Microstructure evolution of the Si3N4/Si3N4 joints brazed using Au-Ni-V filler alloys with different V content

Au-Ni-V filler alloys with different vanadium contents were designed to braze Si₃N₄ ceramic at 1373 K for 30 min, and the microstructures of brazing seams were investigated by SEM and TEM. When the Au-Ni-V filler alloy contains 5 at.% V, round-like Ni[Si, V, Au] precipitates form in the Au[Ni] solid solution matrix and a VN reaction layer with 0.5 μm thickness appears on Si₃N₄ interface. When the V content increases to 10 at.%, a new polygonal Ni₂SiV₃ phase occurs in the seam, and the Ni[Si, V, Au] precipitate coarsens and VN layer thickens. With increase of V contents to 15 and 20 at.%, laminar Ni[Au] and polygonal Ni₃V precipitates form. With 25 at.% V content in the filler alloy, the Ni₂SiV₃ and Ni₃V precipitates distribute homogenously in the brazing seam. These microstructure evolutions were attributed to the reaction between Si₃N₄ and vanadium, which forms VN reaction layer and releases Si into the molten alloy.     

Al/Ni metal intermetallic composite produced by accumulative roll bonding and reaction annealing

In this research, Al/Ni multilayers composites were produced by accumulative roll bonding and then annealed at different temperatures and durations. The structure and mechanical properties of the fabricated metal intermetallic composites (MICs) were investigated. Scanning electron microscopy and X-ray diffraction analyses were used to evaluate the structure and composition of the composite. The Al₃Ni intermetallic phase is formed in the Al/Ni interface of the samples annealed at 300 and 400 °C. When the temperature increased to 500 °C, the Al₃Ni₂ phase was formed in the composite structure and grew, while the Al₃Ni and Al phases were simultaneously dissociated. At these conditions, the strength of MIC reached the highest content and was enhanced by increasing time. At 600 °C, the AlNi phase was formed and the mechanical properties of MIC were intensively degraded due to the formation of structural porosities.     

Coke formation and metal dusting of electroplated Ni3Al-CeO2-based coatings in CO-H2-H2O

Coke formation and metal dusting of electrodeposited pure, 5 μm CeO₂-dispersed, and 9-15 nm CeCO₂-dispersed Ni₃Al coatings were investigated in CO-H₂-H₂O at 650 °C for a period of 500 h. All Ni₃Al coatings showed the inferior long-term resistance to coke formation and metal dusting to the Fe-Ni-Cr alloy due to failure to form a continuous Al₂O₃ scale. CeO₂-dispersed Ni₃Al coatings, especially 9-15 nm CeCO₂-dispersed coatings, exhibited more severe coke formation and metal dusting than the pure Ni₃Al coating. The detrimental effect of CeO₂ is believed to be caused by the enhanced formation of NiO/Ni crystals on the coating surfaces or at the grain boundaries, which catalysed the carbon deposition and promoted the carbon attack on Ni₃Al coatings.     

Formation of periodic patterns in the (Ni,W)/Al diffusion couples

Formation of periodic patterns was observed in the (Ni,W)/Al finite diffusion couples after annealing at 600 °C. The individual layers in the patterns grew parallel to the original interface between the counterparts of the diffusion couples. A combination of the scanning electron microscopy, energy dispersive analysis of characteristic X-rays and X-ray diffraction revealed that the periodic patterns consisted of Al₄W/Al₅W layers, which were embedded either in the Al₃Ni or in the Al₃Ni₂ matrix depending on the local Al concentration. The Al₃Ni matrix developed in the Al-rich part of the sample, the Al₃Ni₂ matrix in the (Ni,W)-rich part of the sample. The microstructure analysis was accompanied by nanoindentation experiments that were performed in selected phases, which developed in the samples. The kinetic of the formation of the periodic patterns was investigated with the aid of diffusion couples that were annealed for 1, 3, 6 and 24 h. It was confirmed that the formation of the periodic patterns in the (Ni,W)/Al diffusion couples at 600 °C is a diffusion-controlled process. A model was suggested to describe this process and compared with existing models from literature.     

The phase equilibria of the Al–Ce–Ni system at 500 °C

The phase equilibria at 500 °C in the Al–Ce–Ni system in the composition region of 0–33.3 at.% Ce are investigated using XRD and SEM/EDX techniques applied to equilibrated alloys. The previously reported ternary phases and the variation of the lattice parameters versus the composition for different solid solution phases are investigated. It is confirmed that τ₂(Al₂CeNi) exists at 500 °C, while τ₃(Al₅Ce₂Ni₅) does not exist at 500 °C. A new compound τ₉ with composition of about Al₃₅Ce_16.5Ni_48.5 is found. The solubility of Ni in Al₁₁Ce₃ and αAl₃Ce is generally about 1 at.%, while the solubility of Ni in Al₂Ce is measured to be 2.7 at.%. The solubility of Ce in Al₃Ni, Al₃Ni₂, AlNi and AlNi₃ is all less than 1 at.%. The solubility of Al in CeNi₅, Ce₂Ni₇ and CeNi₃ is measured to be 30.4, 4.8 and 9.2 at.%, respectively, while there is no detectable solubility for Al in CeNi₂. A revised isothermal section at 500 °C in the Al–Ce–Ni system has been presented.     

Enhanced glass forming ability and refrigerant capacity of a Gd55Ni22Mn3Al20 bulk metallic glass

In this work, a small amount of Mn was added to a Gd₅₅Ni₂₅Al₂₀ glass forming alloy, as a replacement for Ni, and a Gd₅₅Ni₂₂Mn₃Al₂₀ bulk metallic glass (BMG) was obtained by suction casting. Its glass forming ability (GFA) was characterized by X-ray diffraction and differential scanning calorimetry, and its magnetic properties were measured using a magnetic property measurement system. It is found that the minor Mn addition can significantly improve both the GFA and the magnetocaloric effect (MCE) of the alloy. The refrigerant capacity (RC) of the BMG can reach a high value of 825 J kg⁻¹ under a field of 3979 kA/m, which is about 29% larger than that of a Gd₅₅Ni₂₅Al₂₀ BMG. The effect of the minor Mn addition on the GFA and MCE of the BMG was investigated in the study.     

Influence of alloy chemistry on microstructure and properties in NiCrBSi overlay coatings deposited by plasma transferred arc welding (PTAW)

The microstructures and performance of two NiCrBSi alloy overlays deposited by plasma transferred arc welding are studied. The coatings consist of a γ-Ni primary dendritic phase with harder Ni + Ni₃B or Ni + Ni₃Si eutectics and Cr-based particles (CrB, Cr₃C₂, and Cr₇C₃) situated at the interdendritic regions. It was found that the volume fraction of the soft primary dendritic phase drastically decreased and the proportion of chromium borides and carbides increased with an increase of C, B, Si, and Cr content. Microhardness testing revealed that the primary Ni dendrite, interdendritic, and Cr-particle phases had average hardness values of 405, 860, and 1200 HV respectively. An increase in the volume fraction of hard eutectics and Cr-particles lead to a substantial increase in hardness and wear resistance.     

Nanocrystalline matrix Al3Ni2–Al–Al3Ni composites produced by reactive hot-pressing of milled powders

Mechanically alloyed nanocrystalline Al₆₃Ni₃₇ powder with a metastable structure of NiAl intermetallic phase was mixed with 30 vol.% of Al powder. This powder mixture was consolidated under the pressure of 7.7 GPa at 600, 700, 800 and 1000 °C for 15 and 180 s. During the consolidation, in all cases, the metastable NiAl phase transformed into the equilibrium Al₃Ni₂ intermetallic. Moreover, a solid-state reaction between the intermetallic matrix and Al occurred, yielding an Al₃Ni phase. Progress of this reaction depended on the consolidation temperature and temperature exposure time, thus Al₃Ni₂–Al, Al₃Ni₂–Al–Al₃Ni or Al₃Ni₂–Al₃Ni composites were produced by hot-pressing with various parameters. The mean crystallite size of the Al₃Ni₂ intermetallic matrix in the composites is 39–67 nm, depending on consolidation parameters. The composites hardness is in the range of 6.02–7.51 GPa.     

Effects of SiC volume fraction and aluminum particulate size on interfacial reactions in SiC nanoparticulate reinforced aluminum matrix composites

The SiC nanoparticulate reinforced Al-3.0 wt.% Mg composites were fabricated by combining pressureless infiltration with ball-milling and cold-pressing technology at 700 °C for 2 h. The effects of SiC nanoparticulate volume fractions (6%, 10% and 14%) and Al particulate sizes (38 μm and 74 μm) on interfacial reactions were investigated by SEM, TEM and X-ray diffraction. The results show that the MgO at the interface between SiC nanoparticulate and molten Al can provide a barrier for the diffusion of Si, C and Al. Using Al particulate (74 μm) as raw material, the Al₄C₃ phase was not found in the composites containing 6 vol.% and 10 vol.% SiC, but presented in the composites containing 14 vol.% SiC. When SiC content up to 14 vol.%, the products of MgO around SiC nanoparticulate are not enough to provide effective protection from the reaction between SiC and molten Al, therefore the diffusion of Si, C and Al can take place to produce Al₄C₃ and Si phases. Using 38 μm Al particulate as raw material, the fine Al particulate possesses the high reaction activity and can easily be embedded into the gap among the big Mg particulate segregated at the interface, resulting in the appearance of exposure surface of SiCp to the Al and the forming of diffusion channels for the atomics C, Si and Al. So, the formations of Al₄C₃ and Si phases were occurred.     

Influence of powder size on the crystallization behavior during laser solid forming Zr55Cu30Al10Ni5 bulk amorphous alloy

The Zr₅₅Cu₃₀Al₁₀Ni₅ bulk metallic glasses (BMGs) were prepared using laser solid forming (LSF) process from the plasma rotating electrode process (PREP) powder. The effect of the powder size on the crystallization behavior of the remelted zone (RZ) and heat affected zone (HAZ) was investigated. It was found that the as-prepared powders were composed of the amorphous phase and Al₅Ni₃Zr₂-type phase. The RZ mainly kept the amorphous state after LSF. The residual Al₅Ni₃Zr₂-type phase could be observed in RZ only if the powder size was larger than 106 μm. Meanwhile, the NiZr₂-type nanocrystals at the boundary of RZ primarily formed from the solidification of remelted liquid. With the increase of the powder size, the lower overheating temperature and shorter existing time of the molten pool enhanced the heredity of Al₅Ni₃Zr₂ clusters and other intermetallic clusters in remelted alloy melt, which decreased the thermal stability of the already-deposited layer. The volume fraction of crystallization in the deposit increased with the increase in powder size. There was no crystallization occurred in the HAZ between the adjacent tracks and layers for the deposit prepared by the powder with the size range of 53–75 μm. However, the wide crystalline band with Al₅Ni₃Zr₂-type faceted phase, CuZr-type dendrite, CuZr₂-type spherulite and NiZr₂-type nanocrystal were observed in the entire HAZ for the deposit prepared by the powder with the size range of 106–150 μm. The finer powder was benefit to prepare the BMGs by LSF.     

Microstructural changes of aluminized Alloy 617 during high-temperature aging

In this study, aluminized Alloy 617 was prepared by Al-pack cementation of high temperature high Al activity process. The microstructure evolution and microstructural changes of aluminide coating were investigated after Al-pack cementation and high-temperature aging. The aluminide coating was composed of Ni-aluminide layers, such as δ-Ni₂Al₃, β-NiAl, Cr₂Al, Al_3 + xMo_1 − x, and inter-diffusion zone by pack cementation. After high-temperature aging, the aluminide coating was transformed from the δ-Ni₂Al₃ to the β-NiAl because of outward Ni diffusion from substrate. The Cr₂Al and the Al_3 + xMo_1 − x were dissolved during aging. On the other hand, the α-(Cr, Mo) particles were precipitated during aging due to the low solubility of alloying elements in the β-NiAl. The β-NiAl newly formed by the outward Ni diffusion during aging and resulted in the formation of the inter-diffusion zone. The inter-diffusion zone consisted of β-NiAl, Ni₃(Al, Ti), Cr-rich M₂₃C₆ carbide, and sigma phases.     

Field electron emission experiments with plasma sprayed layers

The paper summarizes the investigation of field electron emission properties of plasma sprayed layers. The tested layers were Cr₂O₃ + SiO₂, Al₂O₃ + TiO₂, TiO₂, Cr₃C₂ + NiCr and Cr₃C₂ + NiCrAlY. The deposition methods were APS and SPS. The Cr₂O₃ + SiO₂, Al₂O₃ + TiO₂ and TiO₂ layers were laser engraved after deposition. Structure investigations of the deposited layers revealed them as composite. The obtained results show, that field electron emission from plasma sprayed layers is comparable with emission from DLC and other carbon-based layers. The best turn-on field was 12 V/μm and has been obtained for laser engraved Al₂O₃ + 13 wt.% TiO₂ layers. Investigation of temperature dependence of emission showed a remarkable influence of hot (Schottky) emission. The emission of Al₂O₃ + TiO₂ layers decreased in elevated temperatures. This has been attributed to the temperature dependence of TiO₂ permittivity, which increases with temperature. Computer calculations supported this explanation. The emission parameters suggested very high local electric fields. This probably was the result of field enhancement due to presence of dielectric-conductor-vacuum junction points in addition to that caused by the surface topography.     

Resistivity, thermopower, and thermal conductivity of nickel doped compounds Cr1−xNixSb2 at low temperatures

Substitutional compounds Cr_1−xNi_xSb₂ (0 ≤ x ≤ 0.1) were synthesized, and the effect of Ni substitution on transport and thermoelectric properties of Cr_1−xNi_xSb₂ were investigated at the temperatures from 7 to 310 K. The results indicated that the magnitudes of the resistivity and thermopower of Cr_1−xNi_xSb₂ decreased greatly with increasing Ni content at low temperatures, owing to an increase in electron concentration caused by Ni substitution for Cr. Experiments also showed that the low-temperature lattice thermal conductivity of Cr_1−xNi_xSb₂ decreased substantially with increasing Ni content due to an enhancement of phonon scattering by the increased number of Ni atoms. As a result, the figure of merit, ZT, of lightly doped Cr_0.99Ni_0.01Sb₂ was improved at T > ∼230 K. Specifically, the ZT of Cr_0.99Ni_0.01Sb₂ at 310 K was approximately ∼29% larger than that of CrSb₂, indicating that thermoelectric properties of CrSb₂ can be improved by an appropriate substitution of Ni for Cr.     

Structural, magnetic and electrical properties of Al substituted Ni-Zn ferrite nanoparticles

Nanocrystalline ferrite materials having the general formula Ni_0.7Zn_0.3Fe_2−xAl_xO₄ (0.0 ≤ x ≤ 0.5) have been synthesized by citrate-gel auto combustion method and characterized using X-ray diffraction (XRD), energy dispersive X-ray (EDX), field emission scanning electron microscopy (FE-SEM), dc magnetization, dielectric and impedance spectroscopy measurements. XRD studies confirm that all the samples show single phase cubic spinel structure. The crystallite size of Ni_0.7Zn_0.3Fe_2−xAl_xO₄ (0.0 ≤ x ≤ 0.5) nanoparticles calculated using the Debye-Scherrer formula was found in the range of 13-17 nm. The value of lattice parameter ‘a’ is found to decrease with increasing Al³⁺ content. EDX patterns confirm the compositional formation of the synthesized samples. FE-SEM micrographs show that all the samples have nano-crystalline behavior and particles show spherical shape. The variation of dielectric properties ?′,?″, and tan δ with frequency shows the dispersion behavior which is explained in the light of Maxwell-Wagner type of interfacial polarization in accordance with the Koop's phenomenological theory. The dc magnetization studies infer that magnetic moment of Ni_0.7Zn_0.3Fe_2−xAl_xO₄ (0.0 ≤ x ≤ 0.5) nanoparticles was found to decrease with Al doping. Impedance spectroscopy techniques have been used to investigate the effect of grain and grain boundary on the electrical properties of the synthesized compounds.     

Microstructure, billet surface quality and tensile property of (Al2O3 + Al3Zr)p/Al composites in situ synthesized with electromagnetic field

The aluminum matrix composites reinforced by Al₂O₃ and Al₃Zr particulates were fabricated via in situ chemical reaction between Al-15 wt.% Zr(CO₃)₂ systems. In the process of in situ reaction, a low frequency electromagnetic field (EMF) is employed to improve the conditions of reaction between reactants powder and melt. The optimized electromagnetic density of low frequency EMF is 0.025 T. During the direct chill casting process of composites melt, the custom-designed electromagnetic fields are introduced to control the microstructures and improve the billet surface quality. XRD analysis shows that Al₂O₃ and Al₃Zr reinforcement phases have been obtained. The Lorenz force improves the kinetic condition and accelerates the nucleation of endogenetic particulates. Microstructure analysis by SEM indicates that the average size of particulates and grain size of matrix are refined to 0.5-1 μm and 20-40 μm, respectively. The surface quality of round billet is greatly improved by the high frequency EMF. The results of tensile properties test show that the tensile strength of composites in situ fabricated with EMF is 254.6 MPa, which is increased by about 104 MPa and 69.4% compared with those of composites in situ fabricated without EMF.     

Formation and characterization of aluminide coatings on alloy 800 substrate

Fe-Ni-Cr based Alloy 800 substrates were pack aluminized at 1273 K for 8 hours in argon atmosphere. The cross-section of aluminized substrates indicated formation of multilayer of aluminides as revealed by microscopy, electron probe microanalysis and X-ray diffraction. The outermost layer (~ 150 μm) was found to consist of FeAl + Fe₂Al₅ type phases, while the adjoining layer (~ 250 μm) was composed of an FeAl type phase. The innermost layer (~ 60 μm), which was a solid solution zone, was found to consist of ~ 43 at.% Fe, 38 at.% Cr, 11 at.% Ni containing about 6 at.% Al. In microhardness test, a hardness variation of 213 to 1098 Knoop hardness number along the cross-section was obtained. Scratch test along the cross-section at load levels ranging from 0.9 to 10 N with a loading rate of 30 N/min showed a maximum penetration depth of 12 μm indicating a good adherence of aluminide coatings.     

An investigation of the Al–Ni–Rh phase diagram between 50 and 100 at% Al

The Al-rich part of Al–Ni–Rh was studied between 800 and 1080 °C. The Al₉Rh₂ phase was found to contain up to 8 at% Ni. The orthorhombic Al–Rh ɛ₆-phase extends up to 17.5 at% Ni, high-temperature cubic C-Al₅Rh₂ up to about 10 at% Ni while low-temperature hexagonal H-Al₅Rh₂ extends up to 4 at% Ni. The Al₇Rh₃ phases contained up to 3 at% Ni. The solubility of Rh in Al₃Ni is up to 3 at% and in Al₃Ni₂ up to 5 at%. The isostructural binary AlNi and AlRh phases probably form a continuous β-range of the CsCl-type solid solutions. A ternary hexagonal phase similar to Al₂₈Ir₉ (a = 1.213 and c = 2.626 nm) was found to be formed between Al₇₆Ni₄Rh₂₀ and Al₇₆Ni₁₃Rh₁₁. The formation of the high-temperature stable decagonal phase was confirmed. Another ternary phase, whose structure is not yet clarified, was revealed around Al₇₀Ni₁₁Rh₁₉. Partial 1080, 1000, 900 and 800 °C isothermal sections of the Al–Ni–Rh phase diagram are presented.     

Degradation kinetics at 650 °C and lifetime prediction of Ni2Al3/Ni hybrid coating for protection against high temperature oxidation of creep resistant ferritic steels

The degradation kinetics of the Ni₂Al₃/Ni hybrid coating formed on creep resistant ferritic steel is studied by prolonged isothermal annealing experiments at 650 °C. The sequence of formation and subsequent consumption of all the intermediate phase layers are determined by intermittent analysis of the specimen at various annealing intervals. It is shown that the Ni₅Al₃ phase layer starts to form only when the outer Ni₂Al₃ phase layer is completely consumed and the growth kinetics of each of the intermediate layers differs from that of its subsequent consumption. A model is subsequently formulated to predict the lifetime of the coating studied.     

Growth and stability of CVD Ni3N and ALD NiO dual layers

Multilayers of combinations of NiO, Ni₃N and Ni have been grown by ALD and CVD techniques at 250 °C. Layers of low thermodynamical stability have been modified to reach the target structures. The Ni layers have been formed by decomposition of metastable Ni₃N layers, i.e., the Ni₃N layers act as precursor for Ni film growth. This new reaction route enables production of Ni/NiO layer structures by chemical means for the first time. By choosing suitable low temperature annealing conditions like 180 °C in a 1 Torr hydrogen atmosphere, good control of the interfaces is obtained.It has also been shown that it is possible to grow multilayers which are ordered both with respect to each other, the substrate and the Ni films. For instance the following structure Ni (111)/NiO (111)/α-Al₂O₃ (00l) has been grown. Moreover, another new reaction route is deposition of thin epitaxial seed layers of NiO (111) for subsequent growth of Ni₃N at a high rate. Single phase Ni (111) films could then be obtained by decomposition at 350 °C of the Ni₃N layers. The demonstrated reaction routes for film growth in the Ni-O-N system can also be applied in several similar systems.     

Influence of metastable Al9Ni2 phase on the sequence of phase transformations initiated by heating of Al/Ni multilayer foils produced by EBPVD method

Al/Ni multilayer foils (MF) undergo a cascade of phase transformations at heating, initiated by diffusion interaction of Al and Ni layers. It is found that phase transformations sequence at initial stages depends on the method of producing MF: at sputtering or ion-beam deposition of elements, metastable Al₉Ni₂ phase forms at phase transformations initial stages, and in the case of MF produced by electron-beam physical vapour deposition (EBPVD) method or cold rolling of laminates, this is Al₃Ni phase. Such a difference in phase transformations sequence is associated with the influence of the method of MF production on the possibility of intermixed zone (IZ) formation on layer interfaces. In the study it was suggested that such anomalously high diffusion mobility of atoms can be achieved in the presence of excess vacancies in MF structure. With this purpose, MF structure was produced by high-rate (up to 30 nm/s) layer-by-layer deposition of elements by EBPVD method. Phase transformations and MF were studied by the method of X-ray diffraction (XRD) and differential-thermal analysis (DTA). It is shown that irrespective of MF composition and modulation period, at their heating phase transformations start with formation of metastable Al₉Ni₂ phase. At further MF heating, a stable Al₃Ni phase forms alongside Al₉Ni₂ phase. Later on, Al₉Ni₂ and Al₃Ni phases turn into stable intermetallics characteristic of MF chemical composition.