Papers by Keyword: NiAl

Abstract: An investigation is performed of the filtrational combustion of gaseous hydrocarbons inside a spherical nozzle manufactured from porous Ni-Al material. The porous material is produced using the process of self-propagating high temperature synthesis, which allows an article of the desired configuration, chemical composition and porosity to be fabricated within a single technological stage. A combustion mode is demonstrated, wherein the combustion wave is localized inside the wall of the porous nozzle. Under this condition, up to 70 % of the total heat value of the fuel mixture is converted into IR flux radiated from the surface of the nozzle, with the maximum lying within the wavelength region 3–11 μm. A number of promising applications of new porous materials for designing new energy conversion devices are shown.

Abstract: Intermetallic compound NiAl is considered as a potential candidate material for bond coat in thermal barrier coating (TBC) system due to its capability of forming a continuous and uniform alumina scale at temperatures even high than 1200 °C, but its cyclic oxidation is rather poor. Previous study has found that cyclic oxidation resistance of NiAl alloys and coatings can be drastically improved by minor Dy doping as reactive element. In this paper, NiAlDy alloys were produced by vacuum arc-melting and the effects of various Dy contents on the microstructure and hardness of NiAl alloys were investigated. The results suggest that Dy tends to segregate at grain boundaries and precipitate within grains as brittle DyNi₂Al₃ phase with little DyNiAl needles in it. The addition of minor Dy resulted in grain refinement. The grain size of NiAl alloy were reduced from ~1 mm to ~300 µm, with increasing the content of Dy to 0.5 at.%. The addition of ~0.1 at.% Dy caused a 10 % improvement in both the microhardness and macrohardness due to Dy solid-solution and grain-boundary segregation, but the alloy revealed decreased microhardness and macrohardness with further increasing the content of Dy to 0.5 at.% as the formation of Dy-rich phase.

Abstract: The microstructure of as-cast NiAl-33.5Cr-0.5Zr alloy is the eutectic colonies of NiAl and Cr phase with non-continuous Heusler (Ni2AlZr) phase segregating at the cell boundaries. After hot isostatic press (HIP) processing at 1573K, 100MPa for 2h, the Ni2AlZr phase changes into Zr-rich phase. The elevated temperature tensile strength and elongation of the HIP-ed alloy are much higher than those of as-cast alloy. It was noted that this alloy either as-cast or HIP processing exhibits a brittle to ductile transition behavior, which is difference between as-cast and HIP-ed alloy. HIP processing decreases the BDTT of the alloy, especially in high strain rates, and lower the range of BDTT. When the strain rate increases by two orders of magnitude, the BDTT has an approximate increase from150K for as-cast alloy to 100K for HIP-ed alloy. The mechanism responsible for elevated temperature deformation has also been discussed.

Abstract: Microstructure of the Ni-30Al-8Mo-2Nb alloy is dendrite of NiAl, Mo and small amount of Ni3Al phases, the Ni-40Al-8Mo-2Nb alloy is also dendrite of NiAl and Mo phases, and the Ni-50Al-8Mo-2Nb alloy is irregular dendrite of NiAl, Mo and small amount of NiAlNb(laves) phases. Upon the experiment, NiAlNb(laves) phase is only found in the Al-rich alloy. Compressive yield strength of the Ni-40Al-8Mo-2Nb alloy is higher than that of the Ni-30Al-8Mo-2Nb alloy, and much higher at low temperature or fast initial strain rate. TEM observation shows that the high temperature deformation of the Ni-40Al-8Mo-2Nb alloy is controlled by dynamic recovery and dynamic recrystallization.

Abstract: This small review deals mainly with three issues regarding the nature and protectiveness of alumina scales grown during high-temperature oxidation: (1) sequences of phase transportation of alumina scales formed on Fe-Cr-Al and NiAl alloys, and a few aluminides, (2) combined additions of reactive element (RE) and (3) convolution of α-Al₂O₃ scales. Though the general phase transformation sequence of alumina scales is γ to θ to α phases at intermediate temperatures, variations have been reported. Directional growth of transient aluminas such as γ-Al₂O₃ and θ-Al₂O₃ is discussed with a particular emphasis on its driving force. Parabolic rate constants for the growth of α-Al₂O₃ scales are smaller when the period of transient alumina is longer because of larger α-Al₂O₃ grains. The effect of RE in slowing the parabolic oxidation saturates at a certain concentration, however combined addition further decreases the oxidation rate. The α-Al₂O₃ scales on Fe-Cr-Al alloys without RE are highly convoluted, however those on NiAl and other aluminides are not so convoluted. The α-Al₂O₃ layer beneath the outer NiO layer or NiAl₂O₄ layer is flat in the oxidation of Ni₃Al. Directions for future work are proposed.

Abstract: Generalized gradient approximation (GGA) of the density function theory (DFT) was applied to calculate many properties including density of states, population analysis and electron density in NiAl and NiAl(Fe) to investigate the mechanism of improving room temperature ductility of B2-NiAl by Fe. It was shown that the strong bond to Al p and Ni d hybridization, which leads to the embrittlement of B2-NiAl at room temperature. Addition of Fe, which is beneficial to improve ambient ductility of B2-NiAl, weakens the impact of the bond to Al p and Ni d hybridization and enhances the interaction among next-nearest-neighbor Ni atoms to make the charge distribution uniform along <100>.

Abstract: The microstructure, high temperature tensile behavior of the DS NiAl-Cr(Mo)-W/Nb alloy have been investigated. The transverse microstructure of the alloy consists typically of eutectic colonies of NiAl and Cr(Mo) phase with white phases segregating at the cell boundaries. The white phase is possibly Cr(Mo) phase containing large amount of Nb and W elements. The longtitudinal microstructure of the alloy is lamellar with the direction basically parallel to the directional solidification direction. The tensile yield strength and ultimate strength of the DS NiAl-Cr(Mo)-W/Nb alloy are much higher than that of the general cast NiAl-Cr(Mo)-W/Nb alloy, and the elongation of the alloy is also higher than that of the general cast alloy. The fracture is debonding along phase boundary and cleavage.

Abstract: This paper presents a brief review, followed by some new results from recent diffusion simulations in Ni-base superalloy systems, performed by means of a thermodynamic and kinetic modeling approach as taken in the commercial finite-difference code DICTRA. The DICTRA code solves the multi-component diffusion equations, combining assessed thermodynamic and kinetic data in order to determine the full composition dependent interdiffusion matrix. The link between fundamental physics based models and critically assessed data allows simulations to be performed with realistic conditions on alloys of practical importance. Emphasis in this paper is on modeling and simulation of interdiffusion occurring between NiAl coatings and Ni-base superalloy substrates. For this purpose we have used the so-called homogenization approach to diffusion in multi-phase systems, recently implemented into the DICTRA software. The simulation results have been validated against experimental data and the agreement is very satisfactory given the complexity of the problem.

Abstract: Ni-Al intermetallic compound coatings no visible cracks, pores or inclusions are prepared on mild steel substrate by Laser- cladding in situ synthesis with mixed powders of nickel-coated aluminum, Ni-based alloy and Al. The results show that the microstructure of the coating consists mainly of two phases of NiAl and Ni₃Al, and the former amount is more than the latter, with NiAl being uniformity and staggered fine dendrites which are surrounded continuously by Ni₃Al, and the atom ratios of Ni∶Al∶Fe∶Si in NiAl and Ni₃Al are respectively 54∶39∶2∶5 and 70∶21∶3∶6. Compared with cladding nickel-coated aluminum, the coatings quality are improved, and the dilution of iron from substrate into coating are greatly reduced, as a result of lower cladding powers due to self-fluxing of Ni-based alloy and exothermic reaction between Ni and Al. The hardness of coatings are up to HV490-HV540.

Abstract: Equal-channel angular pressing (ECAP) was applied to polycrystalline NiAl at temperatures around the brittle-to-ductile transition temperature (BDTT). NiAl rods encapsulated in a steel jacket were ECAP-processed in a die with a channel angle of 120°. The microstructure and texture were characterized by electron backscatter diffraction with a scanning electron microscope. The volume fraction of the texture components typical for simple shear in the intersection plane of the channels changes in the range of the BDTT.