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.     

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.     

Solid-state reactive diffusion between Ni and W

Various alloys are interconnected with W using a Ni intermediate layer by the diffusion bonding (DB) technique. During solid-state heating in the DB technique, however, reactive diffusion occurs at the interconnection between Ni and W. In order to examine the kinetics of the reactive diffusion, sandwich Ni/W/Ni diffusion couples were isothermally annealed at temperatures of T = 1023-1173 K for various times up to t = 366 h. Owing to annealing, Ni₄W is produced as a layer at the Ni/W interface in the diffusion couple and grows predominantly towards Ni. The thickness of the Ni₄W layer increases in proportion to a power function of the annealing time. The exponent of the power function is close to 0.5 at T = 1173 K and gradually decreases with decreasing annealing temperature. Furthermore, grain growth takes place in Ni₄W due to annealing. Therefore, volume diffusion is the rate-controlling process for the growth of Ni₄W at T = 1173 K. At T < 1173 K, however, boundary diffusion contributes to the rate-controlling process, and the contribution of boundary diffusion increases with decreasing annealing temperature. On the other hand, a region alloyed with W is formed in Ni from the Ni₄W/Ni interface by diffusion-induced recrystallization (DIR). The growth rate of the DIR region is much greater than the penetration rate of W into Ni by volume diffusion, and the concentration of W in the DIR region at the Ni₄W/Ni interface is equal to the solubility of W in Ni. Such growth behavior of the DIR region was numerically analyzed using a mathematical model. The analysis indicates that the growth of the DIR region is controlled by the interface reaction at the moving boundary of the DIR region as well as the boundary diffusion along the grain boundaries across the DIR region under the present annealing conditions.     

The corrosion of Ni3Al in a combustion gas with and without Na2SO4-NaCl deposits at 600–800°C

The corrosion behavior of Ni₃Al containing small additions of Ti, Zr, and B in combustion gases both with and without Na₂SO₄–NaCl deposits at 600–800°C has been studied for times up to four days. The corrosion of the saltfree Ni₃Al leads to the formation of very thin alumina scales at 600°C but of mixed NiO–Al₂O₃ scales containing also some sulfur compounds at higher temperatures, while the rate increases with temperature up to 800°C. The presence of the salt deposits considerably accelerates the corrosion rate, especially at 600 and 800°C. The duplex scales formed at 600°C are composed mostly of a mixture of NiO and unreacted salt in the outer layer and of alumina and aluminum sulfide with some nickel compounds in the inner layer. The scales grown at 700°C contain only one layer of complex composition, while those grown at 800°C are similar but have an additional outer layer containing similar amounts of nickel and aluminum. At 600 and 700°C NiSO₄ can be detected also in the salt layer. The samples corroded at 700°C and 800°C also show an Al-depleted zone containing titanium sulfide precipitates at the surface of the alloy. The hot corrosion of Ni₃Al involves a combination of various mechanisms, including fluxing of the oxide scale as well as mixed oxidation-sulfidation attack. At all temperatures Ni₃Al shows poor resistance to hotcorrosion attack as a result of the formation of large amounts of Ni compounds in the scales.     

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.     

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.     

Oxidation Behavior of Ni3Al and Fe3Al: II. Early Stage of Oxide Growth

Oxidation treatments of Ni₃Al and Fe₃Al were performed at room temperature in 0.2 atm O₂ for 5 min and at 300 and 500°C in air for 5 min, and 6, 50, 100, and 200 hr. The oxides were analyzed by XPS, AES, and SEM. A model explaining the initial stages of oxide formation is suggested. At room temperature and 300°C, islands of Al₂O₃ and NiO combined with NiAl₂O₄ formed on Ni₃Al. At 500°C, the Ni oxides grow laterally and cover the Al₂O₃ islands. Islands of Al₂O₃ and Fe₂O₃ mixed with Fe(Fe, Al)₂O₄ formed on Fe₃Al at room temperature. At 300 and 500°C the scale is composed of an outer layer rich in Fe oxides and an inner layer rich in Al oxides. During long time exposure, islands of Fe₂O₃ and Fe(Fe, Al)₂O₄ formed at the surface by diffusion of Fe cations through the alumina layer. The oxide growth on Fe₃Al reaches a steady-state regime after formation of the continuous alumina layer. At 300°C, the oxide formed on Fe₃Al is thicker than on Ni₃Al, whereas it is reverse at 500°C.     

Co-effect of heat and direct current on growth of intermetallic layers at the interface of Ti-Ni diffusion couples

The growth of the intermetallic layers at the interface of Ti-Ni diffusion couples was investigated under the co-effect of heat and direct current. Isothermal diffusion treatments for Ti-Ni couples were conducted at 500, 600 and 700 °C for 5, 10 and 15 h with and without the passage of DC current of 10 A intensity. It was found that both Ti₂Ni and TiNi₃ layer form at the Ti-Ni interface in all couples treated by different process, but TiNi layer forms in the couples annealed above 600 °C without current or at 500 °C with current. The growth of the whole interfacial layer shows a parabolic relationship with time. The apparent activation energy of growth for the whole interfacial layer is 83.76 kJ/mol in the couple treated by heating without a current, and it decreases to 42.11 kJ/mol in the couple treated with a direct current of 10 A during heating. The effect of the current on the growth of different intermetallic layers varies with its direction.     

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.     

Annealing effects on the intermetallic compound formation of cold sprayed Ni,Al coatings

The annealing of Ni and Al coatings under various conditions on substrates fabricated by a cold gas dynamic spray process (CDSP) were investigated. The powder particles were accelerated through a standard De Laval-type nozzle with air used as the main carrying gas. The coatings were annealed at 450–550 °C in either argon or air atmospheres for 4 h. In the case of Ni coatings during annealing both in argon and air atmospheres, intermetallic compound layers such as Al₃Ni and Al₃Ni₂ were observed at the interfaces between the Ni coating and Al substrate. Also, the intermetallic layer formation of Al₃Ni and Al₃Ni₂ at the interfaces depended on the solid-state diffusion and the annealing temperature. The intermetallic compound AlNi was obtained at the interface of Al coating on a Ni substrate by low-temperature annealing under the melting temperature.     

Phase transformation behavior of nanocrystalline Ni-W-P alloys containing various W and P contents

In the present investigation, electroless (EL) ternary Ni-W-P coatings were prepared using hypophosphite based alkaline bath by varying sodium tungstate as tungsten source (5-80 g/L). Maximum amount of W incorporation (8.2 ± 1 wt.%) was obtained when the bath contained about 20 g/L of tungsten source. At very high concentrations of W source in the bath the deposit contained about 4 wt.% W and 2 wt.% P. All the as-deposited ternary coatings exhibited nodular surface morphology. X-ray diffractograms (XRD) obtained for as-deposited EL NiWP alloys indicated that crystallinity of the coatings increased with decrease in phosphorus content. Calculated grain size for the deposits varied from 1.2 to 12.7 nm when the tungsten source varied from 5 to 80 g/L in the bath. Higher crystallization temperatures were obtained due to W codeposition in NiP matrix. Presence of metastable phases such as Ni₅P₂ and NiP apart from stable Ni and Ni₃P was identified for the heat treated deposits (400 °C/1 h) containing lower amount of W and higher amount of P. Whereas other ternary deposits after the heat treatment predominantly revealed face centered cubic (f.c.c.) Ni (111) peak. Activation energy for the crystallization of all the alloys has been carried out by modified Kissinger method. Microhardness measurements were carried out on all the deposits isothermally heat treated at 600 °C for 1 h.     

Influence of rotating magnetic field on the microstructure and phase content of Ni-Al alloy

Rotating magnetic field is introduced in the production process of Ni-Al precursor alloy of skeletal Ni catalyst. The results showed that the big dendrites of Al₃Ni₂ disappeared, the size of Al₃Ni₂ decreased from 64.5 μm to 37.2 and 35.5 μm, phase content of Al₃Ni₂ decreased while Al₃Ni increased after applying field current of 80 A and 140 A, respectively. The change of phase content is probably caused by the increase of surface area between the Al₃Ni₂ phase and fluid which is favorable to the peritectic reaction.     

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.     

Fatigue behavior of (graded) (Ti, Al)N-coated 1Cr11Ni2W2MoV stainless steel at high temperature

The high-cycle fatigue properties of graded (Ti, Al)N- and Ti_0.7Al_0.3N-coated 1Cr11Ni2W2MoV at 500 °C have been investigated using a rotating bending fatigue testing machine. The results show that fatigue strength and life of 1Cr11Ni2W2MoV stainless steel were apparently increased by the presence of Ti_0.7Al_0.3N coating. The fatigue life and strength was also improved to some extent by the presence of the graded coating at higher stress levels (475 MPa-525 MPa). The fracture morphologies were analyzed by SEM. It is concluded that the presence of the coatings, which were well adhered and have high compressive stress, restricted plastic deformation of the substrate, thus improved the fatigue properties of the steel.     

Hot corrosion behaviour of Ni3Al in sulphate-chloride mixtures in the atmosphere

Hot corrosion of Ni₃Al intermetallic compound in the presence of sulphate-chloride mixtures was studied. A comminuted Ni₃Al mixed with NaCl-Na₂SO₄, NaCl-Li₂SO₄, LiCl-Na₂SO₄, LiCl-Li₂SO₄ additions was oxidized in the air up to 1000 °C with linearly increasing temperature and isothermally within the temperature range of 500-700 °C. The corrosion process was observed by thermogravimetric method using Mettler thermoanalyzer.The experiments indicated that LiCl (∼10 wt.%)-Li₂SO₄ mixture was the most corrosive agent. It was also found that by addition of MgO the corrosion of Ni₃Al was reduced. Phase composition of the corrosion products was established by X-ray diffraction analysis; there were detected Al₂O₃, Al₂S₃, NaAlO₂ (or LiAlO₂) as intermediate products, nickel sulphides, NiO and NiAl₂O₄. NiAl₂O₄ spinel was formed only at the highest temperatures, while at lower temperatures alumina was present instead of spinel.Hot corrosion behaviour of Ni₃Al in sulphate-chloride mixtures in air atmosphere.     

Solid state reactions in Al/Ni alternate foils induced by cold rolling and annealing

Al/Ni composites made of alternate foils having overall composition Al₅₀Ni₅₀ and Al₆₆Ni₃₄ were rolled up to 75 times folding them after every rolling pass to restore approximately the original thickness. It was found that the deformation of the composite is sustained by the Ni with Al acting as transmitting medium. The logarithmic reduction of foil thickness scales with the number of rolling passes. A nanocrystalline state of the elements, particularly Ni, is progressively reached. No detectable reaction is caused by repeated co-deformation. Reactions in the composites occur on annealing. The sequence of phases obtained at Al/Ni interfaces via nucleation and growth, and identified by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, reproduces that found on annealing deposited multilayers and ball-milled powders: Al₃Ni, Al₃Ni₂, NiAl. All reactions are strongly activated by deformation, i.e. they occur at lower temperature as revealed by continuous heating experiments in a differential scanning calorimeter. The overall set of experimental results is consistent with reaction mechanisms of nucleation and growth with grain-boundary interdiffusion as the rate-determining step. This view is supported by comparison with a collection of data for the activation energy of diffusion, grain growth, and ordering in Al–Ni phases.     

Cyclic oxidation of an ultrafine-grained and CeO2-dispersed δ-Ni2Al3 coating

A novel ultrafine-grained and CeO₂-dispersed δ-Ni₂Al₃ coating was fabricated through aluminizing a CeO₂-nanoparticle-dispersed nanocrystalline Ni matrix film using an NH₄Cl-activated pack cementation method at 600 °C. Two CeO₂-free δ-Ni₂Al₃ coatings, one coarse-grained and the other ultrafine-grained, were also prepared. Compared with the later, the ultrafine-grained and CeO₂-dispersed coating profoundly increased scale spalling resistance during cyclic oxidation in air at 1000 °C. The dispersed CeO₂, together with the ultrafine-grained coating structure, helped prevent the formation of cavities at the scale/coating interface, which was proposed as a main cause for improvement of the cyclic oxidation resistance.     

Effect of chromium on early stages of oxidation of Ni3Al alloys at 600°C

Initial oxide formation at 600°C in air on Ni₃Al alloys with and without chromium additions was studied by TEM. Significant lateral nickel diffusion (apparently stress-induced) occurred in both alloys producing bands of nickel and nickel oxide-enriched hillocks. Chromium additions clearly alleviate dynamic embrittlement in Ni₃Al; chromium additions were previously assumed to affect the oxidation process. Chromium additions significantly reduced the oxidation rate of the alloy. However, a continuous film of pure Cr₂O₃ had not yet formed after 45 sec oxidation. Grain boundaries preferentially oxidized to form Al₂O₃ or Cr₂O₃ and rejected nickel along both the surface and the grain boundary, deeper into the specimen. The dramatic effect of chromium on improving the ductility of Ni₃Al when tested at high temperature in air is apparently a result of a process that occurs at or near the tip of a propagating crack that is both faster and on a finer scale than that studied here.     

Microstructure and composition of the grain/binder interface in WC–Ni3Al composites

The microstructure and composition of WC/Ni₃Al interface were studied. An orientation relationship of [100]_WC//[110]_Ni3Al and (001)_WC//(001)_Ni3Al with a good coherence, besides many random orientation relationships between WC and Ni₃Al, has been repetitively found by selected area electron diffraction and high resolution TEM observations. The XRD pattern of WC–Ni₃Al composites indicated that the major binder phase was Ni₃Al and showed possibility of coherence between WC and Ni₃Al as common interplanar spacings existed. Electron probe microanalysis results revealed that the atomic ratio of Al:Ni is close to 1:3 in binder phase and WC/Ni₃Al interface in the WC–Ni₃Al composites has a sharper compositional gradient and a smaller width of transition region than the WC/Co interface in WC–Co composites.