Consolidation of Al2O3/Al Nanocomposite Powder by Cold Spray

While the improvement in mechanical properties of nanocomposites makes them attractive materials for structural applications, their processing still presents significant challenges. In this article, cold spray was used to consolidate milled Al and Al₂O₃/Al nanocomposite powders as well as the initial unmilled and unreinforced Al powder. The microstructure and nanohardness of the feedstock powders as well as those of the resulting coatings were compared. The results show that the large increase in hardness of the Al powder after mechanical milling is preserved after cold spraying. Good quality coating with low porosity is obtained from milled Al. However, the addition of Al₂O₃ to the Al powder during milling decreases the powder and coating nanohardness. This lower hardness is attributed to non-optimized milling parameters leading to cracked particles with insufficient Al₂O₃ embedding in Al. The coating produced from the milled Al₂O₃/Al mixture also showed lower particle cohesion and higher amount of porosity.     

Characterization and sinterability of oxide-dispersion strengthened nickel powder produced by mechanical alloying

Among the main requirements for the Ni/8% yttria stabilized zirconia (Ni/8YSZ) material, currently used for manufacturing solid oxide fuel cell (SOFC) anodes, fine homogeneous microstructure, considerable structural and mechanical stability, and sufficient gas permeability are of primary concern. In the present investigation, oxide-dispersion strengthened composite Ni powders containing 2, 5, and 10 vol.% 8YSZ were produced by mechanical alloying (MA) in air using a planetary milling machine and ZrO₂ milling media. The progress of the MA process was followed by particle size analysis, optical metallography, and x-ray diffraction (XRD) techniques. Results showed that dispersion of the oxide particles and structural refinement reached a significant point after milling for 180 h. The crystallite size and lattice distortion showed considerable dependence on the processing parameters. The mechanically alloyed powders were sintered at 1100° to 1350 °C. The mechanically alloyed powder containing 10 vol.% 8YSZ exhibited maximum densification. The minimum sintered density was observed for the composite powder containing 2 vol.% 8YSZ.     

Al2O3-nanodiamond composite coatings with high durability and hydrophobicity prepared by aerosol deposition

We attempted the room-temperature fabrication of Al₂O₃-based nanodiamond (ND) composite coating films on glass substrates by an aerosol deposition (AD) process to improve the anti-scratch and anti-smudge properties of the films. Submicron Al₂O₃ powder capable of fabricating transparent hard coating films was used as a base material for the starting powders, and ND treated by 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFOTES) was added to the Al₂O₃ to increase the hydrophobicity and anti-wear properties. The ND powder treated by PFOTES was mixed with the Al₂O₃ powder by ball milling to ratios of 0.01 wt.%, 0.03 wt.%, and 0.05 wt.% ND. The water contact angle (CA) of the Al₂O₃-ND composite coating films was increased as the ND ratio increased, and the maximum water CA among all the films was 110°. In contrast to the water CA, the Al₂O₃-ND composite coating films showed low transmittance values of below 50% at a wavelength of 550 nm due to the strong agglomeration of ND. To prevent the agglomeration of ND, the starting powders were mixed by attrition milling. As a result, Al₂O₃-ND composite coating films were produced that showed high transmittance values of close to 80%, even though the starting powder included 1.0 wt.% ND. In addition, the Al₂O₃-ND composite coating films had a high water CA of 109° and superior anti-wear properties compared to those of glass substrates.     

Microstructural evolution of nanostructured Ti0.9Al0.1N prepared by reactive ball-milling

Nanocrystalline stoichiometric Ti_0.9Al_0.1N powder has been prepared by ball-milling the α-Ti (hcp) and aluminum (fcc) powders under N₂ at room temperature. Initially, α-Ti phase partially transformed to the transient cubic β-Ti phase and Ti_0.9Al_0.1N (fcc) phase is noticed to form after 3 h of milling. Nanocrystalline stoichiometric Ti_0.9Al_0.1N phase is formed after 7 h of milling. After 1 h of milling, all Al atoms are diffused into the α-Ti matrix. The transient β-Ti phase is noticed to form after 1 h of milling and disappears completely after 7 h of milling. Microstructure characterization of unmilled and ball-milled powders by analyzing XRD patterns employing the Rietveld structure refinement reveals the inclusion of Al and nitrogen atoms into the Ti lattice on the way to formation of Ti_0.9Al_0.1N phase. Microstructure of ball-milled samples is also characterized by HRTEM. The particle size of Ti_0.9Al_0.1N phase, as obtained from XRD method, is ∼5 nm which is very close to that obtained from HRTEM.     

The effect of Al2O3 addition on the milling behavior of NiAl powder

The milling behavior of nickel aluminide, NiAl, powder in the presence of a fine Al ₂ O ₃ powder was investigated in the present study. The milling was carried out in an attrition mill. The size and shape of NiAl particles were not changed after milling while only NiAl powder was milled. When fine Al ₂ O ₃ powder was added to the NiAl powder, the Al ₂ O ₃ particles attached to the surface of NiAl particles during milling. As a consequence, the size of NiAl particles was reduced after milling. The shape of NiAl particles also changed. The presence of fine Al ₂ O ₂ particles enhanced the milling efficiency. The Al ₂ O ₃ particles on the surface of NiAl powder can be removed by washing repeatedly in an ultrasonic bath.     

A study on mechanochemical behavior of B2O3–Al system to produce alumina-based nanocomposite

One possible route for producing the fine and homogenous distribution of hard particles in composite microstructure is the mechanochemical processing in which high-energy ball milling promotes the reaction in a mixture of reactive powders. In this study mechanochemical reaction of B₂O₃ and Al powder during ball milling was studied. The phase transformation and microstructure of powder particles during ball milling were investigated by X-ray diffractometry and scanning electron microscopy. The results showed that during ball milling the B₂O₃–Al reacted with a combustion mode producing Al₂O₃–AlB₁₂ nanocomposite. The crystallite size of Al₂O₃ and AlB₁₂ was 40 and 25 nm, respectively. This structure appeared to be stable upon annealing.     

Mechanical behavior and microstructure of Al-10wt.%Nb alloy fabricated by Mechanical Alloying

Al-10%Nb alloy powders were fabricated by mechanical alloying and their mechanical behavior and microstructure were investigated by means of tensile testing, differential scanning calorimetry, X-ray diffraction and electron microscopy. An intermetallic compound of Al₃Nb was partially formed in the mechanically alloyed powders. The grain size was 50 run after mechanical alloying for 20 hours, and increased to 500 nm after hot extrusion at 400°C. However the 20 size of the intermetallic compounds of precipitated Al₃Nb in an Al matrix, did not vary with hot extrusion. The density of the consolidated Al-Nb alloy was over 97% relatively with hot extrusion. Both the tensile strength and elongation decreased at the elevated temperature. As the temperature increased, the dimples in the fracture surface were large and of coalescent shape, and the fracture was caused by the precipitated phases of Al₃Nb.     

Characterization and Formation Mechanism of Ni3Si–Al2O3 Nanocomposite Prepared by Mechanochemical Reduction Method

In this present work, Ni₃Si–Al₂O₃ nanocomposite powders were synthesized by mechanical milling using NiO, Si and Al as raw materials. The phase transformation, formation mechanism and microstructure evolution of the powders during mechanical milling were investigated by X-ray diffraction (XRD), differential thermal analysis (DTA), scanning electron microscopy (SEM), transition electron microscopy (TEM) and microhardness measurements. Results showed that the Ni₃Si, Al₂O₃ and Ni₃₁Si₁₂ phases formed after 5 h of milling with a rapid mechanically induced self-propagating synthesis mode. The average grain size and internal strain of Ni₃Si and Al₂O₃ after 30 h of milling were (16.8 nm, 1.27%) and (19.6 nm, 0.94%), respectively. The maximum microhardness value of 813 HV was obtained in the 30 h milled powder. The relationship between the hardness and grain size of the powders satisfies the Hall–Petch relationship. Ni₃Si–Al₂O₃ nanocomposite powders are very stable during heating at 950 °C. By annealing of the milled powders leads to grain growth, internal strain and microhardness of Ni₃Si powder decrease and transformation of disordered structure to an ordered state. A long-range ordering parameter (LRO) of 0.97 for the ordered Ni₃Si can be achieved after annealing at 950 °C for 2 h.

     

Effect of C and milling parameters on the synthesis of WC powders by mechanical alloying

In the current study, the amount of carbon and the effects of milling parameters in production of tungsten-carbide (WC) powder were evaluated. Mechanical alloying (MA) of elemental W and C powders at different carbon-rich and carbon-deficient compositions was studied. XRD results showed that the higher the carbon content the longer the milling period for the formation of WC powder. We also report on the effect of milling parameters on the phase formation. In stoichiometric composition, WC was synthesized faster than in compositions with higher carbon amount. Furthermore, W₂C phase was observed in compositions with higher carbon content milled at low speed and ball-to-powder ratio (BPR), as well as in carbon-deficient composition milled for shorter period. The ab initio calculations were performed in attempt to explain the destabilization of W₂C on further milling.     

Wear and oxidation resistance of Al2O3 particle dispersed Al3Ti composite with a nanostructure prepared by pulsed electric current sintering of mechanically alloyed powders

Al₃Ti-matrix composite layers containing Al₂O₃ particles were formed on Ti substrate by pulsed electric current sintering (PECS) of mechanically alloyed (MA) powders to improve the wear and oxidation properties of the Ti substrate. Reducing the grain size of each element by MA makes the combustion synthesis of Al₃Ti possible at a lower temperature. The grain size formed by the combustion synthesis of Al–Ti–Al₂O₃ powder mechanically alloyed for 720 ks was about 10 nm and its growth during sintering was suppressed by the existence of Al₂O₃. The densification behavior of the powder was investigated quantitatively. The obtained Al₃Ti/Al₂O₃ composite layer showed better wear and oxidation resistance than the monolithic Al₃Ti layer.     

Synthesis behavior of nanocrystalline Al-Al2O3 composite during low time mechanical milling process

In this work, four different volume fractions of Al₂O₃ (10, 20, 30 and 40 vol.%) were mixed with the fine Al powder and the powder blends were milled for 5 h. Scanning electron microscopy analysis, particle size analysis and bulk density measurements were used to investigate the morphological changes and achieving the steady state conditions. The results showed that increasing the Al₂O₃ content can provide the steady state particle size in 5 h milling process. It was found that increasing the volume fraction of Al₂O₃ leads to increasing the uniformity of Al₂O₃. Standard deviations of microhardness measurements confirmed this result. The XRD pattern and XRF investigations depicted that increasing the Al₂O₃ content causes an increase in the crystal defects, micro-strain and Fe contamination during 5 h milling process of nanocrystalline composite powders while the grain size is decreased. To investigate the effect of milling time, Al-30 vol.% Al₂O₃ (which achieved steady state during 5 h milling process) was milled for 1-4 h. The results depicted that the milling time lower than 5 h, do not achieve to steady state conditions.     

Synthesizing Aluminum alloys by double mechanical alloying

A new synthesis technique, namely double mechanical alloying (dMA), has been developed to fabricate aluminum alloys containing the finely distributed intermetallic compounds and inert dispersoids Al₄C₃ and Al₂O₃ The technique consists mainly of three steps: a primary milling stage of elemental powders (MAI) followed by a heat treatment to promote the formation of intermetallic phases, a secondary milling stage (MA2) to refine the microstructure, and consolidation of the produced powders. The results of mechanical and tribological properties of the resulting materials indicate that the dMA is a promising technique for the fabrication of aluminum alloys for applications requiring wear resistance and high-temperature performance.     

Microstructure and morphological study of ball-milled metal matrix nanocomposites

Due to the difficulty of preparation and beneficial properties achievable, copper and iron matrix nanocomposites are materials for which fabrication via the powder metallurgy route is attracting increasing research interest. The presence of ceramic nanoparticles in their matrix can lead to considerable changes in the microstructure and morphology. The effects of the type of metallic matrix and ceramic nanoparticle on the distribution of nano reinforcements and the morphology of ball-milled composite powders were evaluated in this study. For this purpose, 25 wt % of Al₂O₃ and SiC nanoparticles were separately ball-milled in the presence of iron and copper metals. The SEM, FESEM, and XRD results indicated that as-received nanoparticles, which were agglomerated before milling, were partially separated and embedded in the matrix of both the metals after the initial stages of ball milling, while prolonged milling was not found to further affect the distribution of nanoparticles. It was also observed that the Al₂O₃ phase was not thermodynamically stable during ball milling with copper powders. Finally, it was found that the presence of nanoparticles considerably reduce the average size of metallic powder particles.     

Formation of NiAl–Al2O3 intermetallic-matrix composite powders by mechanical alloying technique

This study investigated the feasibility of preparing intermetallic-matrix composite powders (NiAl/Al₂O₃) by mechanical alloying of Ni, Al and Al₂O₃ powder mixtures with various compositions of (NiAl)_x(Al₂O₃)_100–x. The as-milled powders were examined by X-ray diffraction, scanning electron microscopy, and differential thermal analysis. The formation of NiAl phase was noticed after 5 h of milling. Intermetallic-matrix composite powders (NiAl/Al₂O₃) were prepared successfully at the end of milling for (NiAl)_x(Al₂O₃)_100–x (x=79, 66, and 49), but no alumina phase was detected for (NiAl)₉₅(Al₂O₃)₅. It is suspected that the additions of alumina hampered the cold welding and fracturing process. The thermal analysis of (NiAl)_x(Al₂O₃)_100–x powders after 1 h of milling revealed that the transition temperature of NiAl phase increased with increasing amount of Al₂O₃ additions.     

Magnetic multiresonance behavior of Fe@Al2O3 nanoembedments and microstructural evolution during mechanosynthesis

The embedding of metal nanoparticles into an insulating ceramic matrix can provide encapsulation and prevent their oxidation and agglomeration. A nanoembedment powder with the Fe nanoparticles embedded into Al₂O₃ matrix is prepared by high-energy ball milling. Starting from the highly exothermic reactant mixture of magnetite and aluminum, Fe nanoparticles were in situ formed in Al₂O₃ matrix by mechanochemical reaction. It is found that a post-reaction milling significantly narrows the size distribution of Fe nanoparticles. The mechanism for the two-stage milling process was proposed. The microwave permeability of nanoembedments exhibited a multiresonance behavior, which was the evidence for monodispersed Fe nanoparticles. The Fe@Al₂O₃ nanoembedments are potential candidates as microwave absorber and left-handed materials.     

Phase analysis of the mechanically alloyed Fe−Si powder by differential dissolution technique

The binary Fe?Si elemental powders mixture (1∶2 in atomic proportion) has been milled for different milling times in an attrition mill. The phase characterization of mechanically alloyed powder was investigated using the chemical method of differential dissolution (DD) and the X-ray diffraction (XRD) method. In powder specimens milled for more than 15 hr, ∈-FeSi and unreacted Si were observed. The formation of a supersaturated solid solution of Si in ∈-FeSi induced by mechanical alloying (MA) was also verified. The lattice parameter of the ∈-FeSi of as-milled powders changed from 4.4876 Å to 4.4668 Å according to the increase of MA time. Based on the results of the DD analysis, unreacted Si could be classified as (1) crystalline Si, (2) Si supersaturated in ∈-FeSi, or (3) amorphous Si. Therefore formation of the β-FeSi₂ after annealing could be explained by the reaction between the ∈-FeSi and the Si classified into types (1) and (2). It seemed that the amorphous Si induced by MA did not react with the ∈-FeSi during annealing at 700°C.     

Preparation of Ti3AlC2 by mechanically activated sintering with Sn aids

The mechanically activated sintering process was adapted to synthesize Ti₃AlC₂ using 3Ti/Al/2C/0.05Sn powder mixtures. The result showed that the powders containing TiC, Ti₃AlC₂ and Ti₂AlC were obtained by mechanical alloying (MA) 3Ti/Al/2C powders. Addition of appropriate Sn reduced the content of Ti₂AlC and enhanced the synthesis of Ti₃AlC₂ significantly. The powders with highest content of Ti₃AlC₂ were obtained by MA 3Ti/Al/2C/0.05Sn powders. Through pressureless sintering the mechanical alloyed powders at 900–1100 °C for 2 h, the high purity Ti₃AlC₂ material with fine organization was produced.     

Difference between mechanical alloying and mechanical disordering in the amorphization reaction of Al50Ta50 in a rod mill

Amorphous Al₅₀Ta₆₀ alloy powders have been synthesized by mechanical alloying (MA) from elemental powders of aluminium and tantalum, and mechanical disordering (MD) from crystalline intermetallic compound powders of AlTa respectively using the rod milling technique. The mechanically alloyed and the mechanically disordered alloy powders were characterized by X-ray diffraction, scanning electron microscopy, electron probe microanalysis, transmission electron microscopy, differential thermal analysis, differential scanning calorimetry and chemical analysis. The results have shown that the crystal-to amorphous transformation in the MD process occurs through one stage, while the crystallineto-amorphous formation in the MA process occurs through three stages. At the early and intermediate stages of the MA time, heating the alloy powders to 700 K leads to the formation of an amorphous phase by a solid-state amorphizing reaction. At the final stage of the MA time, the amorphous phase is crystallized through a single sharp exothermic peak. Contrary to this, amorphous alloy powders produced by MD are crystallized through two broad exothermic peaks.     

Preparation and microwave absorbency of Fe/epoxy and FeNi3/epoxy composites

The focus of this study was placed on the lightness of microwave absorbing effective metal/epoxy composites. For such a focus, high aspect ratio of flake iron powder and high absorbing FeNi₃ were prepared. The iron powder particle size was reduced significantly through wet milling, comparing to dry milling. The FeNi₃ alloy powders were synthesized by mechanical alloying (MA); then, the particle size was reduced through wet milling. The iron powder and FeNi₃ alloy were characterized by scanning electron microscopy (SEM) and X-ray diffraction. SEM of the metal particles showed the flake and small structure by wet milling. The microwave absorbing effectiveness of metal/epoxy composite was affected by the structure, loading and dispersion of metal materials. The polyvinylpyrrolidone (PVP) plays an important role in suspending metal powders in wet milling to reduce powder size. Besides, the PVP will be a coupling agent in inhibiting the aggregation and enhancing the interfacial interaction between metal and epoxy. Results suggested that after the above manufacturing process, the microwave absorbency was enhanced substantially. Composite films of Fe/epoxy and FeNi₃/epoxy 1.6 mm in thickness possessed a microwave absorbency above 10 dB at 9.2-15.2 GHz and 13.1-16.2 GHz, respectively.     

Nanocrystalline NiAl Coating Prepared by HVOF Thermal Spraying

Nanocrystalline NiAl intermetallic powder was prepared by mechanical alloying (MA) of Ni₅₀Al₅₀ powder mixture and then deposited on low carbon steel substrates by high velocity oxy fuel (HVOF) thermal spray technique using two sets of spraying parameters. X-ray diffraction (XRD), scanning electron microscopy (SEM), transition electron microscopy (TEM), differential scanning calorimetry (DSC), and hardness test were used to characterize the prepared powders and coatings. The MA of Ni₅₀Al₅₀ powder mixture led to the formation of NiAl intermetallic compound. The resulting powder particles were three dimensional in nature with irregular morphology and a crystallite size of ~10 nm. This powder was thermally sprayed by HVOF technique to produce coating. The deposited coating had a nanocrystalline structure with low oxide and porosity contents. The hardness of coatings was in the range of 5.40-6.08 GPa, which is higher than that obtained for NiAl coating deposited using conventional powders.