Synthesis of a NiTi2-AlNi-Al2O3 nanocomposite by mechanical alloying and heat treatment of Al-TiO2-NiO

The aim of the present study was to investigate the phases formed during ball milling of Al-TiO₂-NiO. For this purpose, a mixture of Al-TiO₂-NiO with a molar ratio of 6:1:1 was used. Characterization of the milled powders by X-ray diffraction, differential thermal analysis, field-emission scanning electron microscopy, and transmission electron microscopy showed the formation of nanocrystalline NiTi₂ along with AlNi. A thermodynamical investigation confirmed that NiO was reduced by Al during ball milling, which consequently promoted TiO₂ reduction and the formation of NiTi₂. Al is capable of reducing NiO either during ball milling or at temperatures above the melting point of Al; by contrast, TiO₂ can be reduced by Al only by milling.     

Influence of B source materials on the synthesis of TiB2-Al2O3 nanocomposite powders by mechanical alloying

An Al2O3-TiB2 nanocomposite was successfully synthesized by ball milling of Al, TiO2 and two B source materials of B2O3 （system （1）） and H3BO3 （system （2））. Phase identification of the milled samples was examined by Xray diffraction. The morphology and microstructure of the milled powders were monitored by scanning electron microscopy and transmission electron microscopy. It was found that the formation of this composite was completed after 15 and 30 h of milling time in systems （1） and （2）, respectively. More milling energy was required for the formation of this composite in system （2） due to the lubricant properties of HaBO3 and also its decomposition to HBO2 and B2O3 during milling. On the basis of X-ray diffraction patterns and thermodynamic calculations, this composite was formed by highly exothermic mechanically induced self-sustaining reactions （MSR） in both systems. The MSR mode took place around 9 h and 25 h of milling in systems （1） and （2）, respectively. At the end of milling （15 h for system （1） and 30 h for system （2）） the grain size of about 35-50 nm was obtained in both systems.     

Microstructures and mechanical properties of nanocrystalline NiTi intermetallics formed by mechanosynthesis

The formulation of nanocrystalline NiTi shape memory alloys has potential effects in mechanical stimulation and medical implantology. The present work elucidates the effect of milling time on the product's structural characteristics, chemical composition, and microhardness for NiTi synthesized by mechanical alloying for different milling durations. Increasing the milling duration led to the formation of a nanocrystalline NiTi intermetallic at a higher level. The formation of nanocrystalline materials was directed through cold fusion, fracturing, and the development of a steady state, which were influenced by the accumulation of strain energy. In the morphological study, uninterrupted cold diffusion and fracturing were visualized using transmission electron microscopy. Particle size analysis revealed that the mean particle size was reduced to~93 μm after 20 h of milling. The mechanical strength was enhanced by the formation of a nanocrystalline intermetallic phase at longer milling time, which was confirmed by the results of Vickers hardness analyses.     

Structure and corrosion resistance of Al–Cu–Fe alloys

The microstructure and electrochemical properties of Al–Cu–Fe alloys with the atomic compositions of Al_(65)Cu_(20)Fe_(15),Al_(78)Cu_7Fe_(15)and Al_(80)Cu_5Fe_(14)Si_1have been studied.The alloys were produced by induction melting of pure elements with copper mold casting.The microstructure of the alloys was analyzed by X-ray diffraction and high-resolution transmission electron microscopy.The formation of quasicrystalline phases in the Al–Cu–Fe alloys was confirmed.The presence of intermetallic phases was observed in the alloys after crystallization in a form of ingots and plates.The electrochemical measurements were conducted in 3.5%NaCl solution.The electronic structure of the alloys was determined by X-ray photoelectron spectroscopy.The post corrosion surface of the samples was checked using a scanning electron microscope equipped with the energydispersive X-ray detector.It was observed that the Al_(65)Cu_(20)Fe_(15)alloy had the highest corrosion resistance.The improved corrosion resistance parameters were noted for the plate samples rather than those in the as-cast state.And the hardness of the Al_(65)Cu_(20)Fe_(15)alloy was significantly higher than the other alloy samples.     

Bulk nanocrystalline Al prepared by cryomilling

Bulk nanocrystalline Al was fabricated by mechanically milling at cryogenic temperature (cryomilling) and then by hot pressing in vacuum. By using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), the microstructure evolution of the material during cryomilling and consolidation was investigated. With increasing the milling time, the grain size decreased sharply and reduced to 42 nm when cryomilled for 12 h. The grains had grown up, and the columnar grain was formed under the hot pressing and extrusion compared with the cryomilled powders. The grain size of as-extruded specimen was approximately 300-500 nm. The reason of high thermal stability of this bulk was attributed primarily to the Zener pinning from the grain boundary of the AlN arising from cryomilling and the solute drag of the impurity. Tensile tests show that the strength of nanocrystalline Al is enhanced with decreasing grain size. The ultimate tensile strength and tensile elongation were 173 MPa and 17.5%, respectively. It appears that the measured high strength in the cryomilled Al is related to a grain-size effect, dispersion strengthening, and dislocation strengthening.     

Influence of chromium on the initial corrosion behavior of low alloy steels in the CO_2–O_2–H_2S–SO_2 wet–dry corrosion environment of cargo oil tankers

The influence of Cr on the initial corrosion behavior of low-alloy steels exposed to a CO2–O2–H2S–SO2 wet–dry corrosion environment was investigated using weight-loss measurements, scanning electron microscopy, N2 adsorption tests, X-ray diffraction analysis, and electrochemical impedance spectroscopy. The results show that the corrosion rate increases with increasing Cr content in samples subjected to corrosion for 21 d. However, the rust grain size decreases, its specific surface area increases, and it becomes more compact and denser with increasing Cr content, which indicates the enhanced protectivity of the rust. The results of charge transfer resistance(Rct) calculations indicate that higher Cr contents can accelerate the corrosion during the first 7 d and promote the formation of the enhanced protective inner rust after 14 d; the formed protective inner rust is responsible for the greater corrosion resistance during long-term exposure.     

Interface microstructure and formation mechanism of ultrasonic spot welding for Al–Ti dissimilar metals

The present study focuses on interface microstructure and joint formation. AA6061 aluminum alloy(Al) and commercial pure titanium(Ti) joints were welded by ultrasonic spot welding(USW). The welding energy was 1100–3200 J. The Al–Ti joint appearance and interface microstructure were observed mainly via optical microscopy and field emission scanning electron microscopy. Results indicated that a good joint can be achieved only with proper welding energy of 2150 J. No significant intermetallic compound(IMC) was found under all conditions. The high energy barriers of Al–Ti and difficulties in diffusion were the main reasons for the absence of IMC according to kinetic analysis. The heat input is crucial for the material plastic flow and bonding area, which plays an important role in the joint formation.     

Effect of homogenization process on the hardness of Zn–Al–Cu alloys

The effect of a homogenizing treatment on the hardness of as-cast Zn–Al–Cu alloys was investigated. Eight alloy compositions were prepared and homogenized at 350 ℃ for 180 h, and their Rockwell “B” hardness was subsequently measured. All the specimens were analyzed by X-ray diffraction and metallographically prepared for observation by optical microscopy and scanning electron microscopy. The results of the present work indicated that the hardness of both alloys (as-cast and homogenized) increased with increasing Al and Cu contents; this increased hardness is likely related to the presence of the θ and τ' phases. A regression equation was obtained to determine the hardness of the homogenized alloys as a function of their chemical composition and processing parameters, such as homogenization time and temperature, used in their preparation.     

Morphology characterization of periclase–hercynite refractories by reaction sintering

A periclase?hercynite brick was prepared via reaction sintering at 1600°C for 6 h in air using magnesia and reaction-sintered hercynite as raw materials. The microstructure development of the periclase?hercynite brick during sintering was investigated using X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy in combination with energy-dispersive X-ray spectroscopy. The results show that during sintering, Fe2+, Fe3+ and Al3+ ions in hercynite crystals migrate and react with periclase to form(Mg1-xFex)(Fe2-yAly)O4 spinel with a high Fe/Al ratio. Meanwhile, Mg2+ in periclase crystals migrates into hercynite crystals and occupies the oxygen tetrahedron vacancies. This Mg2+ migration leads to the formation of(Mg1-uFeu)(Fe2-vAlv)O4 spinel with a lower Fe/Al ratio and results in Al3+ remaining in hercynite crystals. Cation diffusion between periclase and hercynite crystals promotes the sintering process and results in the formation of a microporous structure.     

Synthesis of nanocrystalline NiCrC alloy feedstock powders for thermal spraying by cryogenic ball milling

Nanocrystalline NiCrC alloy powders with a qualified particle size distribution for thermal spraying were synthesized using the cryogenic ball milling (cryomilling) method. The morphology, microstructure, size distribution, and phase transformation of the powders were characterized by scanning electron microscopy (SEM), laser scattering for particle size analysis, X-ray diffraction (XRD), and transmission electron microscopy (TEM). After cryomilling for 20 h, the average grain size of the as-milled powders approached a constant value of 30 nm by XRD measurement. The average particle size slightly increased from 17.5 to 20.3 μm during the 20-h milling. About 90vol% of the powders satisfied the requirement for thermal spraying with the particle dimension of 10-50 μm, and most of the powders exhibited spherical morphology, which were expected to have good fluidity during thermal spraying. The Cr₂O₃ phase formed during the cryornilling process as revealed in the XRD spectra, which was expected to enhance the thermal stability of the as-milled powders during the followed thermal spraying or other heat treatment.     

A TEM study on the microstructure of spark plasma sintered ZrB_2-based composite with nano-sized SiC dopant

Sintering behavior of ZrB_2 ceramic with nano-sized SiC dopant was studied. ZrB_2-25 vol% nano-sized SiC was selected as the starting mixture to fabricate the composite. The manufacturing process was accomplished at 1800℃ for 5 min under 25 MPa via spark plasma sintering(SPS). The as-sintered sample reached a relative density of 99%. Besides the initial phases, namely ZrB_2 and SiC, the high-resolution X-ray diffraction(HRXRD) was used to study the formation of an in-situ ZrC phase. The possible chemical interactions during the ZrC phase formation were scrutinized. The microstructure of the composite was studied by the field emission scanning electron microscopy(FESEM) and transmission electron microscopy(TEM). Elemental analysis through FESEM evaluations revealed the formation of amorphous phases, rich in Zr, C, Si, B, and O elements, which was in harmony with the thermodynamical assessments. TEM studies endorsed the formation of such phases, containing a glassy bed of Si–B–O with ZrC and C islands dispersed therein.     

Effect of Zn addition on microstructure and mechanical properties of an Al–Mg–Si alloy

In the present work, an Al–0.66Mg–0.85Si–0.2Cu alloy with Zn addition was investigated by electron back scattering diffraction(EBSD), high resolution electron microscopy(HREM), tensile and Erichsen tests. The mechanical properties of the alloy after pre-aging met the standards of sheet forming. After paint baking, the yield strength of the alloy was improved apparently. GP(Ⅱ) zones and η’phases were formed during aging process due to Zn addition. With the precipitation of GP zones, β″ phases, GP(Ⅱ) zones and η’phases, the alloys displayed excellent mechanical properties.     

Preparation of NiO nanoparticles from Ni（OH）2-NiCO3-4H2O precursor by mechanical activation

A mechanical activation process was introduced as a facile method for producing nickel oxide nanopowders. The precursor compound Ni（OH）2-NiCO3-4H2O was synthesized by chemical precipitation. The precursor was milled with NaCl diluent. A high-energy ball milling process led to decomposition of the precursor and subsequent dispersal in NaCl media. Nickel oxide nanocrystalline powders were produced by subsequent heat treatment and water washing. Milling rotation speed, milling time, ball-to-powder ratio （BPR）, and nickel chlo-ride-to-precursor ratio were introduced as influential parameters on the wavelength of maximum absorption （λmax）. The effects of these pa-rameters were investigated by the Taguchi method. The optimum conditions for this study were a milling rotation speed of 150 r/min, a mill-ing time of 20 h, a BPR of 15/1, and a NaCl-to-powder weight ratio （NPR） of 6/1. In these conditions,λmax was predicted to be 292 nm. The structural properties of the samples were determined by field emission scanning electron microscopy, X-ray diffraction, and energy dispersive spectrometry.     

In situ(α-Al_2O_3+ZrB_2)/Al composites with network distribution fabricated by reaction hot pressing

In situ(α-Al_2O_3+ZrB_2)/Al composites with network distribution were fabricated using low-energy ball milling and reaction hot pressing. Differential thermal analysis(DTA) was used to study the reaction mechanisms in the Al–Zr O2–B system. X-ray diffraction(XRD) and scanning electron microscopy(SEM) in conjunction with energy-dispersive X-ray spectroscopy(EDX) were used to investigate the composite phases, morphology, and microstructure of the composites. The effect of matrix network size on the microstructure and mechanical properties was investigated. The results show that the optimum sintering parameters to complete reactions in the Al–Zr O2–B system are 850°C and 60 min. In situ-synthesized α-Al2O3 and Zr B2 particles are dispersed uniformly around Al particles, forming a network microstructure; the diameters of the α-Al2O3 and Zr B2 particles are approximately 1–3 μm. When the size of Al powder increases from 60–110 μm to 150–300 μm, the overall surface contact between Al powders and reactants decreases, thereby increasing the local volume fraction of reinforcements from 12% to 21%. This increase of the local volume leads to a significant increase in microhardness of the in situ(α-Al2O3–Zr B2)/Al composites from Hv 163 to Hv 251.     

Preparation of in situ and ex situ reinforced Fe–10Cr–1Cu–1Ni–1Mo–2C containing NbC particles by milling and hot pressing

An in situ and ex situ reinforced powder metallurgy(PM) steel was prepared by the combination of high-energy ball milling and subsequent hot pressing of elemental mixed powders of Fe–10Cr–1Cu–1Ni–1Mo–2C by mass with the addition of Nb C particles. A 40-h milling pretreatment makes the powder particles nearly equiaxed with an average diameter of ~8 μm, and the ferrite grain size is refined to ~6 nm. The sintered density reaches 99.0%–99.7% of the theoretical value when the sintering is conducted at temperatures greater than 1000°C for 30 min. In the sintered bulk specimens, the formation of an in situ M7C3(M = Cr, Fe, Mo) phase is confirmed. M7C3 carbides with several hundred nanometers in size are uniformly distributed in the matrix. Some ultra-fine second phases of 50–200 nm form around the ex situ Nb C and in situ M7C3 particles. The sintered steel exhibits an excellent combination of hardness( Hv 500) and compressive strength(2100–2420 MPa).     

Facile synthesis of nanosized sodium magnesium hydride, NaMgH3

The ternary magnesium hydride NaMgH 3 has been synthesised via reactive milling techniques.The method employed neither a reactive H2 atmosphere nor high pressure sintering or other post-treatment processes.The formation of the ternary hydride was studied as a function of milling time and ball:powder ratio.High purity NaMgH 3 powder(orthorhombic space group Pnma,a 5.437(2),b 7.705(5),c 5.477(2) ;Z 4) was prepared in 5 h at high ball:powder ratios and characterised by powder X-ray diffraction(PXD),Raman spectroscopy and scanning electron microscopy/energy dispersive X-ray spectroscopy(SEM/EDX).The products formed sub-micron scale(typically 200-400 nm in size) crystallites that were approximately isotropic in shape.The dehydrogenation behaviour of the ternary hydride was investigated by temperature programmed desorption(TPD).The nanostructured hydride releases hydrogen in two steps with an onset temperature for the first step of 513 K.     

Characterization of NbSi_2–Al_2O_3 nanocomposite coatings prepared with plasma spraying mechanically alloyed powders

The present study characterized NbS i2–Al2O3 nanocomposite powders plasma-sprayed on Ti–6Al–4Vsubstrates. The powders were agglomerated to obtain suitable particle sizes for spraying. The agglomerated powders were then plasma-sprayed using atmospheric plasma spraying. The structural transformations of the powders along with the morphological and mechanical changes of the coatings were examined by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, transmission electron microscopy, and hardness testing. The results showed that after plasma spraying, the grain size increased, and the lattice strain decreased. However, the grain size of this compound after spraying was still in the nanometer range. The coating was uniform and exhibited good adhesion to the substrate. The microhardness and fracture toughness of the nanocomposite coating were higher than those of a nanostructured NbS i2 coating.     

Densification behavior of mechanically milled Cu–8 at% Cr alloy and its mechanical and electrical properties

Synthesis and consolidation behavior of Cu–8 at%Cr alloy powders made by mechanical alloying with elemental Cu and Cr powders,and subsequently,compressive and electrical properties of the consolidated alloys were studied.Solid solubility of Cr in Cu during milling,and subsequent phase transformations during sintering and heat treatment of sintered components were analyzed using X-ray diffraction,scanning electron microscopy and transmission electron microscopy.The milled powders were compacted applying three different pressures(200 MPa,400 MPa and 600 MPa)and sintered in H2atmosphere at 900 1C for 30 min and at 1000 1C for 1 h and 2 h.The maximum densification(92.8%)was achieved for the sample compacted at 600 MPa and sintered for 1000 1C for 2 h.Hardness and densification behavior further increased for the compacts sintered at 900 1C for 30 min after rolling and annealing process.TEM investigation of the sintered compacts revealed the bimodal distribution of Cu grains with nano-sized Cr and Cr2O3precipitation along the grain boundary as well as in grain interior.Pinning of grain boundaries by the precipitates stabilized the fine grain structure in bimodal distribution.     

Kinetics of discontinuous precipitation in Cu–20Ni–20Mn alloy

The morphology and growth kinetics of discontinuous precipitation (DP) in a Cu–20Ni–20Mn alloy were investigated in the temperature range of 523–673 K by optical microscopy, scanning electron microscopy, and transmission electron microscopy. A lamellar mixed structure consisting of alternating lamellae of a matrix and NiMn phase was observed in DP colonies. The volume fraction of regions formed by a DP reaction was determined by quantitative metallographic measurements. The kinetics of DP was evaluated on the basis of the Johnson–Mehl–Avrami–Kolmogorov equation, which resulted in a time exponent of approximately 1.5. We confirmed that the nucleation of the discontinuous precipitate was confined to grain edges or boundaries at an early stage of the reaction. The activation energy of DP process was determined to be approximately (72.7 ±7.2) kJ/mol based on the Arrhenius equation; this result suggests that DP is controlled by grain boundary diffusion. The hardness values exhibited good correlation with the volume fraction of DP; this correlation was attributed to the presence of the ordered NiMn phase.     

In situ reaction mechanism of MgAlON in Al–Al_2O_3–MgO composites at 1700°C under flowing N_2

The Al–Al_2O_3–MgO composites with added aluminum contents of approximately 0wt%, 5wt%, and 10wt%, named as M_1, M_2, and M_3, respectively, were prepared at 1700°C for 5 h under a flowing N_2 atmosphere using the reaction sintering method. After sintering, the Al–Al_2O_3–MgO composites were characterized and analyzed by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The results show that specimen M_1 was composed of MgO and MgAl_2O_4. Compared with specimen M_1, specimens M_2 and M_3 possessed MgAlON, and its production increased with increasing aluminum addition. Under an N_2 atmosphere, MgO, Al_2O_3, and Al in the matrix of specimens M_2 and M_3 reacted to form MgAlON and AlN-polytypoids, which combined the particles and the matrix together and imparted the Al–Al_2O_3–MgO composites with a dense structure. The mechanism of MgAlON synthesis is described as follows. Under an N_2 atmosphere, the partial pressure of oxygen is quite low; thus, when the Al–Al_2O_3–MgO composites were soaked at 580°C for an extended period, aluminum metal was transformed into AlN. With increasing temperature, Al_2O_3 diffused into AlN crystal lattices and formed AlN-polytypoids; however, MgO reacted with Al_2O_3 to form MgAl_2O_4. When the temperature was greater than(1640 ± 10)°C, AlN diffused into Al_2O_3 and formed spinel-structured AlON. In situ MgAlON was acquired through a solid-solution reaction between AlON and Mg Al_2O_4 at high temperatures because of their similar spinel structures.