AlC) and 50 m2 g−1 (Ti2AlC). The stability of the coating is assessed by using the coated Nb2AlC as particle filler in ZrO2. The coating is stable after the spark plasma sintering process and covers complex features of the particles. The present work shows the feasibility to encapsulate reactive MAX particles with a continuous oxide shell. The coated particles can be used as ductile crack bridging or healing filler in reactive matrixes requiring a spatial separation between filler and matrix. Higher surface charges by Al–O bonds on the surface facilitate the dispersion of the carbide particles in aqueous slurries.

Autonomous high‐temperature healing of surface cracks in Al2O3 containing Ti2AlC particles

AbstractIn this work, the oxidation‐induced crack healing of Al2O3 containing 20 vol.% of Ti2AlC MAX phase inclusions as healing particles was studied. The oxidation kinetics of the Ti2AlC particles having an average diameter of about 10 μm was studied via thermogravimetry and/or differential thermal analysis. Surface cracks of about 80 μm long and 0.5 μm wide were introduced into the composite by Vickers indentation. After annealing in air at high temperatures, the cracks were filled with stable oxides of Ti and Al as a result of the decomposition of the Ti2AlC particles. Crack healing was studied at 800, 900, and 1000°C for 0.25, 1, 4, and 16 hours, and the strength recovery was measured by 4‐point bending. Upon indentation, the bending strength of the samples dropped by about 50% from 402 ± 35 to 229 ± 14 MPa. This bending strength increased to about 90% of the undamaged material after annealing at 1000°C for just 15 minutes, while full strength was recovered after annealing for 1 hour. As the healing temperature was reduced to 900 and 800°C, the time required for full‐strength recovery increased to 4 and 16 hours, respectively. The initial bending strength and the fracture toughness of the composite material were found to be about 19% lower and 20% higher than monolithic alumina, respectively, making this material an attractive substitute for monolithic alumina used in high‐temperature applications.

Structure, mechanical and magnetic properties of Al4C3 reinforced nickel matrix nanocomposites

A new type of nanocomposite, Ni-Al4C3 was prepared using Al4C3 as reinforcement by cathodic co-deposition at different current densities (1.0 to 5.0 A dm−2) from a nickel acetate-N-methyl formamide (non-aqueous) bath. Influence of current density and incorporation of Al4C3 particles in nickel matrix on the structure and properties of the composite coatings was investigated. Surface morphology and composition of the deposits were determined by SEM and EDAX. Crystallographic structure and orientation of the electrodeposited Ni-Al4C3 composite were studied by x-ray diffraction. Compared to nickel metal, these nanocomposites exhibited finer grains, higher microhardness, improved corrosion resistance and enhanced soft magnetic properties. Composite deposited at higher current densities (>2 A dm−2) shows mild texturing along (200) plane. The effect of heat treatment on the microstructure, texture and microhardness of the nanocomposites was also investigated.

An Investigation on Duplex Nucleation in AZ91 Magnesium Alloy and Its Influence on High Temperature Mechanical Properties

The grain refinement of Mg–Al alloy AZ91 via carbon inoculation, including the significant role of Mn in advanced nucleation, was analyzed, and the corresponding mechanical properties and aging behavior were investigated. To this end, various amounts of C were added into the liquid at the desired temperatures. Al8Mn5 particles, which are suitable nucleation sites for α-Mg, were identified as the primary grain refiners. In situ particle formation, along with appropriate wetting and a suitable orientation relationship (OR), facilitated the grain refinement mechanism. Al4C3 particles contributed to heterogeneous nucleation by providing suitable Al8Mn5 nucleation sites. Mn removal resulted in poor grain refinement in the Mg–Al alloy. The Hall–Petch relationship, high-temperature tensile behavior, and aging mechanism of the samples refined by 1 wt % C addition (as the best grain refiner) are discussed and compared with industrial practice.

Effect of gaseous carbon dioxide on grain refinement in Mg-8Al alloy

ABSTRACTA Mg-Al alloy was treated with gaseous carbon dioxide (CO2) and a refining process, and the grain refinement at each stage was investigated. The results indicated that CO2 and the subsequent refining process affect the grain refinement in the Mg-Al alloy. Moreover, the average grain size decreased from 254 to 89 µm, and the microhardness increased from 59.7 to 66.0 HV upon the addition of 9 litres of CO2. Increasing the time of the subsequent refining process to 30 min further decreased the grain size and increased the microhardness. Based on scanning electron microscopy, energy-dispersive spectroscopy and X-ray diffraction analyses, the observed Al4C3 particles, which are attributed to the inoculation of CO2, are responsible for the grain refinement.This paper is part of a thematic issue on Light Alloys.