ball mill aluminium powder hindi

ball milling - an overview | sciencedirect topics

Ball milling technique, using mechanical alloying and mechanical milling approaches were proposed to the word wide in the 8th decade of the last century for preparing a wide spectrum of powder materials and their alloys. In fact, ball milling process is not new and dates back to more than 150 years. It has been used in size comminutions of ore, mineral dressing, preparing talc powders and many other applications. It might be interesting for us to have a look at the history and development of ball milling and the corresponding products. The photo shows the STEM-BF image of a Cu-based alloy nanoparticle prepared by mechanical alloying (After El-Eskandarany, unpublished work, 2014).

Ball milling is often used not only for grinding powders but also for oxides or nanocomposite synthesis and/or structure/phase composition optimization [14,41]. Mechanical activation by ball milling is known to increase the material reactivity and uniformity of spatial distribution of elements [63]. Thus, postsynthesis processing of the materials by ball milling can help with the problem of minor admixture forming during cooling under air after high-temperature sintering due to phase instability.

Ball milling, a shear-force dominant process where the particle size goes on reducing by impact and attrition mainly consists of metallic balls (generally Zirconia (ZrO2) or steel balls), acting as grinding media and rotating shell to create centrifugal force. In this process, graphite (precursor) was breakdown by randomly striking with grinding media in the rotating shell to create shear and compression force which helps to overcome the weak Vander Waal's interaction between the graphite layers and results in their splintering. Fig. 4A schematic illustrates ball milling process for graphene preparation. Initially, because of large size of graphite, compressive force dominates and as the graphite gets fragmented, shear force cleaves graphite to produce graphene. However, excessive compression force may damage the crystalline properties of graphene and hence needs to be minimized by controlling the milling parameters e.g. milling duration, milling revolution per minute (rpm), ball-to-graphite/powder ratio (B/P), initial graphite weight, ball diameter. High quality graphene can be achieved under low milling speed; though it will increase the processing time which is highly undesirable for large scale production.

Fig. 4. (A) Schematic illustration of graphene preparation via ball milling. SEM images of bulk graphite (B), GSs/E-H (C) GSs/K (D); (E) and (F) are the respective TEM images; (G) Raman spectra of bulk graphite versus GSs exfoliated via wet milling in E-H and K.

Milling of graphite layers can be instigated in two states: (i) dry ball milling (DBM) and (ii) wet ball milling (WBM). WBM process requires surfactant/solvent such as N,N Dimethylformamide (DMF) [22], N-methylpyrrolidone (NMP) [26], deionized (DI) water [27], potassium acetate [28], 2-ethylhexanol (E-H) [29] and kerosene (K) [29] etc. and is comparatively simpler as compared with DBM. Fig. 4BD show the scanning electron microscopy (SEM) images of bulk graphite, graphene sheets (GSs) prepared in E-H (GSs/E-H) and K (GSs/K), respectively; the corresponding transmission electron microscopy (TEM) images and the Raman spectra are shown in Fig. 4EG, respectively [29].

Compared to this, DBM requires several milling agents e.g. sodium chloride (NaCl) [30], Melamine (Na2SO4) [31,32] etc., along with the metal balls to reduce the stress induced in graphite microstructures, and hence require additional purification for exfoliant's removal. Na2SO4 can be easily washed away by hot water [19] while ammonia-borane (NH3BH3), another exfoliant used to weaken the Vander Waal's bonding between graphite layers can be using ethanol [33]. Table 1 list few ball milling processes carried out using various milling agent (in case of DBM) and solvents (WBM) under different milling conditions.

Ball milling as a mechanochemical technique has been extensively used for grinding of materials to fine particles and for the formation and modification of inorganic solids. Mechanochemistry is a branch of solid-state chemistry in which intramolecular bonds are broken mechanically by using an external mechanical energy followed by additional chemical reactions [109]. Its use in synthetic organic chemistry is comparatively limited but has attained more attention during the last decade. A study proposed the importance of ball milling in synthetic organic chemistry, which has been widely documented [110]. Many reports in the literature have shown that high-speed ball milling (HSBM) is appropriate for a variety of organic transformations and for the expansion of environmentally benevolent chemical reactions [111, 112]. HSBM in solvent-free circumstances is considered as a feasible alternative to wet chemistry. It is based on the similar principles as that of mortar and pestle, which utilizes mechanical actions to convert reactants to products during the course of the reaction [113, 114]. The mills are effective at creating small particle sizes, which has allowed them to demonstrate their amazing characteristics. The ball milling time is an important factor in nanostructure materials synthesis. It has been demonstrated that an increase in the milling time increases microhardness of synthesized materials [115]. Different numbers, sizes, shapes, and materials of the ball bearings used could influence mixing and impact energy and thus the efficiency of the reaction. Using no ball bearing predictably gave the least amount of mixing and energy resulting in the lowest percent conversion to product.

Reactive ball-milling (RBM) technique has been considered as a powerful tool for fabrication of metallic nitrides and hydrides via room temperature ball milling. The flowchart shows the mechanism of gas-solid reaction through RBM that was proposed by El-Eskandarany. In his model, the starting metallic powders are subjected to dramatic shear and impact forces that are generated by the ball-milling media. The powders are, therefore, disintegrated into smaller particles, and very clean or fresh oxygen-free active surfaces of the powders are created. The reactive milling atmosphere (nitrogen or hydrogen gases) was gettered and absorbed completely by the first atomically clean surfaces of the metallic ball-milled powders to react in a same manner as a gas-solid reaction owing to the mechanically induced reactive milling.

Ball milling is a grinding method that grinds nanotubes into extremely fine powders. During the ball milling process, the collision between the tiny rigid balls in a concealed container will generate localized high pressure. Usually, ceramic, flint pebbles and stainless steel are used.25 In order to further improve the quality of dispersion and introduce functional groups onto the nanotube surface, selected chemicals can be included in the container during the process. The factors that affect the quality of dispersion include the milling time, rotational speed, size of balls and balls/ nanotube amount ratio. Under certain processing conditions, the particles can be ground to as small as 100nm. This process has been employed to transform carbon nanotubes into smaller nanoparticles, to generate highly curved or closed shell carbon nanostructures from graphite, to enhance the saturation of lithium composition in SWCNTs, to modify the morphologies of cup-stacked carbon nanotubes and to generate different carbon nanoparticles from graphitic carbon for hydrogen storage application.25 Even though ball milling is easy to operate and suitable for powder polymers or monomers, process-induced damage on the nanotubes can occur.

Ball milling is a way to exfoliate graphite using lateral force, as opposed to the Scotch Tape or sonication that mainly use normal force. Ball mills, like the three roll machine, are a common occurrence in industry, for the production of fine particles. During the ball milling process, there are two factors that contribute to the exfoliation. The main factor contributing is the shear force applied by the balls. Using only shear force, one can produce large graphene flakes. The secondary factor is the collisions that occur during milling. Harsh collisions can break these large flakes and can potentially disrupt the crystal structure resulting in a more amorphous mass. So in order to create good-quality, high-area graphene, the collisions have to be minimized.

The ball-milling process is common in grinding machines as well as in reactors where various functional materials can be created by mechanochemical synthesis. A simple milling process reduces both CO2 generation and energy consumption during materials production. Herein a novel mechanochemical approach 1-3) to produce sophisticated carbon nanomaterials is reported. It is demonstrated that unique carbon nanostructures including carbon nanotubes and carbon onions are synthesized by high-speed ball-milling of steel balls. It is considered that the gas-phase reaction takes place around the surface of steel balls under local high temperatures induced by the collision-friction energy in ball-milling process, which results in phase separated unique carbon nanomaterials.

Ball milling is another technique which was reported very recently for the production of NFC. In this method, a cellulose suspension is placed in a hollow cylindrical container, partially filled with balls (e.g., ceramic, zirconia, or metal). While the container rotates, cellulose is disintegrated by the high energy collision between the balls. Zhang etal. [114] studied the process of NFC production from once-dried bleached softwood kraft pulp suspension at a solid concentration of 1wt% using ball milling. They showed the influence of the process conditions such as the ball size and ball-to-cellulose weight ratio on the morphology of the produced NFC. An average diameter of 100nm was reported for the disintegrated fibers. The control of the processing parameters was necessary to prevent cellulose decrystallization and to produce cellulose nanofibers rather than short particles.

Ball milling is one of the earliest approach for BNNTs synthesis [59]. The process involves extensive ball milling of boron powder for a long period of time (up to 150h) in NH3 gas followed by annealing at high temperature (up to 1300C) in N2 environment. It was suggested that a nitriding reaction was induced between boron powder and NH3 gas due to high energy milling, resulting in metastable disordered BN nanostructures and boron nanoparticles. BNNTs were grown from these reactive phase during a subsequent high-temperature annealing of the powder in ammonia ambient. It is proposed that BN nanoparticles formed during the milling process act as nucleation sites for growth during annealing process. Apart from them, contaminant Fe nanoparticles introduced during the milling process also served as catalyst for the growth. However, the quality and purity of BNNTs grown by ball milling was not satisfactory.

In the following years, various works have been done to increase the throughput and improve quality of BNNTs using ball-milling process. Li et. al. showed that addition of catalyst during the milling process can help to increase the production yield [60]. As an example, boron powder and 10% of Fe(NO3)3 was milled in NH3 atmosphere at 250 KPa pressure. Annealing the milled powder at N2+15% H2 gas environment at 1100C mostly resulted in bamboo-like BNNTs. Heating the same milled powder at 1300C in NH3 environment resulted in the growth of cylindrical BNNTs with diameters approximately 10nm. Other metal-based compounds such as nickel boride (NiBx) [61] and Li2O [62] are also reported as catalysts to enhance the yield of BNNTs growth. Though large quantity of BNNTs can be synthesized via this process, shortcoming was that the BNNTs are usually bamboo-like structured and contain B/BN reactants (amorphous boron particles and BN bulky flakes) as impurities.

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effect of ball size on steady state of aluminum powder and efficiency of impacts during milling - sciencedirect

At a fixed ball to powder ratio, a change in balls size can significantly change the steady state milling time.High energy impacts are preferred over a high number of low energy impacts.Flattening of the particles is a result of accumulation of impacts in a particular direction.

The concept of steady state milling time was examined for ball milling of aluminum powder. Four different set-ups of balls were used while the mill speed and charge ratio were kept fixed. Different criteria (morphology of particles, average particle size, deviation from the average particle size, lattice imperfections and change in crystallographic orientation) were used to study structural evolution of the milled particles and to compare the steady state time for the different milling conditions. Results showed that different criteria may not determine the same steady state time, however, all criteria were consistent in comparing efficiency of the different milling conditions. Moreover, it was found that at a given mill speed and ball to powder ratio (i.e. at a given consumed energy), a change in balls size and filling ratio of vials can improve milling efficiency. Finally, the effect of energy of each impact and the collective energy of all impacts were discussed.

production of metal powders | engineers gallery

Metallic powders possessing different properties can be produced easily. The mostcommonly used powders are copper-base and iron-base materials. But titanium, chromium,nickel, and stainless steel metal powders are also used. In the majority of powders, the sizeof the particle varies from several microns to 0.5 mm. The most common particle size ofpowders falls into a range of 10 to 40 microns. The chemical and physical properties of metalsdepend upon the size and shape of the powder particles. There are various methods ofmanufacturing powders.The commonly used powder making processes are given as under.

In this process, the molten metal is forced through an orifice and as it emerges, a highpressure stream of gas or liquid impinges on it causing it to atomize into fine particles. Theinert gas is then employed in order to improve the purity of the powder. It is used mostlyfor low melting point metals such as tin, zinc, lead, aluminium, cadmium etc., because of thecorrosive action of the metal on the orifice (or nozzle) at high temperatures. Alloy powdersare also produced by this method.

In this process, the compounds of metals such as iron oxides are reduced with CO or H2at temperatures below the melting point of the metal in an atmosphere controlled furnace.The reduced product is then crushed and ground. Iron powder is produced in this way

Powders of W, Mo, Ni and CO can easily be produced or manufactured by reductionprocess because it is convenient, economical and flexible technique and perhaps the largestvolume of metallurgy powders is made by the process of oxide reduction.

Electrolysis process is quite similar to electroplating and is principally employed for theproduction of extremely pure, powders of copper and iron. For making copper powder, copperplates are placed as anodes in a tank of electrolyte, whereas, aluminium plates are placed into the electrolyte to act as cathodes. High amperage produces a powdery deposit of anodemetal on the cathodes. After a definite time period, the cathode plates are taken out from thetank, rinsed to remove electrolyte and are then dried. The copper deposited on the cathodeplates is then scraped off and pulverized to produce copper powder of the desired grain size.

The crushing process requires equipments such as stamps, crushers or gyratory crushes.Various ferrous and non-ferrous alloys can be heat-treated in order to obtain a sufficientlybrittle material which can be easily crushed into powder form.

The milling process is commonly used for production of metallic powder. It is carried outby using equipments such as ball mill, impact mill, eddy mill, disk mill, vortex mill, etc.Milling and grinding process can easily be employed for brittle, tougher, malleable, ductile andharder metals to pulverize them. A ball mill is a horizontal barrel shaped container holdinga quantity of balls, which, being free to tumble about as the container rotates, crush andabrade any powder particles that are introduced into the container. Generally, a large massto be powdered, first of all, goes through heavy crushing machines, then through crushingrolls and finally through a ball mill to produce successively finer grades of powder.

This process can be applied in case of metals, such as Zn, Cd and Mg, which can be boiledand the vapors are condensed in a powder form. Generally a rod of metal say Zn is fed intoa high temperature flame and vaporized droplets of metal are then allowed to condense onto a cool surface of a material to which they will not adhere. This method is not highlysuitable for large scale production of powder.

High hardness oriented metals such as tantalum, niobium and zirconium are made tocombine with hydrogen form hydrides that are stable at room temperature, but to begin todissociate into hydrogen and the pure metal when heated to about 350C. Similarly nickel andiron can be made to combine with CO to form volatile carbonyls. The carbonyl vapor is thendecomposed in a cooled chamber so that almost spherical particles of very pure metals aredeposited.Source Introduction of Basic Manufacturing Processes and Workshop Technology by Rajender Singh.

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microstructure and mechanical properties of aluminium-graphene composite powders produced by mechanical milling | mechanics of advanced materials and modern processes | full text

The SEM observation shows that aluminium particles are firstly flattened into flakes, and then fractured/ rewelded into equiaxed particles as the ball milling progresses. The crystalline size is decreased and the lattice strain is increased during the ball milling, which are also intensified by the added GNSs. The hardness of the composite is increased by 115.1% with the incorporation of 1.0 vol. % GNSs.

The local stress induced by the hard GNSs accelerates the milling process. The X-Ray diffraction patterns show that the intensity ratio of (111) to (200) can reflect the preferred orientation of the particle mixture, and the evolution of I(111)/I(200) agrees well with the observed results using SEM. The increased hardness is mainly attributed to the refined microstructure and Orowan strengthening.

Aluminium matrix composite (AMC) has found wide application in the fields of aerospace, automobile, military, transportation and building, due to its attractive properties such as light weight, corrosion resistance and superior ductility (Bodunrin et al. 2015). Graphene is a very promising reinforcing phase in AMC because of its outstanding properties, including high mechanical strength, modulus, thermal and electrical conductivity (Stankovich et al. 2006; Novoselov et al. 2012; Zhu et al. 2010; Niteesh Kumar et al. 2017; Shin et al. 2015). Bartolucci et al. (2011) are among the pioneer researchers and introduced graphene into AMCs using ball milling in 2011. Graphene is normally added into the matrix in the form of graphene nanosheets (GNSs) with several to tens of layers (Asgharzadeh and Sedigh 2017; Prez-Bustamante et al. 2014; Nieto et al. 2016). Up to 5wt.% GNSs were incorporated into AA2124 alloy, and it was found that the hardness of the composite was increased by 102%; the wear rate decreased 25% with 9% reduction in coefficient of friction (El-Ghazaly et al. 2017). A wet method was utilised to mix aluminium with graphene in the study of Asgharzadeh et al. (2017), which showed that the yield strength and hardness were both enhanced. The possible strengthening mechanism for the GNSs reinforced AMCs were reported to be grain refinement, Orowan strengthening, stress/load transfer and increased dislocation density (Nieto et al. 2016). The strengthening effect of GNSs also highly depends on the uniform dispersion of GNSs among the metal grains. Mechanical milling involves the cold welding, fracturing and rewelding of particles, which is an effect way to uniformly disperse GNSs into aluminium matrix (Nieto et al. 2016; Hu et al. 2016; Suryanarayana and Al-Aqeeli 2013). In the literature, it is noted that Al-Si alloy is widely used in the fields of aerospace and automobile due to its high specific strength, good corrosion resistance and castability (Mazahery and Shabani 2012). However, this alloy is restricted in certain tribological applications owing to the low hardness and wear-resistance. GNSs are in the right place to improve the hardness and tribological behaviour, as GNSs are potential to boost the mechanical properties and are also tribology-favoured (Nieto et al. 2016).

The research on GNSs reinforced Al-Si alloy is still quite limited. The current study focuses on the synthesis and characterisation of GNSs reinforced A355 Al-Si alloy matrix composites. The effect of GNSs on the morphological and microstructural evolution of the composite powder has been investigated during the mechanical milling. The preferred orientation, lattice strain, crystalline size and micro hardness have been studied as well.

The morphologies of the starting materials are shown in Fig.1. The as-received aluminium powder is generally in spherical shape. Commercial A355 Al-Si alloy (Si: 4.6%, Cu: 0.8%, Mg: 0.51% and Fe: 0.15%) powder with an average particle size of 30m was supplied by Haoxi Nanotechnology. The GNSs are characterised with 15nm in thickness and ~5.0m in lateral size, which were bought from XFNANO Materials Tech. as shown in Fig.1(b) and (c).

The SEM morphologies of the as-received (a) aluminium powder. The morphology of the as-received GNSs observed using (b) SEM and (c) TEM. The inset shows a high-resolution micrograph of the lattice of the GNSs, with indicated number of layers

The powder mixture of Al alloy and 1.0 vol.% GNSs was milled in a planetary ball mill, which was carried out in a 500ml stainless steel jar. The confined powders were firstly ball milled at 180rpm for 0.5h for pre-mixing, and then at 250rpm for the following 20h under argon atmosphere. Samples were taken out at 2, 5, 10, 15 and 20h to investigate the effect of ball milling on the microstructure of the powder mixture. In a typical milling campaign, 300g of 5mm stainless steel balls was used with a ball to powder ratio of 15:1 in mass. Stearic acid (2wt.%) flakes were added to work as a process control agent. To avoid the overheating and sticking of the powder mixture, every 5min ball milling was followed with15 min rest in every milling cycle. Pure A355 powder was ball milled under the same conditions for reference.

The ball milled powders at different times were observed on the JEOL JSM-7500FA microscope with an acceleration voltage of 5kV. X-Ray diffraction (XRD) patterns were acquired using GBC MMA XRD diffractometer with Cu-K radiation from 25 to 85. The step size and scanning rate were 0.02 and 1.5 /min respectively. The crystalline size and lattice strain were analysed using William-Hall theory as follows (Wagih 2014):

The ball milled powders were cold pressed at 350MPa and then vacuum hot pressed under 50MPa at 500C for 60min to produce 20mm disks. The disks were degasified to remove the stearic acid at 400C for 2h before the hot pressing. The produced disks were grinded using abrasive papers and polished before the following characterisation. Vickers hardness was measured on a TIME TH715 micro-hardness tester under 9.8N with a dwell time of 10s. At least ten readings were taken for each sample to obtain the average value. Raman tests were conducted on a WITec alpha 300R confocal Raman microscope (532nm laser) to examine the distribution of GNSs. TEM samples were prepared using a FEI Helios nanoLab G3 CX dual beam microscope and then observed on a JEOL JEM-2011microscope.

The SEM micrographs of the ball milled Al alloy and Al-GNSs composite powders are shown in Figs.2 and 3 respectively. Aluminium is a relatively ductile phase in the ball milling system, while Si and GNSs particles are relatively brittle. The ductile aluminium particles are repeatedly flattened, cold welded, fractured and rewelded in the ball milling process (Suryanarayana and Al-Aqeeli 2013). As shown in Fig.2, the starting aluminium particles are in spherical shape with about 30m in diameter and gradually flatten into flakes from 2 to 10h. The lateral size of the aluminium flakes reaches around 80m at 10h. The flakes are fractured into smaller pieces at 15h as shown in Fig. 2(e) and rewelded into equiaxed particles at 20h. The relatively hard phase, Si, could accelerate the fracturing and rewelding, and is embedded into the aluminium matrix (Suryanarayana and Al-Aqeeli 2013). The presence of GNSs can further intensify the localised stress, and thus accelerates the flattening of aluminium powders as shown in Fig.3 (a) to (d). Aluminium flakes with more than 120m in lateral size can been seen at 10h. When the plastically deformed aluminium flakes are work-hardened to a critical level, the localized stress induced by GNSs will promote the fracture and rewelding of powders. As shown in Fig. 3 (e) to (f), aluminium powders are fractured and rewelded into relatively equiaxed shape at 15h and further fractured into smaller particles at 20h. As a result, the size of the Al-GNSs mixture is less than 20m, which is smaller than the size of the Al alloy powder (around 25m) after 20h of ball milling. The GNSs tend to become occluded and trapped in the aluminium particles, and finally get uniformly dispersed inside the matrix (Suryanarayana and Al-Aqeeli 2013).

The XRD patterns of the ball milled Al alloy and Al-GNSs powder mixtures at different milling times are shown in Figs.4 and 5 respectively, revealing the microstructural evolution of the powder mixing during the ball milling. It is seen that the peak intensity of aluminium decreases with the increase of milling times up to 20h. There is no obvious change for the peaks of Si, indicating no significant structural change for this relatively hard phase. As the concentration of GNSs is only 1.0 vol.%, the peak of GNSs is not distinguishable in XRD observation. For FCC metals, it has been indicated that the intensity ratio of (111) to (200), I(111)/ I(200), can reflect the change in crystallographic orientation of particles in the ball milling process. While for BCC metals, I(110)/ I(200) is used (Razavi-Tousi and Szpunar 2015; Alizadeh et al. 2011). As shown in Fig.6, the I(111)/ I(200) firstly drops to a minimum value and then increases to the initial level. This process is faster for the Al-GNSs composite due to the aforementioned localised stress induced by the addition of GNSs. This could be explained by considering the anisotropy in the elastic modulus of a single aluminium crystal (Alizadeh et al. 2011). To be specific, the aluminium grains/particles tend to be deformed in the soft direction (111), which is perpendicular to the collision direction of milling balls. When the powder sample is prepared for the XRD analysis, the flattened flakes arrange themselves parallel to the sample holder. As a result, I(111) decreases and I(200) increases, which is the case for the powders from 2 to 10h. With further ball milling, the flattened particles are fractured and rewelded into equiaxed particles, which means the texture and the preferential orientation are eliminated from 15 to 20h. Meanwhile, the I(111)/I(200) recovers to the initial level. This evolution behaviour agrees very well with the SEM observation results as shown in Figs. 2 and 3, which show the morphological change of the powders.

It is also seen in Figs. 4 and 5 that peak broadening is caused as the milling process progresses, which indicates the refinement of crystalline grains and the generation of lattice strain. The mean crystalline size and lattice strain can be evaluated using the William-Hall theory, and are illustrated versus milling time in Figs.7 and 8 respectively. It is shown in Fig.7 that the crystalline size of aluminium decreases quickly during the initial 5h and decrease slowly in the following milling process. In addition, the crystalline size of Al-GNSs composite is smaller than that of Al alloy at the same milling time, which could be attributed to the intensified stresses by the GNSs. This also causes the increased lattice strain during the ball milling as shown in Fig.8. The GNSs accelerate the deformation of the crystalline lattice and promote the lattice strain rate of Al-GNSs composite.

Figure9 shows the TEM microstructure of the produced bulk samples. For the Al alloy sample, most of the coarse grains are in flake-shaped with an average grain size of about 1m. It is also noticed that there is a small portion of fine grains (around 100nm). This means that the microstructure of Al alloy sample is not uniform, which could be caused by the insufficient deformation during ball milling. The microstructure of the Al-GNSs composite presented in Fig. 9 (c) shows that the addition of 1vol.% GNSs dramatically reduce the grain size and the size of the grains is quite similar (app. 100nm). The grain refinement is firstly attributed to the intensified deformation during ball milling, which greatly reduces the grain size and gets the GNSs well dispersed. Highly deformed regions are marked in Fig. 9(b), where also feature the concentrated sites of dislocations. Secondly, the incorporation of the thin GNSs largely decreases the interplanar distance between GNSs, which could perform pinning effect and restrain the grain growth during hot pressing. It is challenging to directly observe the GNS in the bulk sample using TEM due to the ultrathin profile of the GNSs and the interference from the matrix. Raman spectroscopy is sensitive to carbonaceous materials and offers a reliable tool to probe the GNSs (Ferrari and Basko 2013). The Raman scanning results of G band are shown in Fig. 9 (c), in which the bright areas represent the presence of GNSs. It is seen that GNSs are well distributed among aluminium matrix.

TEM micrographs of the produced (a) Al alloy, and (b) Al-GNSs composite. Grains are indicated using dashed circles; highly deformed regions are marked using solid circles. c The Raman mapping of G band on the Al-GNSs composite

As shown in Fig.10, the hardness of the hot-pressed Al alloy averages at 81.2HV. The hardness of Al-GNSs composite is 115.1% higher and reaches 174.7HV. This can be understood by considering the presence of GNSs, grain refinement, Orowan strengthening mechanism, the effect of thermal mismatch. The presence of GNSs is difficult to deform and hinders the movement of dislocation (Prez-Bustamante et al. 2014; Liu et al. 2016). The grain size of the composite is refined and more uniform, which contributes to the increased dislocation and hardness as well. In this regard, Hall-Petch equation was derived to express the relationship between grain size and hardness as follows by combining with Tabors empirical relationshipH3. (Petch and Iron Steel 1953; Moon et al. 2008)

where H is the overall hardness, and H0 is the hardness of the matrix. k is a modified locking parameter, 0.068MPam-0.5 (Boostani et al. 2015). The HHallpetch is estimated to be 65.8HV by taking the grain size as 100nm for the composite.

where\( {H}_0^{\ast } \) is the intrinsic hardness of the matrix. HOrowan, HCTE, andHL represent the contribution from the Orowan strengthening, thermal mismatch mechanism and load-bearing effect, respectively.

where G and b represent the shear modulus of the aluminium matrix (26.2GPa (Khodabakhshi et al. 2017)) and Burgers vector (0.286nm (Khodabakhshi et al. 2017)) respectively; and is the interparticle spacing between the dispersoids (taken as 100nm). The HOrowan is calculated to be 39.7HV.

Another contributing mechanism is from the thermal mismatch between the GNSs (1106 /K) and the aluminium matrix (~23.6106 /K). The hardness increase HCTE could be expressed as (Khodabakhshi et al. 2017):

where is a proportional constant (~1.25), T is the temperature difference between the sintering temperature and the ambient temperature (475K). fv is the volume fraction of GNSs (1%). The particle size is selected to be 5m. It is estimated that the contribution from this mechanism is limited and only 2.7HV is obtained.

where m represents the yield strength of the matrix. m is not measured in this study, but normally falls in the range of 100300MPa for the aluminium alloy. The HL is estimated to be less than 1HV, which is neglectable.

Therefore, the hardness of the Al-GNSs composite is predicted to be 189.4HV by taking all these effects into account, which is higher than the measured value of 174.4HV. This could be owing to the presence of defects (such as pores), agglomeration of GNSs, and simplified expressions in Eqs. (2)(6).

The ductile aluminium particles are firstly flattened at the initial stage of the ball milling, and then fractured and rewelded into equiaxed particles. The addition of the GNSs accelerates the flattening and fracturing, and a smaller particle size is achieved for the composite powder.

The I(111)/I(200) firstly falls to a minimum value and then recovers to the initial level, indicating the creation and elimination of texture during the ball milling, which is consistent with the SEM observation results.

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The authors acknowledge use of the facilities at the UOW Electron Microscopy Centre. We appreciate Dr. Monika Wyszomirska who performed the TEM sample preparation and helped interpret the observed results.

J Zhang carried out most of the experiments and wrote the manuscript. ZC, J Zhao and ZJ fixed this topic and helped on experimental design, discussion and manuscript revision. All authors read and approved the final manuscript.

Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Zhang, J., Chen, Z., Zhao, J. et al. Microstructure and mechanical properties of aluminium-graphene composite powders produced by mechanical milling. Mech Adv Mater Mod Process 4, 4 (2018). https://doi.org/10.1186/s40759-018-0037-5

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