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Most methods used to synthesize and prepare NiAl intermetallics and their alloys have the disadvantages of complexity and high cost. In this paper, the NiAl alloy was prepared by a Combustion Synthesis and Hot Pressing (CSHP) technique under rapid solidification. The grain size of the NiAl alloy is significantly refined to 60–80 μm, which reduces the stress concentration during deformation and improves the fracture strength and fracture hardness. Moreover, the large internal stress and greater amount of dislocations in the as-cast microstructure are produced by their formation under pressure due to the fast cooling rate in the solidification process. The high dislocation density strengthens the NiAl alloy, giving it higher strength, hardness, and work-hardening ability. The high compression properties are also present in the NiAl intermetallics at room temperature, in which the fracture strength is around 1005 MPa and the fracture strain reaches 21.6%. The compressive fracture strength at room temperature is higher than that of the pure NiAl alloy prepared by the Hot-Pressing-Aided Exothermic Synthesis (HPES, about 632 MPa), while it is slightly lower than that of pure NiAl alloy treated by HPES and Hot Isostatic Pressing (HIP, 1050 MPa). The fracture strain is significantly higher than that of the NiAl alloy prepared by other methods. This study can provide guidance for the successful preparation of the NiAl alloy with high strength and toughness. [ABSTRACT FROM AUTHOR]

Most methods used to synthesize and prepare NiAl intermetallics and their alloys have the disadvantages of complexity and high cost. In this paper, the NiAl alloy was prepared by a Combustion Synthesis and Hot Pressing (CSHP) technique under rapid solidification. The grain size of the NiAl alloy is significantly refined to 60–80 μm, which reduces the stress concentration during deformation and improves the fracture strength and fracture hardness. Moreover, the large internal stress and greater amount of dislocations in the as-cast microstructure are produced by their formation under pressure due to the fast cooling rate in the solidification process. The high dislocation density strengthens the NiAl alloy, giving it higher strength, hardness, and work-hardening ability. The high compression properties are also present in the NiAl intermetallics at room temperature, in which the fracture strength is around 1005 MPa and the fracture strain reaches 21.6%. The compressive fracture strength at room temperature is higher than that of the pure NiAl alloy prepared by the Hot-Pressing-Aided Exothermic Synthesis (HPES, about 632 MPa), while it is slightly lower than that of pure NiAl alloy treated by HPES and Hot Isostatic Pressing (HIP, 1050 MPa). The fracture strain is significantly higher than that of the NiAl alloy prepared by other methods. This study can provide guidance for the successful preparation of the NiAl alloy with high strength and toughness.

Abstract—Fractional gas analysis is used to study the oxide nonmetallic inclusions (NIs) in a nickel alloy based on the NiAl intermetallic compound. Aluminum, lanthanum, and calcium oxides are shown to be the main NIs; the oxygen content in the alloy, which corresponds to them, is 11.6 ± 1.7 ppm. The NI compositions, which are determined with a scanning electron microscope by local energy-dispersion analysis, confirm the fractional gas analysis data for the NiAl-based nickel alloy. [ABSTRACT FROM AUTHOR]

Experimental studies have been carried out with theoretical calculations of wave synthesis in the Ni–Al–Cu system using a mathematical model. Approximate analytical formulas are obtained for synthesis performance evaluation. The inverse problem method was used to get kinetic constants that determine process dynamics based on the experimental data and analytical relationships. It is shown that the combustion front propagation velocity increases monotonically with an increase in the reaction sample relative density in the range of relative density values of 0.4 to 0.6. The depth of copper-melt penetration from the center of the sample into the nickel–aluminum matrix depends on the relative density of the sample and diameter of the copper wire: higher densities and larger diameters lead to an increase in the liquid-phase impregnation area. The rate of nickel and aluminum powder frame wetting with copper melt is limited by the synthesis wave speed. Based on the experimental data and analytical ratios, we estimate the effective kinetic constants describing the high-temperature synthesis of the Ni + Al reaction mixture in the presence of copper additives. The thermal effect of the NiAl intermetallic formation reaction and the preexponential factor in the chemical transformation equation are calculated, the exponent value in the ratio for the mixture thermal conductivity is established, and a constant determining the process of nickel–aluminum matrix impregnation with copper melt is found. The macroscopic approach used to analyze the NiAl intermetallic synthesis makes it possible to determine all the desired physicochemical characteristics and model parameters. The mathematical model is suitable for predictive estimates and experimental data analysis in the macroscopic approximation. Approximate analytical formulas are obtained for calculating the NiAl intermetallic synthesis characteristics. They allow for calculating the through channel characteristics and can be used in the design of NiAl products. [ABSTRACT FROM AUTHOR]

Abstract In this study, the mixture of nickel and aluminum powders has been mechanically milled for different times in a planetary mill and its thermodynamics were investigated. Phase and structural changes of the mixture powders have been investigated by XRD and SEM analysis. The prediction of a stable phase of the milling process was carried out using the Miedema's semi-empirical method. The results showed that Ni 55 Al 45 intermetallic was more stable than a solid solution and amorphous phase after the mechanical milling process. In order to determine the mechanism of forming an intermetallic mixture of milled and unmilled powders, they have been subjected to DTA thermal analysis and were compared the experimental and the theoretical results. The results of the thermal analysis have been showed that the intermetallic formation reaction was completed in four steps, with the formation of the middle phases of Ni 3 Al and Al 2 Ni 3. The grain size and lattice strain changes have been calculated using the Williamson-Hall equation during mechanical alloying. Highlights • Draw phase diagrams and compare their stability with each other.. • Calculation of enthalpy of formation and free energy changes. • Order and disorder intermetallic. • Calculation of enthalpy changes due to the formation of triple-defects by mechanical activation. • Investigation of the structure and the long-range order by mechanical alloying. [ABSTRACT FROM AUTHOR]

A novel mesoporous intermetallic NiAl nanocompound was successfully prepared at 600 °C in a molten LiCl with an assistance of CaH2. The obtained powder had a fine crystal structure assigned to a single phase intermetallic NiAl. The measured BET surface area was as high as 113.9m2/g, originating from the mesopores (3.1 nm on average) and the unique crumpled morphology. Calcium species, such as CaH2, Ca(OH)2, CaO and CaCl2, could work as templates in alloying process and they were readily removable by NH4Cl solution rinsing afterwards. As far as we know, this paper is the first report of mesoporous metal aluminide with a defined crystal structure and a high surface area of >100m2/g. [ABSTRACT FROM AUTHOR]

The article presents a study of the effect of various modes of annealing and process heatings on microstructure and properties of single-crystalline blanks of VKNA-1 intermetallic alloy on the basis of NiAl with crystallographic orientations of 〈001〉 and 〈111〉, obtained by the method of directional crystallization, and billets with polycrystalline structure from the VKNA-1VR alloy, made by the precision investment casting method. [ABSTRACT FROM AUTHOR]

WC and CeO2 particles reinforced in-situ synthesized NiAl matrix (WC/CeO2/NiAl) composite coatings were prepared on aluminum alloys by laser surface alloying. The microstructure, microhardness and fracture toughness as well as the tribological properties at 25°C and 400°C of the composite coatings were investigated. The results show that the WC/CeO2/NiAl composite coating contains the phases of NiAl, WC, W2C, γ-(Fe,Ni), CrB, Cr23C6, Al3Ni2 and CeNi5. Due to the synergistic strengthening effects of WC and CeO2 particles, the average grain size of the composite coating is reduced, and the microhardness and fracture toughness are increased. The friction coefficient and wear loss of the composite coatings rise in ascending order with the temperature, which are smaller than those of the WC/NiAl composite coatings, the NiAl coatings and the substrates. As the temperature increases from 25°C to 400°C, the wear mechanisms of the composite coatings change from micro-cutting wear into multi-plastic deformation wear and slight oxidative wear. [ABSTRACT FROM AUTHOR]

A possibility of determining the corrosion current density in the case that cathodic reaction proceeds under the conditions of mixed kinetics is analyzed using the equation proposed by Nagy and Thomas and the method of fitting. By the examples of model and real corrosion systems, it is shown that, under certain conditions, this approach enables one to estimate the corrosion current density and the cathodic diffusion limiting current density with regard for the contributions of cathodic and anodic reactions to the corrosion current on the stationary electrode, in particular, in the absence of Tafel region in the cathodic polarization curve. [ABSTRACT FROM AUTHOR]

Highlights: [•] Uniaxial tension of NiAl nanofilm with prismatic dislocations is simulated. [•] The NiAl nanofilm demonstrates an extraordinary high elastic strain. [•] In a region of elastic strain deformation of the nanofilm is discontinuous. [•] The effect can be explained by the competing interaction of different sort atoms. [ABSTRACT FROM AUTHOR]

A series of iron-doped NiAl ternary alloys is investigated by resistivity measurements and differential thermal analysis. Temperatures of phase transformations and disordering onset are determined. A hysteresis phenomenon is observed in the liquid state in polytherms of resistivity upon heating the alloy under study to the critical temperature. [ABSTRACT FROM AUTHOR]

The well-known concept of γ surfaces originally suggested by Prof. Vitek is extended to the case of a shift of one part of a crystal with respect to the other part in two adjacent {111} planes of a stacking fault. The proposed approach is used to construct the effective γ surfaces in the {111} plane in fcc Ni and L1 NiAl of intermetallic compound. It is shown that NiAl can have five metastable stacking faults in this plane, namely, an intrinsic superlattice stacking fault, an extrinsic superlattice stacking fault, a complex stacking fault, an antiphase boundary, and a complex extrinsic stacking fault. The existence of the latter in the {111} plane in the L1 NiAl intermetallic compound has not been reported previously. The shift of the crystal in two close-packed planes in fcc Ni makes it possible to avoid the formation of a high-energy stacking fault which arises when constructing an ordinary γ-surface, as well as to self-consistently include the extrinsic stacking fault into consideration. [ABSTRACT FROM AUTHOR]

Abstract: NiAl intermetallic compound was synthesized by mechanical alloying technique in planetary and attritor mills. The starting powders consisted of elemental mixtures of Ni and Al at Ni50Al50 (at%) composition. In the planetary mill, compound formation occurred gradually during mechanical alloying, while the occurrence of a mechanically induced self-propagating reaction (MSR) can be suggested in the attritor mill. The NiAl obtained in both mill types was partially disordered with long-range order parameter not inferior to 0.66. Quantitative phase analysis using the Rietveld method was performed in as-milled samples, and this method was also employed to estimate changes in crystallite size and lattice strain of the NiAl produced during mechanical alloying. [Copyright &y& Elsevier]

Abstract: In this work we first time investigate the reaction mechanism and possibility of synthesizing NiAl intermetallic compound under the activated combustion mode. A halogen containing organic polymer, namely, polytetrafluoroethylene – PTFE is used as promoter. It is shown that the single-phase target product is obtained when amount of the promoter is 0.1%. Differential thermal (DTA), thermogravimetric (TG) and XRD analyses testify that reaction proceeds stepwise. It is shown that in first stage decomposition products of PTFE remove the oxide film from Al particles; then follows reaction between Ni and pure Al. This reaction undergoes two different modes. First the solid-state reaction takes place, then Al melts and reaction continues between solid Ni and molten Al. It is established that the increase in the promoter amount higher than 1% shifts kinetic activation mode to thermal-kinetic mode. The latter leads to misbalance of green mixture and formation of undesired by-products. The optimal conditions of chemically activated synthesis for NiAl intermetallic are established. [Copyright &y& Elsevier]

Abstract: The microstructure, compression property and work-hardening effect of the NiAl-matrix composite with 1.7wt.% NbB2 and Nb x C (x =1 or 2) fabricated by the combustion synthesis and hot pressing technique have been investigated in the study. The NiAl-matrix composite has strong strain rate sensitivity. The yield strength of the NbB2–Nb x C/NiAl composite decreases while the fracture strain and the work-hardening capacity (H c) increase with decreasing strain rate. A high true ultimate strength of 1472MPa, a fracture strain of 20.1% and H c =1.41 were obtained under the strain rate of 2.0×10−5 s−1. [Copyright &y& Elsevier]

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The corrosion behavior of NiAl containing small additions of Ti, Zr, and B in combustion gases both with and without NaSO−NaCl deposits at 600-800°C has been studied for times up to four days. The corrosion of the saltfree NiAl leads to the formation of very thin alumina scales at 600°C but of mixed NiO−AlO 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 NiSO 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 NiAl involves a combination of various mechanisms, including fluxing of the oxide scale as well as mixed oxidation-sulfidation attack. At all temperatures NiAl shows poor resistance to hotcorrosion attack as a result of the formation of large amounts of Ni compounds in the scales. [ABSTRACT FROM AUTHOR]

Abstract: The pure NiAl intermetallic compound was fabricated by the combustion synthesis and hot pressing technique. Microstructure examination showed that the NiAl intermetallic compound contained fine grains. Analysis of the X-ray diffraction and the HRTEM studies showed that the phase in the intermetallics was the only NiAl phase. The NiAl showed prominent compression properties. The true ultimate compression strength and the fracture strain of the NiAl are 1002−94 +72 MPa and 21.6−1.8 +1.8%, respectively. The work-hardening capacity (H c) is 1.40−0.07 +0.09 and the Vickers micro-hardness is 360−19 +15 HV. The finer grains, the high density dislocation and the seriously distorted lattices in the matrix, and the intense interactions between dislocations contribute to the prominent compression properties. [Copyright &y& Elsevier]