Ni 3Al Research Articles

Self-diffusion in the intermetallic compounds NiAl and Ni 3Al: An embedded atom method study

The self-diffusion in NiAl and Ni 3Al are investigated by the molecular dynamics (MD) with an analytical embedded atom method (EAM) model which is fitted by the basic physical properties of pure constituents nickel (Ni) and aluminum (Al), and the characteristic properties of NiAl and Ni 3Al, including the equilibrium lattice constant, the elastic constants, the cohesive energy, and the effective defect formation energies, are calculated. Furthermore, in NiAl and Ni 3Al, the activation energy and migration energy of Ni atom and Al atom are also calculated, respectively. The present calculations are in agreement with the experimental value and the theoretical results obtained from other authors.

Precipitation of Cr-rich phases in rapidly solidified Ni–20Al–8Cr (at.%) powders

The evolution of the microstructure in rapidly solidified Ni–20.9Al–8Cr–0.49B (at.%) powders after different continuous and isothermal heat treatments at temperatures up to 1100 °C has been studied by electron microscopy and microanalysis. Powders in the rapidly solidified condition have a dendritic microstructure consisting of Ni 3Al dendrites and a NiAl phase in the interdendritic regions. Chromium is in solid solution in both phases. This microstructure is stable when heating at 10 K min −1 up to 750 °C. When the powders are heated up to 950 °C, partial dissolution of the NiAl phase and the precipitation of very small chromium-rich particles take place. The microstructure of the powders after annealing at temperatures between 750 and 1100 °C for different times is characterised by the dissolution of the β-NiAl phase and the simultaneous precipitation of various Cr-rich phases. α-Chromium, the metastable X-phase, and dark polygonal Cr 5B 3 precipitates have been identified. The segregation of chromium and boron in the form of borides removes these elements from the intermetallic matrix, so the content of both elements should be optimised to preserve their beneficial influence on the ductility of the γ′-Ni 3Al phase.

Dissimilar autogenous full penetration welding of superalloy K418 and 42CrMo steel by a high power CW Nd:YAG laser

Experiments of autogenous laser full penetration welding between dissimilar cast Ni-based superalloy K418 and alloy steel 42CrMo flat plates with 3.5 mm thickness were conducted using a 3 kW continuous wave (CW) Nd:YAG laser. The influences of laser welding velocity, flow rate of side-blow shielding gas, defocusing distance were investigated. Microstructure of the welded seam was characterized by optical microscopy (OM), scanning electron microscopy (SEM) and X-ray diffraction (XRD) and energy dispersive spectrometer (EDS). Mechanical properties of the welded seam were evaluated by microhardness and tensile strength testing. Results show that high quality full penetration laser-welded joint can be obtained by optimizing the welding velocity, flow rate of shielding gas and defocusing distance. The laser-welded seam have non-equilibrium solidified microstructures consisting of γ-FeCr 0.29Ni 0.16C 0.06 austenite solid solution dendrites as the dominant and very small amount of super-fine dispersed Ni 3Al γ′ phase and Laves particles as well as MC needle-like carbides distributed in the interdendritic regions. Although the microhardness of the laser-welded seam was lower than that of the base metal, the strength of the joint was equal to that of the base metal and the fracture mechanism showed fine ductility.

Reactive processing of nanostructured nickel aluminide intermetallics starting from elemental nanopowders

The current paper investigates the fabrication of nanostructured intermetallic alloys Ni 3Al starting from elemental nanopowders Ni and Al micro-alloyed with different compositions of submicron B and Nb metal powders. Low-energy reaction synthesis (RS) of the nanopowder green's compact in an air furnace was successfully carried out from which a nanostructured micro-alloyed Ni 3Al alloy was successfully produced. The nanopowders were completely reacted to form the intended single-phase nanostructured Ni 3Al with no remaining nanopowders up to 0.3 and 4 wt% compositions of B and Nb, respectively, with a maximum relative density of 90%. Microstructure examination revealed that the micro-alloyed Ni 3Al alloys produced contain very fine grain structures (120–140 nm) which resulted in an almost 200% increase in the microhardness, as well as a considerable increase in the final product ductility as a result of small micro-alloying additions of B (0.1 wt%) and Nb (up to 4 wt%). Both SEM/EDS analysis and XRD were used to examine the phase compositions after the in situ reactive processing.

Dynamical properties of the Ni 3Al low index surfaces with and without Ni or Al adatoms from molecular dynamics simulations

We present molecular dynamics simulation results on the vibrational properties of the Ni 3Al low index surfaces with and without Ni/Al adatoms. We found that the mean square displacements of both surface atoms and adatoms depend linearly upon temperature, the adatoms on the (1 1 0) face exhibiting vibrational amplitudes twice as large as on the other two surfaces. In addition, we found that all surfaces studied are expanded and rumpled, the rippling effect being more pronounced in the case of the (0 0 1) face with the Al atoms occupying the topmost position. In the cases of the (1 1 0) and the (1 1 1) surfaces the rippling is visible at several atomic layers beneath the surface. The adatoms are found to be contracted at ambient conditions, an effect that is significant when deposited on the (1 1 0) face, attaining the value of −10%. Moreover, from the calculated phonon density of states (DOS) and phonon spectral densities at room temperature, we found that the presence of surfaces modifies the bulk phonon DOS, exhibiting new modes some of which are located in the bulk phonon gap. The present results are in line with available experimental and theoretical results.

Preparation of Ni–Al alloys at reduced temperature for fuel cell applications

Ni–Al alloys have been fabricated from the physical mixture of nickel and aluminum powders at temperatures below the melting point of aluminum using AlCl 3 vapor as an activator. Pure Ni 3Al intermetallic compound has been synthesized from the Ni–Al powder mixtures containing 15 wt% aluminum at 600 °C. From the powder mixture containing 5 wt% aluminum, a compound consisting of Ni–Al solid solution and Ni 3Al was produced at 600 °C, but this compound was converted entirely to a Ni–Al solid solution after annealing at 700 °C. The particle size and morphology of the Ni–Al solid solution were not greatly different from original nickel powders. It was observed that oxidation resistance was significantly enhanced by alloying of nickel with aluminum. The Ni–Al alloys synthesized in this study can be used as anode materials for high-temperature fuel cells which need improved resistance toward creep, sintering and redox cycling.

Failure behaviors of TBCs on Ni 3Al base alloy IC6A during room temperature tensile test and stress rupture test under the condition of 1100 °C/100 MPa

The failure behaviors of thermal barrier coatings (TBCs) during room temperature tensile test and 1100 °C stress rupture test have been studied in the present investigation. The TBCs were prepared by electron beam physical vapor deposition (EB-PVD). The experimental results showed that during room temperature tensile test cracks initiated from the concave area of the bond coat surface, as well as the interface between the bond coat and the alloy substrate. The cracks may originate from the internal stress arising from the mechanical property differences between the bond coat and the substrate alloy. The similar failure of TBCs and the diffusion of Mo from the alloy substrate to TBCs were observed while in the case of high temperature stress rupture test, however, the failure of TBCs was not as serious as that in the case of room temperature tensile tests and the reason may be attributed to the formation of diffusion zone between the bond coat and the substrate alloy.

Spatially resolved intermetallic reaction of Ni 3Al by laser interference structuring

Some disadvantages of intermetallic compounds are their lack of plasticity, the inferior capacity of anti-creeping, and the environment-sensitive brittleness. In this work, the intermetallic reaction of Ni and Al was driven in a spatially resolved manner, namely by the laser interference pattern. Through laser interference, spatially varied temperature distribution can be easily obtained on the surface of the to be structured samples. Typical patterns given here include two dimensional linear array and three-dimensional dot array; the geometrical parameter of these patterns can be easily adjusted by laser parameters. Thus intermetallic Ni 3Al was developed in the form of dot array or line array, so as to get a thin film with alternative hard and soft properties on its surface. This can be a new way to solve the problems of plasticity and brittleness for the intermetallic compounds.

Mechanical properties of dual multi-phase single-crystal intermetallic alloy composed of geometrically close packed Ni 3X (X: Al and V) type structures

Dual multi-phase intermetallic alloy, which is composed of Ni 3Al(L1 2) and Ni solid solution (A1) phases at high-temperature annealing and is additionally refined by a eutectoid reaction at low temperature aging, according to which the Al phase is transformed into the Ni 3Al(L1 2) + Ni 3V(DO 22) phases, was prepared based on the pseudo-ternary system Ni 3Al–Ni 3Ti–Ni 3V. The high-temperature tensile deformation, fracture behavior and tensile creep were investigated using single crystalline material. The alloy with such a novel microstructure shows extremely high yield and tensile strength with good temperature retention, when compared not only with conventional Ni-based superalloys but also with polycrystalline materials reported previously. Over a broad temperature range fracture occurred along octahedral plane in the major component L1 2 phase, accompanied with high tensile elongation and ductile fracture mode. The tensile creep test conducted at 1173 K and 1223 K showed the presence of threshold stress, and also extremely low creep rate and long creep rupture time when compared with conventional Ni-based superalloys. The obtained results are promising for the development of a new-type of high-temperature structural material.

Superplasticity and cooperative grain boundary sliding in nanocrystalline Ni 3Al

Cooperative grain boundary sliding (CGBS) has been shown to account for the majority of macroscopic strain seen in microcrystalline metallic systems undergoing superplastic deformation. While CGBS has been observed on the surface of microcrystalline samples deforming superplastically through the shifting of diamond scribe lines, there have been few transmission electron microscopy results showing such occurrences in the bulk of the material, or the details behind the micromechanism of CGBS. In this work, nanocrystalline Ni 3Al produced via high-pressure torsion is deformed superplastically in the electron microscope. High-temperature (∼700 °C) in situ tensile testing shows the nature of CGBS at the nanoscale through direct observation of this phenomenon.

Phase equilibria between the B2, L1 2, and fcc phases in the Ir–Ni–Al system

The isotherms of the Ir–Ni–Al in the composition range up to 50 at% Al are presented at 1573 K. The phase constitution and microstructure of the Ir–Ni–Al alloys were examined using X-ray diffractometry (XRD), scanning electron microscopy (SEM) with an electron probe microanalyzer (EPMA), and transmission electron microscopy (TEM) after heat treatment at 1573 K for 168 h. The B2-NiAl and B2-IrAl phases connected with each other at 1573 K. The highest solubility limit of Ir into Ni 3Al was about 3.5 at% in the tested alloys. Then, a wide fcc + B2 and a narrow fcc + L1 2 and B2 + L1 2 two-phase region appeared in the isothermal section. In part of the B2 phase, a martensitic transformation from the B2 to the L1 0 phase was observed.

Texture memory effect in heavily cold-rolled Ni 3Al single crystals

In Ni 3Al the cold-rolled Goss texture changed to a complicated one after primary recrystallization and returned to the original Goss during the subsequent grain growth, which can be referred to as the texture memory effect. In this study, we examined the evolution of grain orientations during the grain growth using the electron backscatter diffraction (EBSD) method. It was found that just after the primary recrystallization most of the grains had a 40°〈1 1 1〉 rotation relationship to the Goss texture, the remaining grains being Goss and other textures. The formation of the 40°〈1 1 1〉 rotated grains can be explained by a multiple twinning mechanism. In the grain growth, the Goss grains, which were surrounded by the 40°〈1 1 1〉 rotated grains, grew preferentially due to the high mobility of the 40°〈1 1 1〉 grain boundaries, leading to the texture memory effect.

Reactive pressing of intermetallics

This paper discusses the reactive pressing (RP) of Ni 3Al from nickel (Ni) and aluminum (Al) elemental powder compacts. The effect of varying pressure and temperature on the microstructure and properties was examined. Three pressures (100, 200 and 300 MPa) were applied at three different temperatures (1000, 1100 and 1200 °C) while the pellet cooled down from the maximum combustion temperature. The results show that as both of the pressure and temperature increase, the product porosity decreases with a corresponding increase in microhardness (HV). The grain size was almost independent of both pressing temperature and pressure. The results were statistically analyzed to arrive at response surfaces that predict residual porosity, grain size and microhardness at any given pressure/temperature combination. X-ray diffraction (XRD), scanning electron microscopy, image analysis and Archimedes principle were used to study the effect of processing parameters on the final residual percentage total porosity, microstructure, average grain size ( d av), and microhardness of RP-Ni 3Al alloys.

Improving the wear resistance of AA 6061 by laser surface alloying with NiTi

To improve the wear resistance of a popular aluminum alloy AA 6061, a 1.5 mm thick hard surface layer consisting of Ni–Al and Ti–Al intermetallic compounds was synthesized on the alloy by laser surface alloying technique. NiTi powder was preplaced on the aluminum alloy substrate and irradiated with a high-power CW Nd:YAG laser in an argon atmosphere. With optimized processing parameters, a modified surface layer free of cracks and pores was formed by reaction synthesis of Al with Ni and Ti. X-ray diffractometry (XRD) confirmed the main phases in the layer to be TiAl 3 and Ni 3Al. The surface hardness increased from below 100 HV for untreated AA 6061 to more than 350 HV for the laser-treated sample. Accompanying the increase in hardness, the wear resistance of the modified layer reached about 5.5 times that of the substrate.

Research on laser welding of cast Ni-based superalloy K418 turbo disk and alloy steel 42CrMo shaft

Exploratory experiments of laser welding cast Ni-based superalloy K418 turbo disk and alloy steel 42CrMo shaft were conducted. Microstructure of the welded seam was characterized by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive spectrometer (EDS). Mechanical properties of the welded seam were evaluated by microhardness and tensile strength testing. The corresponding mechanisms were discussed in detail. Results showed that the laser-welded seam had non-equilibrium solidified microstructures consisting of FeCr0.29Ni0.16C0.06 austenite solid solution dendrites as the dominant and some fine and dispersed Ni3Al γ′ phase and Laves particles as well as little amount of MC short stick or particle-like carbides distributed in the interdendritic regions. The average microhardness of the welded seam was relatively uniform and lower than that of the base metal due to partial dissolution and suppression of the strengthening phase γ′ to some extent. About 88.5% tensile strength of the base metal was achieved in the welded joint because of a non-full penetration welding and the fracture mechanism was a mixture of ductility and brittleness. The existence of some Laves particles in the welded seam also facilitated the initiation and propagation of the microcracks and microvoids and hence, the detrimental effects of the tensile strength of the welded joint. The present results stimulate further investigation on this field.

Surface segregation of Pt in γ′-Ni 3Al: A first-principles study

We performed first-principles slab calculations based on the density functional theory to investigate the surface segregation behavior of Pt on the clean (1 0 0), (1 1 0) and (1 1 1) surfaces of γ′-Ni 3Al. The results clearly indicate that Pt has a strong tendency to surface segregate in all three low-index surface orientations. Such a conclusion is in complete agreement with a recent experimental finding that Pt segregates to the clean Ni 3Al(1 1 1) surface. The tendency of Pt to surface segregate also helps to explain why Pt addition is beneficial to the oxidation resistance of Ni 3Al.

Effect of Ta on microstructure and phase distribution in cyclic oxidized Ir–Ta modified aluminide coatings on nickel base single crystal superalloy

The effect of Ir–Ta coating (with varying amounts of Ta) on microstructure, composition variation and phase distribution in the modified aluminides on Ni-base single crystal superalloy, TMS-75 subjected to cyclic oxidation has been investigated. Ir–Ta coatings with Ta = 14.1, 23.1, 40.7 and 82.9 at.% deposited on 〈1 0 0〉 oriented nickel base single crystal superalloy, TMS-75 were aluminized and subjected to cyclic oxidation at 1373 K, for 600 h. The mass gain as a function of number of cycles data demonstrates an improved oxidation resistance with decreasing Ta content. In contrary to Ta-rich aluminides, Ir-rich aluminides show a large volume fraction of the β-phase, a less amount of γ′-Ni 3Al phase near the surface of the coatings. Rapid decrease in the concentration of Al in the diffusion zone and formation of interconnected γ′-Ni 3Al has been found to be responsible for the lower oxidation resistance of Ta-rich aluminides.

Hot corrosion behaviour of Ni 3Al in sulphate–chloride mixtures in the atmosphere

Hot corrosion of Ni 3Al intermetallic compound in the presence of sulphate–chloride mixtures was studied. A comminuted Ni 3Al mixed with NaCl–Na 2SO 4, NaCl–Li 2SO 4, LiCl–Na 2SO 4, LiCl–Li 2SO 4 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.%)–Li 2SO 4 mixture was the most corrosive agent. It was also found that by addition of MgO the corrosion of Ni 3Al was reduced. Phase composition of the corrosion products was established by X-ray diffraction analysis; there were detected Al 2O 3, Al 2S 3, NaAlO 2 (or LiAlO 2) as intermediate products, nickel sulphides, NiO and NiAl 2O 4. NiAl 2O 4 spinel was formed only at the highest temperatures, while at lower temperatures alumina was present instead of spinel. Hot corrosion behaviour of Ni 3Al in sulphate–chloride mixtures in air atmosphere.

Hot corrosion and oxidation behavior of a novel Pt + Hf-modified γ′-Ni 3Al + γ-Ni-based coating

A new type of Pt + Hf-modified γ′-Ni 3Al + γ-Ni-based coating has been developed in which deposition involves Pt electroplating followed by combined aluminizing and hafnizing using a pack cementation process. Cyclic oxidation testing of both Pt + Hf-modified γ′ + γ and Pt-modified β-NiAl coatings at 1150 °C (2102 °F), in air, resulted in the formation of a continuous and adherent α-Al 2O 3 scale; however, the latter developed unwanted surface undulations after thermal cycling. Type I ( i.e. 900 °C/1652 °F) and Type II ( i.e. 705 °C/1300 °F) hot corrosion behavior of the Pt + Hf-modified γ′ + γ coating were studied and compared to Pt-modified β and γ + β-CoCrAlY coatings. Both types of hot corrosion conditions were simulated by depositing Na 2SO 4 salt on the coated samples and then exposing the samples to a laboratory-based furnace rig. It was found that the Pt + Hf-modified γ′ + γ and Pt-modified β coatings exhibited superior Type II hot corrosion resistance compared to the γ + β-CoCrAlY coating; while the Pt + Hf-modified γ′ + γ and γ + β-CoCrAlY coatings showed improved Type I hot corrosion performance than the Pt-modified β.

Microstructure and abrasive wear performance of chromium carbide reinforced Ni 3Al matrix composite coating

A Ni–Al–Cr 3C 2 welding wire was produced by metal–powder–core technique. When the welding wires were welded on the surface of carbon steel, under the effect of the physical heat of arc, Ni reacted with Al to form Ni 3Al and carbide particles reinforced Ni 3Al matrix composite was formed. Cr 3C 2 was decomposed during welding and dispersed Cr 7C 3 with stripe shape formed, which strengthened the matrix significantly. The Cr 7C 3–Ni 3Al interface has broadened into a zone of interdiffusion and a new phase M 23C 6, which indicates that a good bond has been formed. The pin-abrasion wear test showed that the abrasion resistance of Cr 7C 3/Ni 3Al composite is six times higher than that of Stellite12 alloy at room temperature. The good wear resistance of Cr 7C 3/Ni 3Al composite coating can be attributed to large volume fraction of carbides, high hardness, and good phases interface bond.