Ni Al Cr Research Articles

New possibilities of recently constructed high-temperature solution calorimeter

The calorimeter presented in this paper is constructed under supervision of authors. The most substantial innovation of construction of the device was introduction of intermediate container with ten sockets. As a result the calorimeter is used for determination of enthalpy of formation for alloys in a wide range of temperature (up to 1,400 K). As an example results from alloys from the Ni–Al–Cr system are presented, they are aimed to determine directly γ′ / γ′ + γ / γ phase boundaries with use of this “in situ” calorimetric method. Precise indication of phase boundaries is crucial for determination of phase diagrams.

Wear properties of NiAl based materials

The NiAl based materials including NiAl–TiC–Al2O3 composite, NiAl–Cr(Mo)–Hf–Ho eutectic alloy and NiAl–Cr(Mo)–CrxSy in situ composite were fabricated and their wear properties were tested at different temperatures. The results revealed that the NiAl–TiC–Al2O3 composite, NiAl–Cr(Mo)–Hf–Ho eutectic alloy and NiAl–Cr(Mo)–CrxSy in situ composite exhibited the excellent wear properties between 700°C and 900°C. The microstructure observations exhibited that the self-lubricant films formed on the worn surfaces during the dry sliding test at high temperature, which decreased the wear rate and friction coefficient significantly. TEM observation on the self-lubricant film revealed that it was mainly comprised by ceramic amorphous and nanocrystalline. Compared with the NiAl–TiC–Al2O3 composite, the NiAl–Cr(Mo)–CrxSy in situ composite has lower friction coefficient at low temperature. Such phenomena may be ascribed to the addition of sulfide which contributes much to the formation of self-lubricant, and moreover the TiC addition increase the strength of NiAl based material and its wear resistance.

Influence of composition on monomodal versus multimodal γ′ precipitation in Ni–Al–Cr alloys

This study investigates the influence of alloy composition on γ′ precipitation in Ni–8Al–8Cr and Ni–10Al–10Cr at.% during continuous cooling from a supersolvus temperature. When subjected to the same cooling rate, Ni–8Al–8Cr develops a monomodal population, whereas Ni–10Al–10Cr develops a multimodal (primarily bimodal) population of γ′ precipitates. The bimodal γ′ precipitate size distribution in Ni–10Al–10Cr alloy can be attributed to two successive nucleation bursts during continuous cooling while the monomodal γ′ size distribution in Ni–8Al–8Cr results from a single nucleation burst followed by a longer time—wider temperature window for nucleation resulting in a larger number density of precipitates. Three-dimensional atom probe investigations reveal that while local equilibrium is achieved across the γ/γ′ interface, far-field γ compositions still retain an excess of Al and a depletion of Cr with respect to equilibrium, thus giving direct experimental evidence of the driving force for subsequent nucleation bursts during continuous cooling in case of Ni–10Al–10Cr. Contrastingly, in case of the Ni–8Al–8Cr alloy, while the γ phase retains an excess of Al and a depletion of Cr with respect to equilibrium after continuous cooling to room temperature, the additional driving force is consumed during subsequent isothermal annealing by growth of the large number of γ′ precipitates without any new nucleation.

Fabrication of Ni–Al–Cr alloy anode for molten carbonate fuel cells

Ni-(3,5) wt.% Al-(3–10) wt.% Cr alloys suitable for use as an anode material for molten carbonate fuel cells (MCFCs) were prepared by a solid-gas-solid diffusion method. The creep resistances and sintering resistances of the prepared Ni–Al–Cr alloy samples evaluated in the creep test system were significantly improved with increasing Al and Cr concentration. The Ni-5 wt.% Al-10 wt.% Cr alloy showed the best creep resistance with a creep strain of 1.7% among as-prepared samples. To determine the electrochemical performance of the Ni–Al–Cr alloy anode, a single cell test was carried out. The cell performances of all the Ni–Al–Cr alloy anodes were above 0.8 V at 150 mA cm−2 for 1000 h of operation. It was also verified that the addition of Cr to Ni–Al enhances the ductility of the alloy and thus reduces micro-crack formation in the Ni–Al–Cr anode during cell operation. Therefore, it can be concluded that the Ni–Al–Cr alloy anode is a good candidate anode material for MCFCs.

New NiAl-strengthened ferritic steels with balanced creep resistance and ductility designed by coupling thermodynamic calculations with focused experiments

Two critical issues restricting the applications of NiAl precipitate-strengthened ferritic steels are their poor room temperature ductility and insufficient creep resistance at temperatures higher than 600 °C. In this study, a thermodynamic modeling approach is integrated with experiments focused on investigating the ductility and creep resistance of steel alloys based on the Fe–Ni–Al–Cr–Mo multi-component system. The mechanical property studies showed that the creep resistance increases with increasing the volume fraction of B2-ordered precipitates, while the opposite trend was observed for the ductility. Low solubility of Al in the α-Fe matrix was found to favor a ductility increase. Thermodynamic calculations were used to predict the volume fraction of B2-ordered precipitate and the elemental partitioning to guide the selection of alloy compositions that might exhibit the balanced creep resistance and ductility. Key experiments were then conducted to validate the prediction. This integrated approach was found to be very effective in the alloy development.

Perfect cellular eutectic growth in directionally solidified NiAl–Cr(Mo) hypereutectic alloy

Cellular eutectic microstructures with fully lamellar morphology were observed in the directionally solidified Ni–31Al–32Cr–6Mo (at%) hypereutectic alloy at withdrawal rates of 15, 25 and 50μm/s, but the morphologies of cellular microstructures did not change consecutively with increasing withdrawal rate. The growth interfaces were deep cellular at withdrawal rates of 15 and 50μm/s, but it changed to be shallow cellular at rate of 25μm/s. The reason is that the interface undercooling comes to minimum at the middle rate of 25μm/s. If the interface undercooling decreases, the tendency of constitutional undercooling will be weaken. The small constitutional undercooling will increase the interface stability, so that the interface morphology changes from deep cellular to shallow cellular. The shallow cellular growth interface led to a perfect cellular eutectic microstructure, which was analogous to the planar eutectic microstructure. In this case, the widths of the intercellular regions were narrowest, no coarse or irregular plates existed at the cell boundaries, and the thicknesses of the lamellae were almost uniform. The properties of the alloy may be markedly improved.

Phase-specific elastic/plastic interface interactions in layered NiAl–Cr(Mo) structures

The depth-dependent, as-grown and deformation-induced strain and dislocations partitioned through the interfaces in a two-phase layered NiAl–Cr(Mo) structure are directly measured at the mesoscale using 3-D X-ray microdiffraction. It is demonstrated that in the as-grown, undeformed state, neighboring submicron Cr solid solution and NiAl eutectic lamellae (doped with ∼3% Mo) form a heterointerface with 180° rotation around a 〈112〉 pole. It is shown that the mechanical response to the indentation of a layered composite with alternating Cr(Mo)–NiAl lamellae is distinct from the response of single-phase materials. In the center of the indent, after the load is released, the NiAl lamellae are under compressive forward stresses (with the same sign as the indentation-induced compression) while Cr solid solution lamellae are under tensile back stresses (with opposite sign from the indentation load). The depth-dependent alternation of compressive/tensile residual strains in the neighboring Cr solid solution and NiAl lamellae is understood in the framework of the Mughrabi’s composite model considering two types of structure elements: harder and softer regions. Under indentation, both kinds of lamellae are assumed to deform compatibly. After the load is released, residual forward stresses are formed in the harder lamellae, and back stresses are formed in the mechanically softer lamellae. Line-broadening analysis of the intensity distribution along the diffraction vector reveals a 15-times increase in dislocation density in the near-surface zone in the center of the indent. Such a large increase is typical for severe deformation.

Thermal conductivity mapping of the Ni–Al system and the beta-NiAl phase in the Ni–Al–Cr system

We use two examples to highlight the power of micron-scale-resolution thermal conductivity mapping on graded compositions created in a diffusion-multiple sample in rapid establishment of composition–phase–property relationships. The Ni–NiAl diffusion couple example clearly shows the effects of solid solutioning, order–disorder transition, and compositional point defects on thermal conductivity. Thermal conductivity mapping of the beta-NiAl phase region of an Ni–Cr–NiAl diffusion triple clearly shows that Cr substitutes for both Ni and Al sites in beta-NiAl without a strong preference, which is also found not to be strongly composition dependent.

Improvement of compressive strength and ductility in NiAl–Cr(Nb)/Dy alloy by rapid solidification and HIP treatment

The NiAl–Cr(Nb)/Dy alloy was fabricated by conventionally casting and rapid solidification and treated by hot isostatic pressing treatment (HIP). The results reveal that Cr2Nb phase contains much of Ni and Al elements and retains the C14 crystal structure at room temperature. The rapid solidification refines the NiAl phase, Cr2Nb phase and needle-like Cr2Nb precipitates in NiAl matrix. Moreover the rapid solidification increases α-Cr phase and results in the formation of Cr7Ni3 phase with stacking faults and microtwins inside. The HIP treatment transforms the needle-like Cr2Nb precipitate in the NiAl phase into sphere shape and blunts the tip of the Cr2Nb phases along the NiAl phase boundary. In addition, the HIP treatment leads to some moveable dislocations in the alloy, which is beneficial to the compressive ductility. The mechanical test shows the rapidly solidified alloy with HIP treatment has the best mechanical properties, whose compressive ductility is more than ten times that of the as cast alloy.

Room temperature ductility of NiAl-strengthened ferritic steels: Effects of precipitate microstructure

Abstract The effects of precipitate microstructure on the room temperature ductility of a series of carefully designed Fe–Al–Ni–Cr–Mo steels were investigated. Transmission electron microscopy (TEM), ultra small angle X-ray scattering (USAXS), and atom probe tomography (APT) were conducted to quantify the nano-scaled precipitates. The accuracy of the characterization results was verified by a numerical analysis. Three point bending tests results demonstrated that ductility was a function of the precipitate volume fraction and the Al and Ni concentrations in the Fe matrix, these relationships were discussed in terms of possible mechanisms. The ductility was also found to be independent of the precipitate size and inter-particle spacing in the studied range, which was validated by a theoretical model.

Kinetic pathways for phase separation: An atomic-scale study in Ni–Al–Cr alloys

The kinetic pathways involved in the formation of γ′(L12 structure)-precipitates during aging of concentrated Ni–Al–Cr alloys at 873K, for three distinct alloy compositions, are studied experimentally by atom probe tomography, and computationally with lattice kinetic Monte Carlo (LKMC) simulations using parameters deduced from first-principles calculations of cohesive energies, and from experimental diffusion data. It is found that the compositional evolution of the γ′-precipitate phase does not follow the predictions of a classical mean-field model for coarsening of precipitates in ternary alloys. LKMC simulations reveal that long-range vacancy–solute binding plays a key role during the early stages of γ′-precipitation. With the aid of Monte Carlo techniques using the parameters employed in the LKMC simulations, we compute the diffusion matrix in the terminal solid-solutions and demonstrate that key features of the observed kinetic pathways are the result of kinetic couplings among the diffusional fluxes. The latter are controlled by the long-range vacancy–solute binding energies. It is concluded that, because it neglects flux couplings, the classical mean-field approach to phase separation for a ternary alloy, despite its many qualitatively correct predictions, fails to describe quantitatively the true kinetic pathways that lead to phase separation in concentrated metallic alloys.

Tribological behavior of NiAl matrix composites with addition of oxides at high temperatures

NiAl, NiAl–Cr–Mo alloy and NiAl matrix composites with addition of oxides (ZnO/CuO) were fabricated by powder metallurgy route. It was found that some new phases (such as NiZn3, Cu0.81Ni0.19 and Al2O3) are formed during the fabrication process due to a high-temperature solid state reaction. Tribological behavior was studied from room temperature to 1000°C on an HT-1000 ball-on-disk high temperature tribometer. The results indicated that NiAl had high friction coefficient and wear rate at elevated temperatures, while incorporation of Cr(Mo) not only enhanced mechanical properties evidently but also improved high temperature tribological properties. Among the sintered materials, NiAl matrix composite with addition of ZnO showed the lowest wear rate at 1000°C, while CuO addition into NiAl matrix composite exhibited the self-lubricating performance and the best tribological properties at 800°C.

Effect of withdrawal rate on the microstructure of directionally solidified NiAl–Cr(Mo) hypereutectic alloy

Solidification microstructure and growth interface morphology of directionally solidified Ni–31Al–32Cr–6Mo(at.%) hypereutectic alloy were studied. The experiments were carried out at higher temperature gradient of about 250 K cm −1 with different withdrawal rates of 4–500 μm s −1. When the withdrawal rate was less than 50 μm s −1, the primary Cr(Mo) dendrites were gradually eliminated through competitive growth between the primary phase and the eutectic phase. When the withdrawal rate exceeded 100 μm s −1, no primary phase formed, eutectic phase grew directly. Fully eutectic microstructures with lamellar morphology were observed at all withdrawal rates. With increasing withdrawal rate V, the growth interface changed from planar to cellular and then dendritic, the solidification microstructure also transformed from planar eutectic to two-phase cellular eutectic and dendritic eutectic. The microstructure was refined and the eutectic interlamellar spacing λ decreased according to the relationship of λ = 4.82 V −0.42. Compared to the alloy at eutectic composition, the volume fraction of Cr(Mo) strengthening phase was increased obviously.

Thermodynamic modeling and experimental investigation of the phase stability at the Ni-rich region of the Ni–Al–Cr–Ir system

The effect of adding 3at.% Cr on the phase stability of the Ni–Al–Ir system was studied experimentally at 1250°C. A thermodynamic description of the Ni–Al–Cr–Ir quaternary system in the Ni-rich region was then developed based on the microstructures, the crystal structures and the phase compositions determined by experiment for eight alloys in both as-cast and 1250°C annealed states. The calculated isothermal section at 1250°C using the obtained description was consistent with the phase-equilibrium data obtained in this study. The calculated two-dimensional section of liquidus projection was also in accordance with the primary phases of solidification observed from alloys in the as-cast state. The effects of Cr additions to the Ni–Al–Ir alloys on the as-cast and annealed microstructures were elucidated through Scheil simulation and phase-equilibrium calculation using Pandat.

Room temperature mechanical properties and tribology of NICRALC and Stellite casting alloys

Abstract NICRALC is a family of intermetallic compound-based casting alloys proposed as an economic alternative for abrasion and erosion-resistant cobalt alloys, mainly under more elevated temperature conditions. NICRALC microstructures contain hard chromium carbides dispersed in a continuous Ni–Al–Cr γ′ (L12) ordered intermetallic matrix. In this work, cast NICRALC alloys with different carbon contents, different carbide types and morphological features were compared with one spray-formed NICRALC alloy and one Co-based alloy similar to commercial Stellite 6 ® alloy, with regard to their mechanical and wear behaviour at room temperature. Mechanical properties were evaluated in terms of uniaxial testing in compression, hardness and fracture toughness; wear properties were evaluated in dry-sand–rubber-wheel tests at room temperature and in block-on-ring lubricated sliding tests at 100 °C temperature. The alloys were characterized by optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), and instrumented micro hardness (commonly called nanoindentation). The worn surfaces and fracture path were analyzed by SEM. Results indicated that the mechanical properties, such as yield stress, fracture toughness and hardness, of NICRALC alloys were dependent upon the carbon content. However, their wear behaviour could not be directly related to hardness and yield stress, but varied depending on the type and distribution of carbides. Among the compositions tested eutectic and near eutectic NICRALC alloys demonstrated wear resistance under abrasion and sliding wear similar to those obtained with Stellite alloy. The alloys with lower fracture toughness showed the higher volume loss as under sliding as under abrasion wear tests in despite its higher hardness and higher yield strength.

A novel concept for the preparation of alloy nanowires

AbstractA novel concept for the preparation of alloy nanowires has been developed. By combining two pseudobinary eutectic systems NiAl–W and NiAl–Cr with a completely miscible system Cr–W, a pseudoternary system was created in which there is large two‐phase field between NiAl and W(Cr) solid solutions, separated by a eutectic trough. This enables the fabrication of nanowires with compositions from pure W to pure Cr allowing a free change of the ratio. The directional solidification in the pseudoternary system has been carried out. For the first time alloy nanowires in the W–Cr system have been reported. In addition, nanowires of pure W and Cr were fabricated as well. Moreover, an electrochemical procedure for the release of nanowires from the matrix has been established. magnified image High yield of W–Cr alloy nanowires with equiatomic composition.

Internal oxidation and phase transformations of multi-phase Fe–Ni–Al and Fe–Ni–Al–Cr alloys induced by KCl corrosion

The corrosion of two multiphase Fe–Ni–Al and Fe–Ni–Al–Cr alloys is studied at 650 °C in KCl-contaminated air. The oxidation rate of the alloys in air alone is low. When KCl is introduced, the corrosion is accelerated, producing a thick external scale of iron oxides, an intermediate layer of spinel, and a region of internal oxidation of Al. Potassium chromate is detected on Fe–Ni–Al–Cr surface that accounts for the degradation of protective chromia. An Al-depleted single phase region is observed in the front of the internal oxidation, due to the selective consumption of Al via an “active oxidation” process.

NiAl Matrix High-Temperature Self-Lubricating Composite

A high-temperature self-lubricating composite NiAl–Cr–Mo–CaF2 was fabricated using the powder metallurgy technique, and the tribological behavior of the composite at a wide range of temperatures (room temperature to 1000 °C) was investigated. The results showed that the composite had a favorable friction coefficient of about 0.2 and an excellent wear resistance of about 1 × 10−5 mm3N−1m−1 at the high temperatures tested (800 and 1000 °C). The excellent self-lubricating performance was attributed to the formation of the glaze film on the worn surface consisting mainly of CaCrO4 and CaMoO4 as high-temperature solid lubricants.

Preliminary investigation on strong magnetic field treated NiAl–Cr(Mo)–Hf near eutectic alloy

The Ni-33Al-28Cr-6Mo-0.3Hf near eutectic alloy was treated in a 10 T strong magnetic field at 1073 K and 1173 K for 1.5 h respectively. Microstructure examination reveals that after the strong magnetic field treatment, most Ni 2AlHf Heusler particles have changed into Hf solid solution, and moreover Hf solid solution and Heusler particles along eutectic cell boundaries distribute more uniformly. In addition, the eutectic cellular microstructure of the alloy with strong magnetic field treatment at 1173 K is changed greatly. The eutectic cell has an obvious disintegration and Cr(Mo) plates become coarsening and spheroidizing. The compression tests show that room temperature compressive ductility of the alloys with strong magnetic field treatment improves significantly, compared with the heat-treated alloy.

Improvement of compressive strength and ductility in NiAl based eutectic alloy by uniform high magnetic field treatment

With the development of superconducting technology, high magnetic fields (HMF) provide unique opportunity for materials and physics scientists to extend or exploit high performance of existing materials. In this paper, we report our findings of the improvement of compressive strength and ductility in a NiAl based eutectic alloy induced by high uniform magnetic field treatment. It shows that the compressive strength and ductility of NiAl–Cr(Mo)–(Ti,Hf) eutectic alloy were improved by 100 MPa and 12% respectively after 11T uniform magnetic field treatment. Microstructural characterization and fractography observation suggested that the modification of morphology and dispersed distribution of constituent phases through atomic diffusion induced by the HMF treatment could be responsible for the improvement of mechanical performance.