Synthesis of Sn–Ag binary alloy powders by mechanical alloying

Sn–Ag binary powders of 2–5 wt%Ag were synthesized by mechanical alloying. Structural evolutions, morphologies, particle size distributions and melting points of the milled Sn–Ag powders were studied. The results show that the milled Sn–Ag powders consist of a supersaturated solid solution of Ag in Sn, Sn(Ag), and Ag₃Sn. During ball milling, Sn, Ag particles in the Sn–3.5Ag powders are deformed, overlapped and cold-welded together to form the Sn/Ag composite particles with a lamellar structure, and then the composite particles are fractured into small spherical particles. When increasing the Ag content from 2 to 5 wt%, the average particle sizes of the 60 h milled Sn–Ag powders are changed from 2.2 to 5.7 μm, and the morphologies of them are changed from spherical shape to irregular shape, respectively. It indicates that the cold-welding and agglomeration of the Sn–Ag powders increases with the Ag content during MA. The melting point of the 60 h milled Sn–3.5Ag powders was detected to be 224.23 °C, near to the eutectic point of the Sn–Ag binary system (221 °C).     

Preparation of NiAl–TiC nanocomposite by mechanical alloying

NiAl–TiC nanocomposite was successfully synthesized via a ball-milled mixture of Ni, Al, Ti, and graphite powders. The structural and morphological evolutions of the powders were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Results show that NiAl–TiC composite was obtained after 6 h of milling. The mean grain sizes of 6 and 10 nm were attained for NiAl and TiC at the end of milling, respectively. An annealing of 3 h milled sample at 600 °C led to the formation of Ni (Al, Ti, C) solid solution. NiAl–TiC nanocomposite that was formed in the 12 h milled sample is stable during an annealing at 600 °C. The mean grain size of NiAl at the 12 h milled powder increased during annealing at 600 °C. Maximum micro hardness value of 870 kg/mm² (8.7 GPa) was acquired from the 12 h milled powder. SEM images and particle size measurement showed that very fine spheroid particles (1 μm) were procured at the end of milling.     

Effect of cup and ball types on alumina–tungsten carbide nanocomposite powder synthesized by mechanical alloying

Alumina-based nanocomposite powders with tungsten carbides particulates were synthesized by ball milling WO₃, Al and graphite powders. X-ray Diffraction (XRD) was used to characterize the milled and annealed powders. Microstructures of milled powders were studied by Transmission Electron Microscopy (TEM). Results showed that Al₂O₃–W₂C composite formed after 5 h of milling with major amount of un-reacted W in stainless steel cup. The remained W was decreased to minor amount by increasing carbon content up to 10 wt.%. When milled with ZrO₂ cup and balls, Al₂O₃–W₂C composite was completely synthesized after 20 h of milling with the major impurity of ZrO₂. In the case of stainless steel cup and balls with 10 wt.% carbon, Fe impurity after 5 h of milling (maximum 0.09 wt.%) was removed from the powder by leaching in 3HCl·HNO₃ solution. The mean grain size of the powder milled for 5 h was less than 60 nm. The powder preserved its nanocrystalline nature after annealing at 800 °C.     

Formation mechanism of B4C–TiB2–TiC ceramic composite produced by mechanical alloying of Ti–B4C powders

In this study, the B₄C–TiB₂–TiC composite powder was synthesized by mechanical alloying (MA) of Ti–B₄C powder mixture. For this purpose, four powder mixtures of Ti and B₄C powders with different molar ratios were milled. In order to study the mechanism of Ti–B₄C reaction during milling, structural changes and thermal analysis of powder particles were studied by X-ray diffractometry (XRD) and differential thermal analysis (DTA). Morphology and microstructure of powder particles during milling were studied by scanning electron microscopy (SEM). It was found that during MA, after decomposition of the outer layers of B₄C particles, first, C reacted with Ti and after that, B was diffused in Ti structure and TiC and TiB₂ phases were formed in gradual reaction mode. Also, the results of DTA and thermodynamic analysis confirmed the suggested mechanism for Ti–B₄C reaction.     

Fabrication of Al–Si coating on Ti–6Al–4V substrate by mechanical alloying

Al–Si coatings were synthesized on Ti–6Al–4V alloy substrate by mechanical alloying with Al–Si powder mixture. The as-prepared coatings had composite structures. The effects of Al–Si ratio, milling duration and rotational speed on the microstructure and oxidation behavior of coating were investigated. The results showed that the continuity and the anti-oxidation properties of the coating were enhanced with the increase of Al–Si weight ratio. The thickness of the coating largely increased in the initial 5-hour milling process and decreased with further milling. A rather long-time ball milling could result in the generation of microdefects in coating, which had an adverse effect on the oxidation resistance of coating. Both the thickness and the roughness of the coating increased with the raise of rotational speed. The low rotational speed would lead to the formation of discontinuous coating. The rotational speed had a limited effect on the coating oxidation behavior. Dense, continuous and high-temperature protective Al–Si coatings could be obtained by mechanical alloying with Al–33.3?wt.%Si powder at the rotational speed ranging from 250 to 350?rpm for 5?h.     

Production of Fe3Al–TiC nanocomposite by mechanical alloying of FeTi2–Al–C powders

Abstract

The aim of the present work was to produce Fe₃Al/TiC nanocomposite by mechanical alloying of the FeTi₂30Al10C60 (in at-%) powder mixture. The morphology and the phase transformations in the powder during milling were examined as a function of milling time. The phase constituents of the product were evaluated by X-ray diffraction (XRD). The morphological evolution during mechanical alloying was analysed using scanning electron microscopy (SEM). The results obtained show that high energy ball milling, as performed in the present work, leads to the formation of a bcc phase identified as Fe(Al) solid solution and an fcc phase identified as TiC and that both phases are nanocrystalline. Subsequently, the milled powders were sintered at 873 K. The XRD investigations of the powders revealed that after sintering, the material remained nanocrystalline and that there were no phase changes, except for the ordering of Fe(Al), i.e. formation of Fe₃Al intermetallic compound, during the sintering process.     

Thermal stability and mechanical properties of nanocrystalline Fe–Ni–Zr alloys prepared by mechanical alloying

The thermal stability of nanostructured Fe_100?x?y Ni _x Zr _y alloys with Zr additions up to 4 at.% was investigated. This expands upon our previous results for Fe–Ni base alloys that were limited to 1 at.% Zr addition. Emphasis was placed on understanding the effects of composition and microstructural evolution on grain growth and mechanical properties after annealing at temperatures near and above the bcc-to-fcc transformation. Results reveal that microstructural stability can be lost due to the bcc-to-fcc transformation (occurring at 700 °C) by the sudden appearance of abnormally grown fcc grains. However, it was determined that grain growth can be suppressed kinetically at higher temperatures for high Zr content alloys due to the precipitation of intermetallic compounds. Eventually, at higher temperatures and regardless of composition, the retention of nanocrystallinity was lost, leaving behind fine micron grains filled with nanoscale intermetallic precipitates. Despite the increase in grain size, the in situ formed precipitates were found to induce an Orowan hardening effect rivaling that predicted by Hall–Petch hardening for the smallest grain sizes. The transition from grain size strengthening to precipitation strengthening is reported for these alloys. The large grain size and high precipitation hardening result in a material that exhibits high strength and significant plastic straining capacity.     

Synthesis of Cu–Zr–Al/Al2O3 amorphous nanocomposite by mechanical alloying

Two routes were used to produce Cu–Zr–Al/Al₂O₃ amorphous nanocomposite. First route included mechanical alloying of elemental powders mixture. In second route Cu₆₀Zr₄₀ alloy was synthesized by melting of Cu and Zr. Cu₆₀Zr₄₀ alloy was then ball milled with Al and CuO powder. It was not possible to obtain a fully amorphous structure via first route. The mechanical alloying of Cu₆₀Zr₄₀, Al and CuO powder mixture for 10 h led to the reaction of CuO with Al, forming Al₂O₃ particulate, and concurrent formation of Cu₆₂Zr₃₂Al₄ amorphous matrix. The thermodynamical investigations on the basis of extended Miedema’s model illustrated that there is a strong thermodynamic driving force for formation of amorphous phase in this system. Lack of amorphization in first route appeared to be related to the oxidation of free Zr during ball milling.     

Effects of TiB2 on microstructure of nano-grained Cu–Cr–TiB2 composite powders prepared by mechanical alloying

Nano-grained CuCr25 and CuCr25–(2 wt%–10 wt%)TiB₂ composite powders were prepared by mechanical alloying method. The milled powders were characterized by SEM, FSEM, EDS, XRD, TEM and HTEM. The results indicate that average grain size of Cu, Cr and TiB₂ are less than 50 nm and each component disperses uniformly. The grain size of Cu and Cr decreased and the lattice distortion increased gradually as the TiB₂ content increased from 2 wt% to 10 wt%. The ultrafine TiB₂ particles play the role of “micro-milling balls” to compact, micro-etch, micro-frict and micro-cut with Cu and Cr so that the grain refinement and lattice strain are promoted.     

Synthesis of CuNbC nanocomposites by mechanical alloying

Elemental Cu and NbC powders were mechanically alloyed with either graphite or hexane to form nanocomposite CuNbC powders. These powders were consolidated by hotisostatic-pressing at one-third of the melting temperature of NbC to form a 100% dense compact, while maintaining a crystallite size well within the nano-regime.     

Structure and ferroelectric properties of barium titanate films synthesized by sol–gel method

The barium strontium titanate (Ba_0.7Sr_0.3TiO₃, BST) thin films were synthesized by a sol–gel technique on a silicon nanoporous pillar array (Si-NPA) substrate. SEM observation reveals that the as-prepared BST thin film has uniformly covered the inherited pillar-like surface of the Si-NPA substrate. X-ray diffraction analysis indicates that the perovskite phase was able to be generated in the BST film when the annealing temperature was higher than 600 °C. The remnant polarization (Pr) and coercive field (Ec) values were also found to increase with the annealing temperature, with the maxima of 4.57 μC cm⁻² for Pr and 7.61 kV mm⁻¹ for Ec at 800 °C, respectively. The measurement of leakage current density against voltage applied suggested that the BST films are excellent insulators along with fair resistance to breakdown, and the mechanism of leakage current was discussed.     

Amorphous Ti–Cu–Ni–Al alloys prepared by mechanical alloying

Ti _x (CuNi)_90–x Al₁₀ (x = 50, 55, 60) amorphous powder alloys were synthesized by mechanical alloying technique. The evolution of amorphization during milling and subsequent heat treatment was investigated by scanning electron microscopy, X-ray diffraction, differential scanning calorimetry and transmission electron microscopy. The fully amorphous powders were obtained in the Ti₅₀Cu₂₀Ni₂₀Al₁₀, Ti₅₅Cu_17.5Ni_17.5Al₁₀ and Ti₆₀Cu₁₅Ni₁₅Al₁₀ alloys after milling for 30, 20 and 15 h, respectively. Differential scanning calorimetry revealed that thermal stability increased with the increasing (CuNi) content: Ti₆₀Cu₁₅Ni₁₅Al₁₀, Ti₅₅Cu_17.5Ni_17.5Al₁₀ and Ti₅₀Cu₂₀Ni₂₀Al₁₀. Heating of the three amorphous alloys at 800 K for 10 min results in the formation of the NiTi, NiTi₂ and CuTi₂ intermetallic phases.     

Effects of stearic acid on synthesis of nanocomposite WC–MgO powders by mechanical alloying

The effects of stearic acid as a process control agent (PCA) on the synthesis of WC–MgO by mechanical alloying have been investigated. 0–2.0 wt% of stearic acid is added into the mixture of WO₃, Mg, and graphite powders in high-energy planetary ball milling experiments, and the as-milled powders are characterized by XRD and TEM. Results show that the mechanochemical reaction among WO₃, Mg, and graphite to form WC–MgO can be changed from a mechanically induced self-propagating reaction (MSR) to a gradual reaction by the addition of stearic acid in the range from 1.2 to 1.8 wt%, when other milling parameters are maintained at the same level. It has also been found that with the addition of stearic acid, the crystallite and particle size of WC–MgO powders can be refined, the homogeneity of particle size can be improved and the powder yield can be increased.     

Thermoelectric properties of β-FeSi2 with B4C and BN dispersion by mechanical alloying

The B₄C- and BN-dispersed -FeSi₂ thermoelectric materials were synthesized by mechanical alloying and subsequent hot pressing. The effects of the B₄C and BN dispersion on the thermoelectric properties, such as Seebeck coefficient, electrical resistivity and thermal conductivity etc., of the -FeSi₂ were investigated. For the sample with B₄C and BN addition, a larger amount of the residual phase was detected in the X-ray diffraction patterns than the sample without addition. In the case of the BN addition, the Seebeck coefficient was enhanced by BN addition above 700 K, and the electrical resistivity also increased with increasing amount of BN. This is considered to result from doping of a small amount of B into the phase due to partial decomposition of the BN phase. The fine dispersion of BN particles in the phase matrix was quite effective for reducing the thermal conductivity as compared to the B₄C addition over the entire temperature range. The figure of merit, Z, of the -FeSi₂ was significantly enhanced by BN addition.     

Effect of temperature on dielectric and electrical properties of Co–Zr doped barium hexaferrites prepared by sol–gel method

In the present research, temperature dependence of dielectric properties of cobalt–zirconium substituted barium hexaferrites, fabricated using citric acid sol gel method, has been reported. The dielectric constant, loss tangent and A.C. conductivity were investigated on the circular pellets in temperature range 30–350 °C and frequency range 10 kHz–1 MHz using impedance analyzer. This paper also presents impedance (Z*) and electric modulus (M*) analysis of all the samples. The single semi-circular arcs, observed in impedance Nyquist plots, suggest the dominance of grain boundaries in the conduction process. Dielectric constant and dielectric loss tangent show very small variation up to 200–250 °C temperature and abrupt increase afterwards up to 350 °C. Thus, these ferrites can be successfully implemented in the practical applications like capacitors, microwave devices etc. up to 250 °C, without any significant change in properties.     

Fabrication of ultrafine W – Cu powders by mechanical alloying

Abstract

Ultrafine composite powders of W – 15 wt-%Cu, W – 25 wt-%Cu, and W – 35 wt-%Cu have been fabricated by mechanical alloying. The effects of type of mill, process control agent, temperature of milling, and ball/powder ratio on the final products have been evaluated. The results show that the planetary ball mill possesses a higher impact energy intensity than that of the vibratory ball mill. The optimum milling time is confirmed by the formation of a nanocrystalline microstructure in the planetary ball mill after optimisation of the milling parameters. A steady state between cold welding and fracture is attained with a milling time of up to 25 h in the planetary ball mill under optimised conditions. Crystallites with sizes of 7 – 8 nm for W – Cu composite powders have been obtained after 25 h of ball milling. The powders obtained after mechanical alloying have been characterised in terms of their size, shape, phase constitution, and microstructural features using X-ray diffraction and scanning electron microscopy.     

Investigation on WC–Al composite coatings of AZ91 alloy by mechanical alloying

In this paper, WC–Al composite coatings of AZ91 alloy prepared by mechanical alloying have been investigated in detail. It was found that the premixing process of composite powders has no significant effect in promoting the formation of uniform coating of WC–Al powders. Under the optimised conditions, i.e. the composition of composite powders of (10 g WC–6·5 g Al–3 g AZ91–0·5 g Mg), ball-to-powder weight ratio of 14∶1 and milling duration of 12 h, the average thickness of composite coating can be remarkably increased to 38·01 μm. Compared with the bare substrate, the Brinell hardness of the specimen with WC–Al composite coating can be significantly increased by about 90·01%.     

Microstructures and mechanical properties of spark plasma sintered Cu–Cr composites prepared by mechanical milling and alloying

Nanograined Cu–8 at.% Cr composite was produced by a combination of mechanical milling (MM), mechanical alloying (MA) and spark plasma sintering (SPS). Commercial Cu and Cr powders were pre-milled separately by MM. The milled Cu and Cr powders were then mechanically alloyed with as-received Cr and Cu powders respectively. After milling, the powder mixtures were separately subjected to SPS. It was found that pre-milling Cr can efficiently decrease the size of grain and reinforcement, resulting in remarkable strengthening. The grain size of Cu matrix was about 82 nm after SPS. The Vickers hardness, compressive yield strength and compression ratio of the composite were 327 HV, 1049 MPa and 10.4%, respectively. The excellent mechanical properties were primarily attributed to dispersion strengthening of the Cr particles and fine grain strengthening of the Cu matrix. The strong Cu/Cr interface and dissolved Cr atoms can also contribute to strengthening of the composite.     

Magnetic properties of Ni–Co doped barium strontium hexaferrite

A series of Ni–Co substituted barium strontium hexaferrite materials, Ba_0.5Sr_0.5Ni _x Co _x Fe_12–2x O₁₉ (x = 0.0, 0.2, 0.4, 0.6, 0.8 mol%) was synthesized by the sol–gel method. X-ray diffraction analysis has shown that the Ni–Co substitutions maintain in a single hexagonal magnetoplumbite phase. The room temperature magnetic properties and the cation site preferences of Ni–Co substituted ferrite were investigated by VSM. Substitutions led to decrease in coercivity while saturation magnetization remains the almost same. It indicates that the saturation magnetization (52.81–59.8 Am²/kg) and coercivity (69.83–804.97 Oe) of barium strontium hexaferrite samples can be varied over a very wide range by an appropriate amount of Ni–Co doping contents.     

Development of nanocrystalline Fe–Co alloys using mechanical alloying

Abstract

Nanostructured alloys have considerable potential as soft magnetic materials. In these materials a small magnetic anisotropy is desired, which necessitates the choice of cubic crystalline phases of Fe, Co, Ni, etc. In the present work, Fe–50 at.-%Co alloys were prepared using mechanical alloying (MA) in a planetary ball mill under a controlled environment. The influence of milling parameters on the crystallinity and crystal size in the alloys was studied. The particle size and morphology were also investigated using SEM. In addition, a thermal treatment was employed for partial sintering of some of the MA powders. The crystal size in both MA powders and compacted samples was measured using X-ray diffraction. It was shown that the crystal size could be reduced to less than 15 nm in these alloys. The nanocrystalline material obtained was also evaluated for magnetic behaviour.