Formation Of NiAl Research Articles

Characterization of two-phase nickel aluminides produced by pressure-assisted combustion synthesis

Two-phase (B2+L1 2) nickel aluminide intermetallic compounds were synthesized by the pressure-assisted volume combustion synthesis (CS). The production and characterization of the samples containing NiAl+Ni 3Al were investigated. Aluminum (99% pure, 15 μm) and carbonyl nickel (99.8 pure, 4–7 μm) powders were used. The production of intermetallic compound was carried out at 1050 °C under 150 MPa uniaxial pressure in open air atmosphere in an electrical resistance furnace for 60 min. The formation temperature of intermetallic compound was determined by differential scanning calorimeter (DSC) analysis, and exothermic temperature of powder mixture was determined as 653 °C. The characterization of samples was confirmed by optical microscope, SEM and XRD analysis. It was observed that the structure of compound has very low porosity and the formation of NiAl was completed successfully. The relative density of test materials measured according to Archimedes’ principle was 98.04%. The microhardness of test materials was about 351 HVN 1.

Growth of Ni–Al alloys on Ni(1 1 1), from Al deposits of various thicknesses: (II) Formation of NiAl over a Ni 3Al interfacial layer

This paper describes the second part of a study devoted to the growth of thin Ni–Al alloys after deposition of Al on Ni(1 1 1). In the previous paper [S. Le Pévédic, D. Schmaus, C. Cohen, Surf. Sci. 600 (2006) 565] we have described the results obtained for ultra-thin Al deposits, leading, after annealing at 750 K, to an epitaxial layer of Ni 3Al(1 1 1). In the present paper we show that this regime is only observed for Al deposits smaller than 8 × 10 15 Al/cm 2 and we describe the results obtained for Al deposits exceeding this critical thickness, up to 200 × 10 15 Al/cm 2. Al deposition was performed at low temperature (around 130 K) and the alloying process was followed in situ during subsequent annealing, by Auger electron spectroscopy, low energy electron diffraction and ion beam analysis-channeling measurements, in an ultra-high vacuum chamber connected to a Van de Graaff accelerator. We evidence the formation, after annealing at 750 K, of a crystallographically and chemically well-ordered NiAl(1 1 0) layer (whose thickness depends on the deposited Al amount), over a Ni 3Al “interfacial” layer (whose thickness—about 18 (1 1 1) planes—is independent of the deposited Al amount). The NiAl overlayer is composed of three variants, at 120° from each other in the surface plane, in relation with the respective symmetries of NiAl(1 1 0) and Ni 3Al(1 1 1). The NiAl layer is relaxed (the lattice parameters of cc-B2 NiAl and fcc-L1 2 Ni 3Al differ markedly), and we have determined its epitaxial relationship. In the case of the thickest alloyed layer formed the results concerning the structure of the NiAl layer have been confirmed and refined by ex situ X-ray diffraction and information on its grain size has been obtained by ex situ Atomic Force Microscopy. The kinetics of the alloying process is complex. It corresponds to an heterogeneous growth leading, above the thin Ni 3Al interfacial layer, to a mixture of Al and NiAl over the whole Al film, up to the surface. The atomic diffusion is very limited in the NiAl phase that forms, and thus the progressive enrichment in Ni of the Al film, i.e. of the mean Ni concentration, becomes slower and slower. As a consequence, alloying is observed to take place in a very broad temperature range between 300 K and 700 K. For annealing temperatures above 800 K, the alloyed layer is decomposed, Al atoms diffusing in the bulk of the substrate.

Milling criteria for the synthesis of nanocrystalline NiAl by mechanical alloying

The optimum milling parameters and energy domains for the synthesis of nanocrystalline NiAl by mechanical alloying was determined. The long-range ordered structure of NiAl under various milling conditions was also evaluated. Dominant role of the total energy of the grinding balls over the impact energy of each ball in the phase formation process was observed. A total energy of 200 kJ kg −1 was found necessary to trigger the NiAl formation from an equiatomic Ni–Al blend during mechanical alloying in stainless steel as well as in tungsten carbide media. The completion of the reaction required 550 and 700 kJ kg −1 of total energy in the stainless steel and tungsten carbide media, respectively. The low-energy requirement for the NiAl synthesis in stainless steel media was attributed to the Fe-contamination during ball milling. A close relation between the enthalpy of formation of the compound and the total energy of the grinding balls for its formation was observed.

Fabrication and Characterization of Cold Rolled Ni/Al Multilayer Foils

AbstractNi/Al reactive multilayer foils were fabricated by cold rolling method and self-propagating reactions in these foils were investigated. A two-stage phase formation process was observed in the ignition experiment. The first step is the lateral growth of Al3Ni phase from isolated nucleation sites and the following coalescence into a continuous layer; the second step is the growth of such Al3Ni layers in the perpendicular direction of the interface until all Al is consumed. As there is still Ni available, Al3Ni will continuously react with Ni until no Ni is left, and the final reaction product turns out to be ordered B2 AlNi compound. X-ray diffraction (XRD) experiments showed that the reaction product of the cold rolled foil was the same as the physical vapor deposition (PVD) foil. The reaction process was studied by differential scanning calorimetry (DSC). Three peaks can be identified from the DSC curve. Cold rolled foils were heated to different peak temperatures obtained from DSC curve with the same heating rate as DSC. XRD results for such foils showed that the first two peaks were the exothermic formation of Al3Ni, while the last one was for the formation of AlNi. The enthalpy of the reaction for the cold rolled foil was calculated to be -57.5 KJ/mol, which was in good agreement with the formation enthalpy of AlNi (-59 KJ/mol). The reaction velocities of the first formation stage were measured to be 7mm/s for cold rolled foils, which were much smaller than the reaction velocities of PVD foils (which range between 1-30 m/s).

The enthalpy of formation of NiAl

NiAl and NiAl based ternary metallic systems have attracted much attention in recent years for potential use as high temperature structural materials [1–6]. Alloy development of such systems requires a thorough knowledge of the phase equilibria and thermodynamics of the systems. Using high temperature calorimeters [7–9], the enthalpies of formation for many alloy systems have been measured and the results incorporated into thermodynamic databases for modeling of their phase diagrams using software such as Thermocalc©R [10]. However, there exists some confusion regarding the value for the enthalpy of formation of NiAl. In one reference [11], comparison of published experimental data was made which are referred to different standard states, with the consequent conclusion that there is a wide discrepancy in the experimental NiAl data. In this paper, the enthalpies of formation of NiAl as a function of composition determined by two different calorimetric techniques are compared. The results are also compared to enthalpy of formation data for the stoichiometric alloy from several other sources. Two kinds of calorimeters are widely used in measuring the enthalpies of formation of intermetallic compounds with high melting points: the differential solution calorimeter and the direct synthesis calorimeter. The former is an indirect method because the compound is prepared before the enthalpy of formation is determined, while the latter is a direct one since the enthalpy of formation is determined during alloy formation. Using the differential solution calorimeter, Henig and Lukas [12] determined the enthalpy of formation of NiAl at 1100 K. During their experiment, the element Ni and the compound NiAl were dropped into the liquid aluminum separately and their heat of dissolution were measured in turn. The reactions involved in the measurement are:

Preparation of Nanocrystalline NiAl Compounds and Composites by Mechanical Alloying

During the past ten years, mechanical alloying/ball milling has been employed to successfully synthesize nanocrystalline NiAl intermetallic compounds (NiAl, NiAl (Cr,Co,Ti,Fe)) and composites (NiAl-TiC, NiAl-TiB2, NiAl-HfC, NiAl-HfB2) in our group, namely, by means of self-sustained reaction. During the mechanical alloying process, an abrupt temperature rise has been observed. The large heat release from the formation of NiAl, as well as HfC, HfB2, TiC, TiB2 compounds is suggested to be the driven force of the self-sustained reaction. It is interested to observe that nanocrystalline NiAl exhibits very well microstructure stability during high temperature annealing. The alloying additions can decrease reaction rate and prolong the reaction time. With the increase of alloying additions, supersaturated solid solution even amorphization of NiAl has been formed. The nanocrystalline NiAl compounds show a rise of the compressive strength and the ductility at room temperature compared with that of the cast coarse crystalline compound.

Electrolytic production and heat-treatment of Ni-based composite layers containing intermetallic phases

The influence of the presence of Al powder in the galvanic bath on the chemical composition of Ni + Al layers was investigated. The electrodeposition process was carried out at a temperature of 293K and a current density of jdep = 200–450mAcm−2. It was stated, that Al percentages increase with an increase in the amount of Al powders in the bath and reach the value 8–44% for Al. The surface morphology of the coatings was examined by means of scanning microscopy (SEM method). The thickness of the coatings was estimated using microscope methods on the base of cross-sectional images of the coatings (the thickness of Ni + Al coatings was 150–320μm). X-ray diffraction method was used to determine the phase composition of the layers. Quantitative chemical analysis of the coatings was determined by means of X-ray fluorescence method (XRF) and verified by atomic absorption spectroscopy (AAS). The size of the nickel crystallites has been determined from the half-width of the diffraction peaks (27nm). The thermal treatment of the layers in 900°C/12h in argon atmosphere changes the surface morphology and leads to produce a new kind of materials. They contain Ni2Al3, Ni3Al, NiAl phases embedded in the nickel matrix. About 24h thermal treatment in 900°C leads to stabilization of the system with the formation of NiAl, AlNi3 phase. The mechanism of incorporation of solid particles into the layers’ structure was explained based on Ni2+ adsorption/desorption processes on the metal powder.

Formation of Ni-Al and Ni-Al-X Phases by Mechanical Alloying

Formation of Ni-Al and Ni-Al-X Phases by Mechanical Alloying

Reactive processing of nickel-aluminide intermetallic compounds

NiAl have been fabricated by reactive sintering compacts of ball-milled powder mixtures containing Ni and Al. The reaction mechanism, as well as phase and microstructural development, were investigated by analyzing compacts quenched from different temperatures during reactive hot compaction. It was found that the reaction process was strongly affected by pressure, heating rates, heat loss from the sample to the environment. The application of 50 MPa prior to the reaction resulted in the intermetallic-formation reaction initiating at a temperature (480°C) much lower than that (∼550°C) when no pressure was applied. At high heating rate (50°C/min), when the heat loss is small, the formation of NiAl occurs rapidly via combustion reaction. On the other hand, if the heat loss is significant as in slow heating rate (10°C/min), the reaction process is controlled by solid-state diffusion. The phase formation sequence for the slow solid-state reaction was determined to be: NiAl3→ Ni2Al3→ NiAl → NiAl (Al-rich) + Ni3Al → NiAl.

411 NiAl/α-Al_2O_3 複合材料の機械的性質に及ぼす酸化処理の影響

The strength and fracture toughness of 10vol%NiAl/α-Al_2O_3 composites is 494MPa 3.39MPa・m^<1/2>, and 570MPa and 3.91MPa・m^<1/2> at the sintering temperature of 1550 and 1600℃, respectively. The higher strength and higher fracture toughness exihibited at 1600℃ is due to the high interfacial joining strength between NiAl grains and matrix alumina. The strength of oxidation-derived composite degrades relating to the formation of NiAl_2O_4 on the surface, however, relatively high strength of 470MPa is maintained.

611 アルミナ/NiAl 複合材料の表面自己組織化による強靱化

Tri-layer composites consisting of surface layers with residual compressive stress and a stress-tolerant inner layer have been fabricated by the oxidation of alumina-matrix composites dispersing intermetallic NiAl. The residual surface stress is generated by the volume expansion due to the formation of NiAl_2O_4. The starting Al_2O_3-NiAl powder was prepared by heating the precursor mixture of α-Al_2O_3 powder for the matrix and (Ni, Al)-hydradino-benzoate precipitate of at 1200℃ in Ar. The fracture toughness of. the 10vol%NiAl-Al_2O_3 composites was 6.3MPa・m^<1/2>, indicating the suitability as the inner layer material applied under tensile stress. The measured surface compressive stress for the tri-layer composites was more than 120MPa and this stress improved the fracture toughness to 1.5 times of the dispersion composites at the degree of oxidation of 85%. However, when the surface oxidation progressed to more than 90%, much defects formed at the boundary of the surface and inner layers to relax the stress and degrade the toughness.

Composition dependence of the enthalpies of formation of NiAl

The enthalpy of formation of Ni x Al 1− x as a function of composition has been determined by high temperature reaction calorimetry over the range 0.44≤ x≤0.58. The value for the Ni 0.5Al 0.5 composition is −61.8±1.1 kJ/mol. The present results are compared with previously published results on the enthalpy of formation of Ni x Al 1− x . In several cases the experiments were performed at two different calorimeter temperatures and the difference in heat content values is compared with the heat capacity of NiAl.

Reaction Condition Effects on Nickel Sorption Mechanisms in Illite-Water Suspensions

Nickel sorption in illite suspensions was studied as a function of pH (4.5–8.0), reaction time (3 h, 24 h, and 1 wk) and ionic strength (I = 0.1 and 0.003 M) using x-ray absorption spectroscopy (XAS) to characterize the Ni sorption complexes formed. The formation of Ni-Al layered double hydroxide (LDH) phases was observed at pH values >6.25, with increasing formation of these phases with time, and a faster formation rate with increasing pH. Comparison among samples with the same Ni loading, but different reaction times and pH values, showed larger second neighbor scattering for the samples reacted at higher pH, which had a faster Ni sorption rate. Most likely this was due to a greater importance of Ni-Al LDH precipitation relative to other (mononuclear) Ni sorption mechanisms at higher pH, and/or a higher Al content in the Ni-Al LDH phase formed at lower pH (slower Ni sorption rate). Lowering the ionic strength resulted in increased Ni sorption for the entire pH range studied. Our extended x-ray absorption fine structure spectroscopy (EXAFS) data indicated that this was due to significant outer-sphere Ni sorption occurring at the planar sites at low I, leading to a reduced importance of Ni-Al LDH formation in the overall Ni sorption process at pH > 6.50.

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.

Solid state reactions in Ni–Al–Ti–C system by mechanical alloying

Several Ni–Al–Ti–C compositions from different areas of the Ni–Al phase diagram containing various amounts of Ti and C were prepared by mechanical alloying. Synthesized samples were investigated by X-ray diffraction and differential scanning calorimetry methods. Solid state reactions in Ni–Al–Ti–C powder mixtures with an equiatomic Ni/Al ratio lead to the formation of NiAl and TiC equilibrium phases. Metastable solid solution of Ni(Al,Ti,C) and non-stoichiometric TiC x are formed in the mixtures with C Ni/C Al ratio of 1.7–3. The grain size of metastable intermetallic phases is less than 5 nm, if Ni and Al content in the mixtures corresponds to the dual-phase and Ni 3Al-areas of the Ni–Al phase diagram.

Reaction steps in the combustion synthesis of NiAl/TiB2 composites

The reaction steps in the formation of NiAl/TiB2 composites produced by reaction synthesis have been determined using differential thermal analysis (DTA). The DTA technique reveals that the formation of NiAl/TiB2 composites occurs in a two-step reaction. NiAl forms first at approximately 550 °C, followed by TiB2, which forms at approximately 1050 °C. Both X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS) were performed on composites produced by reaction synthesis in a hot-press facility using the same heating rate used in the DTA experiments. It was found that the formation of NiAl and TiB2 is preceded by the formation of intermediate compounds such as Ni3Al, TiAl, Ti3Al, Ti2Ni, and TiB. The relative density measured in the composites with 0, 0.25, 0.50, and 0.75 volume fractions of TiB2 was in excess of 96 pct of the theoretical density, since, during synthesis, NiAl is a transient liquid that acts as a binder phase for the TiB2 particles.

Evolution of aluminide coating microstructure on nickel-base cast superalloy CM-247 in a single-step high-activity aluminizing process

This study deals with the aluminizing of a directionally cast Ni-base superalloy, namely CM-247, by a single-step process using a high-activity pack. It is observed that significant incorporation of Al into the substrate surface during aluminizing continues over a period of about 1 hour and is not restricted merely to the first few minutes, as reported in the literature. Based on the microstructural details of the coatings formed at various stages of aluminizing, it is concluded that the coating growth in the above process takes place primarily by inward Al diffusion initially, followed by an intermediate stage when the growth involves both inward Al and outward Ni diffusion. In the final stages, the outward diffusion of Ni dominates the coating formation process. The above mechanism of coating formation is different from the one that prevails in the conventional two-step high-activity coating process in that the reaction front for the formation of NiAl remains spatially stationary despite the outward diffusion of nickel during the intermediate stage. It is also shown in the present study that the content of the Al source in the pack affects the coating structure significantly. It is further demonstrated that the microstructure of the aluminide coatings depends not only on the amount of Al incorporated in the sample during aluminizing but also on the time over which the uptake of this Al takes place.

Synthesis of NiAl–TiC nanocomposite by mechanical alloying elemental powders

A NiAl-TiC nanocomposite has been synthesized by mechanical alloying from Ni, Al, Ti, and C powders. During milling, an abrupt reaction occurred, resulting in simultaneous formation of NiAl and TIC phases. It is suggested that two separate exothermic explosive reactions, i.e, Ni + Al --> NiAl and Ti + C --> TiC, were involved. However, the reactions were incomplete with the existence of a small amount of elemental powders. Prolonged milling led to a gradual formation of NiAl and TiC as well as grain refinement. The final grain size for TiC was 3.5 times as large as that for NiAl. The formation mechanism of the NiAl-TiC nanocomposite during mechanical alloying was also discussed. (C) 1998 Elsevier Science S.A. All rights reserved.

Microstructure and wear behaviour of aluminium implanted with nickel

Technical semi-hard aluminium (99.5 wt.% Al) was implanted with nickel at 200 keV ( T impl≤200 °C) and 1 MeV ( T impl≤50 °C). The chosen doses resulted in maximum nickel concentrations of 22–32 at.% and 20–45 at.% for implantation at 200 keV and 1 MeV, respectively. The as-implanted microstructure was characterized by an essentially amorphous matrix with nanocrystalline precipitates of NiAl 3 and NiAl. The size of the NiAl 3 precipitates was in the order of 5–10 nm. For the 200 keV implantation formation of NiAl was observed for all samples with typical particle sizes also of 5–10 nm. After implantation at 1 MeV, NiAl was only observed for maximum nickel concentrations >30 at.%, but with larger precipitates of 40±20 nm. The occurrence of NiAl was explained by the higher resistance against radiation damage of NiAl compared with NiAl 3. By annealing at 400 °C in both sample series a buried continuous layer of NiAl 3 was formed. The metastable NiAl precipitates disappeared. Annealing at 600 °C led to the growth of large NiAl 3 grains with typical dimensions up to the micrometre range. A significant wear reduction by a factor of about 20 was observed for the 200 keV samples with the deep nickel distribution obtained by annealing at 600 °C as well as for the as-implanted state of 1 MeV implantation with high doses resulting in maximum nickel concentrations >40 at.%.

Reaction process during relative sintering of NiAl

Nickel aluminide compounds were synthesized by the thermal explosion mode of the self-propagating high temperature synthesis (SHS). The effects of green density and heating rate on the combustion characteristics and the microstructure of the products were studied. It was found that the combustion can not be ignited with heating rates lower than 5 K min-1. In this case, the formation of NiAl can be achieved by classic reactive sintering. At heating rates higher than 5 K min-1, a precombustion stage exists for compacted samples allowing the ignition of the reaction at low temperature. Combustion temperature were found to be higher than thermodynamic predictions in argon and, moreover in air, because of the formation of aluminium oxide which triggers the NiAl synthesis reaction. © 1998 Chapman & Hall