Longer Milling Times Research Articles

Evaluation of process behavior and crystallite specifications of mechano-chemically synthesized WC–Al2O3 nano-composites

A new route for the synthesis of WC–Al2O3 composite free from unwanted W2C phase through mechano-chemical treatment of WO3–xAl–y(4−1.5x)C mixture has been investigated. It has been concluded that by substitution of 10mol% of 2mol Al (x=1.8) with equivalent stoichiometric amount of C together with 20mol% excess carbon addition (y=1.2), i.e., using a WO3:1.8Al:1.56C initial mixture, W2C free WC–Al2O3 composite can be successfully obtained by about 6h milling. Crystallite size and lattice strain of the WC–Al2O3 composite synthesized in the present study as well as those obtained by the previous successful routes have been reported. The results revealed that a powder whose synthesis route involves lower adiabatic temperature as well as longer milling time would have a smaller crystallite size and higher lattice strain. Furthermore, TEM observations showed that all synthesized powders contain the agglomerates which consist of nano-size particles.

Synthesis of nanostructure tetragonal ZrO2 by high energy ball milling

Phase transformation of monoclinic ZrO2 to tetragonal and cubic ZrO2 was successfully performed during ball milling. X-ray diffraction and high temperature in situ X-ray diffraction were performed to study the progressing of this transformation. Quantity phase analysis, crystallite size and microstrain were calculated by the Rietveld refinement method. Morphology and microstructure were monitored by scanning and transmission election microscopy respectively. The results showed that this transformation was initiated after 30 h milling and progressed gradually at longer milling times. Longer milling led to the smaller crystallites with larger microstrain, which were proposed as the mechanism of stabilisation of tetragonal and cubic ZrO2 at room temperature. A nanostructure powder with a mean crystallite size of <10 nm was obtained at the end of milling on the basis of the Rietveld method, which was confirmed by transmission electron microscopic images.

Homogeneous nanoparticle dispersion prepared with impurity-free dispersant by the ball mill technique

The homogeneous dispersion of nanoparticles in solvents or polymer matrices is essential for practical application of nanocomposites. In this study, the planetary ball milling technique was used to de-agglomerate silica nanoparticles in butyl acetate. The size of the nanosilica aggregates was evaluated by TEM and SEM. With the addition of polyacrylate polymer to the organic solvent, the nanoparticle agglomerates were effectively broken up by planetary ball milling at the proper milling time; however, re-agglomeration occurred after a longer milling time. The results of TGA and FTIR indicated that the polyacrylate molecules could be adsorbed in situ onto the nanoparticles. Behaving similar to a dispersant, the adsorbed polyacrylate reduced the blend viscosity significantly and prevented re-agglomeration of the nanoparticles. Utilizing the polyacrylate polymer both as the dispersant and the polymer matrix, the polyacrylate-based nanocoatings were further prepared. The optical transmittance and haze value of the nanocoatings were found to be sensitive to the dispersion level of the nanoparticles, and the elastic modulus and hardness of the nanocoatings were improved in comparison with those of the neat polymer coating.

Effect of process parameters on micro and macro-properties of an Al-based nanocomposite prepared by means of mechanical milling

Abstract In this research, some Al-based composites reinforced with silica nanoparticles were fabricated using mechanical milling and processing by powder metallurgy. Density, morphology evolution, hardness and strength of the composites were studied as a function of milling time and additive concentration. Microstructural evidence reveals that milling conditions generated a uniform dispersion of nanoparticles in the Al matrix. Noticeable matrix grain refinement was induced by the milling process. An optimum mechanical response was reached with small amounts of reinforcement nanoparticles complemented with medium milling intensities. These manufacturing conditions allowed to achieve an improvement of mechanical properties compared with the reference material (pure milled Al and un-milled composites processed under the same conditions). On the other hand, an adverse effect was observed with longer milling time and higher additive concentration.

Mechanical Milling Behavior and Characteristics of Bi2Te3 Thermoelectric Materials

Abstract In this paper, starting from the gas atomization powders, n-type Bi2Te3 thermoelectric alloys prepared via high energy ball milling and subsequently consolidated by hot extrusion. The effect of ball milling on the microstructure of Bi2Te3 alloys was studied. With increasing the milling time, from 3 min to 90 min, the particles were coarsening due to the agglomerated because of the high surface energy of the fine particles adhesive each other during the high energy ball milling. The preferential orientations of grains indicate that the extrusion process has the significant method to forcing the basal plane to orient preferentially in the extrusion direction were clearly observed by the TEM micrograph. The maximum electrical conductivity 6675.567 Ω−1m−1 measured at 100 °C temperature owing to the increasing of carrier concentration due to the donor defect formation and contamination occurred during the high energy ball milling at longer milling time (90 min).

Thermodynamic analysis of solid solution formation in the nanocrystalline Fe–Ti–Al ternary system during mechanical alloying

The nanocrystalline (Fe,Ti)3Al intermetallic compound was synthesized by mechanical alloying (MA) of elemental powders with the composition of Fe50Al25Ti25. The structural changes of powder particles during MA were studied by X-ray diffractometry. It was found that at early stages of MA, a nanocrystalline Fe(Ti, Al) solid solution is formed. This structure was transformed to disordered (Fe,Ti)3Al intermetallic compound at longer milling times. Thermodynamic analysis of this system revealed that in this ternary system, the least thermodynamic driving force for solid solution formation occurred for those compositions that contained 90% of one element. Fe substitution by Ti in Fe3Al compound decreased the Gibbs free energy change, resulting in a more stable structure.

Characterization of nano-structured Cu–6 wt.% Zr alloy produced by mechanical alloying and annealing methods

The mechanical alloying process is used to produce nano-structured solid solution alloys, which may be further strengthened by the age-hardening process. Milling time is an important parameter during mechanical alloying. In the present research, the optimum milling time for the production of Cu–6wt.% Zr alloy was to be determined. Cu and Zr powder mixtures were milled in a planetary ball mill for different milling times. As milled powder mixtures were investigated by SEM and XRD. Average particle sizes, lattice parameter and average crystal size were determined at different milling times. As milled powder mixtures were subsequently isothermally annealed at different temperatures and their microhardness was measured. The results showed that during milling, average particle size initially increased to a maximum value and decreased at longer times, while average crystal size continuously decreased with milling time. Lattice parameter increased with milling time. The maximum hardness value was obtained after 48h of milling and annealed at 600°C. Longer milling times did not improve hardness of the material.

In-Situ Synthesis of Alumina Reinforced (Fe,Cr)3Al Intermetallic Matrix Nanocomposite

In this study, a mixture of chromium oxide, elemental iron, and aluminum powders was subjected to high energy ball milling to produce Al2O3 reinforced (Fe,Cr)3Al matrix nanocomposite. The structural changes of powder particles during milling were investigated by X-ray diffractometry. Cross-sectional microstructure of powder particles after different milling times was observed by scanning electron microscopy using back scatter mode. The change in morphology of powder particles was followed by scanning electron microscopy observations. The transmission electron microscopy was used to confirm the development of nano-sized structure in the final product. The changes in phase composition, structure and morphology of powder particles suggested that during milling, Al gradually reacts with Cr2O3 leading to the formation of metallic Cr and Al2O3 phases and concurrently, the reduced Cr and remaining Al diffuse into the Fe lattice and as a result, a Fe(Al,Cr) solid solution forms. This structure was transformed to (Fe,Cr)3Al intermetallic compound at longer milling times.

Influence of ball milling parameters on the structure of the mechanically alloyed SiBCN powder

SiBCN powders were prepared by an improved mechanical alloying technique, using cubic silicon, graphite and hexagonal boron nitride powders as raw materials. The influences of ball milling parameters on the structures and the particle size distribution of the prepared powders were carefully studied by XRD, HRTEM, EFTEM and particle-size analysis. Results show that well amorphized SiBCN powder can be prepared when the rotation speed of the vials is between 600 and 800rpm, the ball-to-powder mass ratio is larger than 20:1, and the milling time is longer than 20h. A higher rotation speed and a larger ball-to-powder mass ratio may enhance the power input into the powder, and hence facilitate the formation of SiC crystallites. While a longer milling time has little influence on the powder microstructure, it promotes the reaction between silicon and carbon atoms. The optimized ball milling parameters are 600rpm, 20:1, 30 or 40h, respectively. The prepared SiBCN powders mainly consist of near-spherical agglomerates, 4.6–5.3μm in the average particle size. The particle size distribution of these powders is slightly affected by the variation of ball milling parameters.

Real time indicators of material refinement in an attritor mill

Power, torque, and rotation speed of the impeller of an attritor mill were measured during preparation of metal matrix composite powders. Compositions of Al·MgO and Al·B4C were selected to include ductile (Al) and brittle components. Brittle components, MgO, and B4C, were respectively softer and harder than the milling media (hardened steel). Yield strength of pressed powder compacts and x‐ray diffraction peak width were treated as indicators of the achieved structural refinement. Both peak width and yield strength increased for both materials at longer milling times. Torque and power increased for Al·MgO but decreased for Al·B4C at longer milling times. The results were interpreted considering differences in hardness of MgO and B4C. Amplitudes of rapid oscillations of the milling process parameters were also monitored and were observed to correlate with the material refinement. Utility of real‐time milling progress indicators for different types of materials is demonstrated. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1088–1095, 2013

Structural Evaluation of Mechanically Alloyed and Heat-Treated Ti-25at-%Si Powders

This work discusses on the structural evaluation of mechanically alloyed and heat-treated Ti-25at%Si powders. The milling process was conducted in a planetary ball mill using stainless steel balls/vials, 200 rpm and ball-to-powder weight ratio of 5:1, whereas the heat treatment was conducted under Ar atmosphere at 1100°C for 4 h. Samples were characterized by X-ray diffraction, differential scanning calorimetry, scanning electron microscopy and energy dispersive spectrometry. The Si peaks disappeared after milling for 30h, indicating that the Si atoms were dissolved into the Ti lattice in order to form an extended solid solution. The Ti peaks were broadened and their intensities reduced for longer milling times whereas a halo was formed in Ti-25Si powders milled for 200h suggesting that an amorphous structure was achieved. The crystallite size was decreased with increasing milling times. A large Ti3Si amount was found in mechanically alloyed Ti-25at%Si powders after heating at 1100°C for 4h.

Thermodynamic analysis of nanocrystalline and amorphous phase formation in Nb–Al system during mechanical alloying

Mixtures of pure elemental Al and Nb powders of Nb–75 at-%Al and Nb–25 at-%Al composition were mechanically alloyed. Phase transformation of the powder mixtures was investigated by X-ray diffraction technique. It was found that mechanical alloying of the Nb–75 at-%Al powder mixture led to the formation of Nb(Al) solid solution and subsequent formation of DO22-NbAl3 intermetallic compound, and no amorphous phase was formed after longer milling times. The results showed that mechanical alloying of Nb–25 at-%Al for 8 h led to the formation of amorphous phase, and Nb3Al intermetallic was not formed during milling. A thermodynamic analysis was carried out based on the semiempirical theory of Miedema. This analysis showed that there is a driving force for the Nb–Al binary to form a solid solution for all compositions while the amorphous phase is stable in the range of 0·1⩽xAl⩽0·95.

Preparation and Crystallization of Nd&lt;sub&gt;2&lt;/sub&gt;Fe&lt;sub&gt;14&lt;/sub&gt;B/&lt;i&gt;α&lt;/i&gt;-Fe Permanent Magnetic Nanomaterial by Mechanical Milling

Nanocomposite Nd2Fe14B/α-Fe magnetic material had been prepared by mechanical milling Nd8Fe86B6 as-cast alloy in Argon and subsequent crystallization. Effect of mechanical milling and crystallization technique on its structure had been investigated by means of X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM), Transmission Electron Microscopy (TEM) and Differential Scanning Caborimetry (DSC), etc. The crystallization behavior of amorphous Nd8Fe86B6 powders had also been studied. The results showed that Nd2Fe14B grain refined quickly and the grain size of α-Fe decreased while extending milling time. After milling for 25h, grain size of α-Fe reached a constant (about 7nm). Higher temperature of complete crystallization as well as finer and more homogeneous grains resulted from longer milling time. During the process of crystallization, the total amorphous phase crystallized into four phases firstly, i.e., Nd4.4Fe77.8B17.8, Nd1.1Fe4B4, Nd2Fe14B and α-Fe. At higher temperature, final mixture was consisted of Nd2Fe14B, α-Fe and a few undecomposed Nd1.1Fe4B4 phase.

Formation mechanism and characterization of nanostructured Ti6Al4V alloy prepared by mechanical alloying

In recent years, researches on properties of nanocrystalline materials in comparison with coarse-grained materials have attracted a great deal of attention. The present investigation has been based on production of nanocrystalline Ti6Al4V powder from elemental powders by means of high energy mechanical milling. In this regard, Ti, Al and V powders were milled for up to 90h and heat treated at different temperatures. The structural and morphological changes of powders were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential thermal analysis (DTA) and microhardness measurements. The results demonstrated that Ti(Al) and Ti(Al, V) solid solutions with grain size of 95 and 20nm respectively form during mechanical alloying. In addition, an amorphous structure was obtained at longer milling times. The crystallization of amorphous phase upon annealing led to the formation of nanostructured Ti6Al4V phase with a grain size of 20–50nm. The as-milled Ti6Al4V powder with amorphous structure exhibited a high microhardness of ∼720Hv. Upon crystallization the hardness value reduced to ∼630Hv which is higher than those reported for Ti6Al4V alloys processed by conventional routes.

Structural characterization of mechanically alloyed nanocrystalline Cu-Fe: Strain broadening due to dislocations

Nanocrystalline Cu(Fe) solid solution was successfully synthesized by using high-energy mechanical milling. The structural and morphological changes during mechanical milling were investigated by X-ray diffraction and scanning electron microscopy. The patterns so obtained were analyzed using the X'Pert High Score Plus program. The final product of the mechanical alloying process was nanocrystalline FCC Cu(Fe) solid solution with a mean crystallite size in the range of few nanometers. The final microstructure, especially the high levels of lattice strains was explained by the presence of dislocations, with a dislocation density of about 7.4×10 16 m �2 . The identified steady-state saturation values of these parameters can be related to accumulate strain hardening of the powder material during longer milling times.

Effect of Milling Time on the Physical and Mechanical Properties of Celsian-Mullite Composites Synthesized from Coal Fly Ash

ABSTRACTFour Celsian (Ba0.75Sr0.25Al2Si2O8)/Mullite (Al6Si2O13) composites, with potential structural applications at high temperatures, are synthesized from coal fly ash (byproduct of a Mexican coal-burning power plant, constituted mainly by SiO2 and Al2O3). Nominal Celsian/Mullite weight ratios studied are 80/20, 60/40, 40/60 and 20/80. Mullite is synthesized separately at 1600ºC/2h and then mixed with a Celsian precursor mixture previously calcined at 900°C/5h. During this process the Celsian phase is formed by a solid state reaction at 1100-1400ºC/5h. Prior to this, the reacting mixture is milled in a planetary mill during 1 or 2h and then compacted by uniaxial and cold isostatic pressing. The microstructure and phase composition of the synthesized composites are characterized by X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM/EDS). Their dynamic Young’s modulus is measured by an ultrasonic technique, and their mechanical strength is evaluated from flexural tests carried out at room temperature. The expected phases are obtained in all cases, although with some differences with respect to their expected relative proportions, according to the studied nominal compositions. In general, the longest milling time employed produced samples with the largest degree of crystallinity and density, as well as with the best microstructural characteristics and mechanical properties.

Synthesis of Fine Co/ WC Composite and Application to Supersonic Plasma Spraying

Co/ WC composite powders (with 10 wt% content of Co) were synthesized by direct mechanical grinding in a rotary-vibration mill. The powders with different mill time were evaluated. WC and WC/Co composite coatings were prepared by supersonic plasma spraying. The results showed that the milled powders consist of composite particles that were formed in the first 2h of milling. Longer milling times improve the distribution of phases inside the composite particles. The formation of the composite particles involves sequential steps of deformation, fragmentation, cold welding, work hardening and piercing of particles of the hard phase in the soft phase. X-ray spectra of the sprayed coating are shown that only very weak W2C and Co6W6C peaks are observed. Compared with WC coating, the Co/ WC coating is denser, and less large pores within composite coating.

Potassium Zinc Borohydrides Containing Triangular [Zn(BH4)3]− and Tetrahedral [Zn(BH4)xCl4–x]2– Anions

Three novel potassium-zinc borohydrides/chlorides are described. KZn(BH 4) 3 and K 2Zn(BH 4) xCl 4-x form in ball-milled KBH 4:ZnCl 2 mixtures with molar ratios ranging from 1.5:1 up to 3:1. On the other hand, K 3Zn(BH 4) xCl 5-x forms only in the 2:1 mixture after longer milling times. The new compounds have been studied by a combination of in situ synchrotron powder diffraction, thermal analysis, Raman spectroscopy, and the solid state DFT calculations. Rhombohedral KZn(BH 4) 3 contains an anionic complex [Zn(BH 4) 3] - with D 3 (32) symmetry, located inside a rhombohedron K 8. KZn(BH 4) 3 contains 8.1 wt % of hydrogen and decomposes at ∼385 K with a release of hydrogen and diborane similar to other Zn-based bimetallic borohydrides like MZn 2(BH 4) 5 (M = Li, Na) and NaZn(BH 4) 3. The decomposition temperature is much lower than for KBH 4. Monoclinic K 2Zn(BH 4) xCl 4-x contains a tetrahedral complex anion [Zn(BH 4) xCl 4-x] 2- located inside an Edshammar polyhedron (pentacapped trigonal prism) K 11. The compound is a monoclinically distorted variant of the paraelectric orthorhombic ht-phase of K 2ZnCl 4 (structure type K 2SO 4). K 2Zn(BH 4) xCl 4-x releases BH 4 starting from 395 K, forming Zn and KBH 4. As the reaction proceeds and x decreases, the monoclinic distortion of K 2Zn(BH 4) xCl 4-x diminishes and the structure transforms at 445 K into the orthorhombic ht-phase of K 2ZnCl 4. Tetragonal K 3Zn(BH 4) xCl 5-x is a substitutional and deformation variant of the tetragonal (I4/mcm) Cs 3CoCl 5 structure type possibly with the space group P4 2/ncm. K 3Zn(BH 4) xCl 5-x decomposes nearly at the same temperature as KZn(BH 4) 3, i.e., at ∼400 K, with the formation of K 2Zn(BH 4) xCl 4-x and KBH 4, indicating that the compound is an adduct of the two latter compounds. © 2011 American Chemical Society.

Effect of Milling Time and Dispersant on Microstructures of Alpha Alumina Nanopowder Synthesized from Aluminium Dross Waste

The paper presents a study on the effect of milling time and addition of SLS dispersant on the crystallite and particle size reduction of α-alumina produced from aluminium dross waste. Wet milling was performed in high velocity planetary mill at speed of 1100 rpm and milled for 2 to 5 hours. XRD analysis shows both peak broadening and shifting occurred especially at longer milling time. The peak broadening effect was then used to calculate both the crystallite size and lattice strain. Samples milled with dispersant gave smaller crystallite size and the smallest crystallite value was 39.4nm obtained using 5 hours milling time. Similarly the mean particle size obtained for samples milled with dispersant gave smaller value and the smaller mean particle size obtained was 0.771 μm after 5 hours of milling.

Preparation of C/Cu Composite Powders by High-Energy Ball-Milling

Cu-4% mixed-powder consisting of rough copper powder and graphite powder was separately mechanical alloyed by high-energy ball milling. The phases and micrograph of these powders were determined by X-ray diffraction and scanning electron microscopy (SEM). The results show an increase in the lattice parameter of copper with milling times, up to a saturation value of about 24h; There was an absence of graphite reflections from X-ray diffractograms after longer milling times.