Intermetallic NiAl Research Articles

Precipitation behavior of intermetallic NiAl particles in Fe-6 at.%Al-4 at.%Ni analyzed by SANS and 3DAP

Three-dimensional atom probe (3DAP) measurements and in-situ small-angle neutron scattering (SANS) were conducted in order to study the precipitation behavior of an Fe-6 at.%Al-4 at.%Ni alloy. The radius, number density and volume fraction of intermetallic NiAl precipitates were determined by a cluster search algorithm applied on the 3DAP data as well as by the evaluation of scattering curves obtained from SANS. The chemical compositions of the NiAl precipitates, found by 3DAP, and the experimental R-value obtained from SANS, were used to calculate the nuclear scattering contrasts. The introduction of 3DAP results enables an accurate evaluation of magnetic SANS curves by providing information on the magnetic scattering contrast. It is shown that a high amount of Fe is incorporated in the NiAl precipitates which gives rise to a magnetic moment. Finally, the results on the precipitation kinetics gained by 3DAP and SANS were combined and subsequently used to model the hardness behavior of the model alloy.

Asymmetry in structural and thermo-mechanical behavior of intermetallic NiAl nanowire under tensile/compressive loading: A molecular dynamics study

The asymmetric stress–strain behavior under tension/compression in an initial 〈100〉 B2-NiAl nanowire is investigated considering two different surface configurations i.e., 〈100〉/(0 1 0) (0 0 1) and 〈100〉/(0 1 1) (0 −1 1). This behavior is attributed to two different deformation mechanisms namely a slip dominated deformation under compression and a known twinning dominated deformation under tension. It is also shown that B2 → BCT (body-centered-tetragonal) phase transformation under tensile loading is independent of the surface configurations for an initial 〈100〉 oriented NiAl nanowire. Under tensile loading, the nanowire undergoes a stress-induced martensitic phase transformation from an initial B2 phase to BCT phase via twinning along {110} plane with failure strain of ∼0.30. On the other hand, a compressive loading causes failure of these nanowires via brittle fracture after compressive yielding, with a maximum failure strain of ∼−0.12. Such brittle fracture under compressive loading occurs via slip along {110} plane without any phase transformations. Softening/hardening behavior is also reported for the first time in these nanowires under tensile/compressive loadings, which cause asymmetry in their yield strength behavior in the stress–strain space. Result shows that a sharp increase in energy with increasing strain under compressive loading causes hardening of the nanowire, and hence, gives improved yield strength as compared to tensile loading.

A STUDY ON THE SYNTHESIS OF ULTRA-FINE IN SITUNi3Al–Al2O3/TiC COMPOSITE POWDER

Titanium carbide ( TiC ) has a low density, relatively high thermal and electrical conductivity. TiC is also very stable, with a melting temperature of 3100°C and does not undergo phase transformations. These properties have attraction to apply cutting tool extensively. Recently, in order to improve the properties of cermet tool materials, most researches have been carried out on the development of new binder. Ni3Al intermetallic is an interesting material because it has a great advantage of application for some structural materials, especially at the high temperatures. Ni3Al has low density, high strength and good corrosion resistance properties, which recommend them for the application of high temperature structural material. In this study, we had investigated the effects of mechanical alloying time and annealing temperature on the Ni3Al–Al2O3/TiC phase formation. Raw powders to fabricate Ni3Al–Al2O3/TiC composite powders were Ni , Al , TiH2 and activated carbon. These powders were ball-milled in planetary milling jar in argon atmosphere. Ball-milled powders were annealed up to 1200°C in Ar + 4%H2 atmosphere. The mean particle size of 300 nm was obtained after 10 h ball-milling. Ni3Al–Al2O3/TiC composite powders were formed by annealing for 1 h over 1100°C.

Catalytic Activity of Ni<sub>3</sub>Al Foils in Methanol Reforming

Intermetallic Ni3Al–based alloys (doped with zirconium and boron) represent a group of advanced materials with potential outstanding physical and chemical properties (such as high catalytic activity and structural stability in corrosive environments) that make them a considerable candidate for many high-tech applications. In this paper, the catalytic activity of fully dense Ni3Al-based thin foils (as thin as 50 m) possessing structures with micrometer or nanometer grain sizes is discussed. The examined material, without any additional catalytic coating, was successfully produced from as-cast coarse-grained sheets by heavy cold rolling and recrystallisation with an appropriately chosen set of parameters. The examination focuses on methanol and methanol/water mixture decomposition into H2 and CO at temperatures up to 530OC in a quartz reactor. Except for these products, a small amount (below 1%) of CO2 and dimethyl ether was observed. The catalyzed reaction began effectively at about 400OC, with a methanol conversion of about 90% or higher.

Influence of nickel content on catalytic activity and stability of the systems, based on intermetallic Ni3Al in the conversion of natural gas using carbon dioxide

In CO2 reforming of methane catalytic systems based on Ni3Al with various nickel and aluminum content were investigated. Catalysts were obtained using the self-propagating high-temperature synthesis. The XRD technique was used to investigate the phase content of the studied systems. It was demonstrated that the catalytic activity of the samples grows upon an increase in the nickel content of the system. By means of scanning electron microscopy, elemental analysis, and differential thermal analysis, it was demonstrated that these catalytic systems undergo structural changes and carbonize negligibly with remaining catalytic activity in time.

On Core Structure Properties and Peierls Stress of Dissociated Superdislocations in Aluminides: NiAl and FeAl

The study of dislocation properties in B2 structure intermetallics NiAl and FeAl is crucial to understand their mechanical behaviors. In this paper, the core structure and Peierls stress of collinear dissociated (111}{110} edge superdislocations in NiAl and FeAl are investigated with the modified P-N dislocation equation. The generalized stacking fault energy curve along (111) direction in {110} slip plane contains two modification factors that can assure the antiphase energy and the unstable stacking fault energy to change independently. The results show that the core width of superpartials decreases with the increasing unstable stacking fault energy, and increases with the increasing antiphase boundary energy. The calculated Peierls stress of (111}{110} edge superdislocations in NiAl and FeAl are 475 MPa and 3042 MPa, respectively. The values of Peierls stress in NiAl is in accordance in magnitude with the experimental and the molecular statics simulations results.

A Self-Propagating Foaming Process of Porous Al-Ni Intermetallics Assisted by Combustion Reactions

The self-propagating foaming process of porous Al-Ni intermetallics was investigated. Aluminum and nickel powders were blended, and titanium and boron carbide powders were added as reactive exothermic agents. The blended powder was extruded to make a rod-shape precursor. Only one end of the rod precursor was heated to ignite the reaction. The reaction propagated spontaneously throughout the precursor. Pore formation took place at the same time as the reaction occurred. Adding the exothermic agent was effective to increase the porosity. Preheating the precursor before the ignition was also very effective to produce porous Al-Ni intermetallics with high porosity.

On the shock response of cubic metals

The response of four cubic metals to shock loading is reviewed in order to understand the effects of microstructure on continuum response. Experiments are described that link defect generation and storage mechanisms at the mesoscale to observations in the bulk. Four materials were reviewed; these were fcc nickel, the ordered fcc intermetallic Ni3Al, the bcc metal tantalum, and two alloys based on the intermetallic phase TiAl; Ti–46.5Al–2Cr–2Nb and Ti–48Al–2Cr–2Nb–1B. The experiments described are in two groups: first, equation of state and shear strength measurements using Manganin stress gauges and, second, postshock microstructural examinations and measurement of changes in mechanical properties. The behaviors described are linked through the description of time dependent plasticity mechanisms to the final states achieved. Recovered targets displayed dislocation microstructures illustrating processes active during the shock-loading process. Reloading of previously shock-prestrained samples illustrated shock strengthening for the fcc metals Ni and Ni3Al while showing no such effect for bcc Ta and for the intermetallic TiAl. This difference in effective shock hardening has been related, on the one hand, to the fact that bcc metals have fewer available slip systems that can operate than fcc crystals and to the observation that the lower symmetry materials (Ta and TiAl) both possess high Peierls stress and thus have higher resistances to defect motion in the lattice under shock-loading conditions. These behaviors, compared between these four materials, illustrate the role of defect generation, transport, storage, and interaction in determining the response of materials to shock prestraining.

Micro-XRF and micro-XAFS studies of an Al matrix Fe–Ni composite

We report on the distribution and local coordination of Fe and Ni in an Al matrix Fe–Ni composite, by means of X-ray Fluorescence mapping (XRF), micro- (μ-) and conventional Extended X-ray Absorption Fine Structure (EXAFS) spectroscopies. The μ-XRF maps reveal that Fe segregates and forms Fe-rich islands which are depleted of Ni. The combined μ-EXAFS and EXAFS results reveal that both metals are bonded only to Al. More specifically, the Fe-rich islands are identified as FeAl 3 microcrystallites while in the Fe-poor regions, Fe belongs to an intermetallic FeAl phase. The bonding environment of Ni is also modified due to the variations in the distribution of the metals. In the region with high Ni concentration, i.e. Fe-poor regions, the Ni atoms are bonded to 10.5 ± 1.1 Al atoms that substitute Ni in fcc Ni. On the contrary, in the Ni-poor regions, where the Fe concentration exhibits maxima, the number of Al atoms in the first nearest shell of Ni is equal to 7.8 ± 0.9, i.e. in this region, Ni forms intermetallic NiAl. Finally, the atomic percentage of the Fe and Ni atoms that belong to the Fe-rich and Fe-poor islands is determined from the Fe- K and Ni- K edge EXAFS analysis. The majority of the Fe atoms (∼80 at%) belongs to the FeAl 3 microcrystallites, embedded into a FeAl matrix. On the contrary, the same atomic percentage of Ni atoms (∼50 at%), occupies sites in both the NiAl regions as well as in the matrix of the (Ni, Al) solid solution.

Morphological characteristic of the conventional and melt-spun Al-10Ni-5.6Cu (in wt.%) alloy

The Al-10Ni-5.6Cu alloy was prepared by conventional casting and further processed melt-spinning technique. The resulting conventional cast and melt-spun ribbons were characterized using X-ray diffraction, optical microscopy, scanning electron microscopy together with energy dispersive spectroscopy, differential scanning calorimetry and microhardness techniques. The X-ray diffraction analysis indicated that ingot samples were α-Al, intermetallic Al 3Ni and Al 2Cu phases. The optical microscopy and scanning electron microscopy results show that the microstructures of rapidly solidified ribbons are clearly different from their ingot alloy. Al-10Ni-5.6Cu ribbons reveal a very fine cellular structure with intermetallic Al 3Ni particles. Moreover, at high solidification rates the melt-spun ribbons have a polygonal structure dispersed in a supersaturated aluminum matrix. The differential scanning calorimetry measurements revealed that exothermic reaction was between 290 °C and 440 °C which are more pronounced in the ternary Al-10Ni-5.6Cu alloy.

Preparation of Ni-Al Intermetallics Bonded TiC Porous Cermets by SHS Method

Porous cermets composites were prepared from mixed powders (Ni, Al, Ti and C) by SHS (Self-propagating High-temperature Synthesis) method. When Ni-Al intermetallics bonded TiC porous cermets substitute for TiC ceramic as perform in the process of manufacturing Ni-Al/TiC composites by pressureless melt infiltration, the wetting of the ceramic material and Ni-Al intermetallics can be improved. In this study, the microstructure and mechanical properties (density and compressive resistance strength) were reported. It is learnt from the study that the relation between the open porosity and the pore size does not exist, the former depends on the green density, and the latter depends on the particle size of the nickel powder. The porous cermets with porosity ranging from 40% to 50%, pore size from 1μm to 300μm, and the compressive resistance strength about 15 MPa can be prepared by SHS. SEM observations of fracture surface suggest the fracture mode is brittleness fracture, and the microstructures of the porous cermets are very homogenous.

Solid Solution or Intermetallics in a High‐Entropy Alloy

The modulated structure of the AlCrFeCoNiCu high-entropy alloy consists of NiAl intermetallics (β′) and a (α-Fe, Cr) solid solution (β). The formation of the NiAl intermetallics greatly affects the strengths and magnetic properties of the AlxCrFeCoNiCu alloys. Evidently, the AlCrFeCoNiCu alloy cannot be treated as a solid-solution alloy.

Formation of porous Ni–Al intermetallics through pressureless reaction synthesis

Pressureless reaction synthesis method was used to produce porous NiAl and Ni 3Al intermetallics using elemental Ni and Al powder mixture in this investigation. The sintering behavior of Ni and Al mixed powder compacts was investigated by applying XRD, SEM and EDS in the temperature range of 510–1000 °C. It has been found that the formation of porous Ni 3Al and NiAl was accompanied by the volume expansion due to the formation of intermediates and pores during the reaction synthesis procedure. The intensive reaction and phase transformation procedures were similar in the Ni–14 wt% Al and Ni–30 wt% Al. However, the maximum pore size, expansion ratio and porosity have been found to be varied with the sintering temperature and Al content in the compacts. The Al content in the green compacts could effectively influence the volume expansion and pores of the products: the volume expansion and maximum pore size of NiAl are much higher than that of Ni 3Al under the same sintering procedure.

Mechanical, corrosion, and sliding wear behavior of intermetallics reinforced austenitic stainless steel composites

The present work investigates the effect of supersolidus sintering and intermetallics (Ni3Al, Fe3Al) additions on the densification, mechanical, tribological, and electrochemical behavior of sintered austenitic (316L) stainless steels. The performances of the supersolidus liquid phase sintered (SLPS) compacts are compared with the conventional solid-state sintered (SSS) compacts of similar compositions. Correspondingly, the sintering was carried out at two different temperatures 1200 °C (SSS) and 1400 °C (SLPS). Supersolidus sintering results in significant improvement in densification, wear resistance, corrosion resistance, strength, and ductility in both straight as well as aluminide added composites.

Spectroscopic study of interfaces in Al/Ni periodic multilayers

Using electron-induced X-ray emission spectroscopy (XES), we have studied two Al/Ni periodic multilayers that differ only by their annealing temperature: as-deposited and annealed at 115 °C. Our aim is to show that XES can provide further details about the chemistry at the metal-metal interface, in addition to what is obtained by X-ray diffraction. The distribution of valence states exhibiting Al 3p and Ni 3d character is determined from the analysis of the Al and Ni emission bands respectively. The multilayer emission bands are compared to those of reference materials: pure Al and Ni metals as well as Al 3 Ni, Al 3 Ni 2 and AlNi intermetallics. We provide evidence that, for temperatures up to 115 °C, Al 3 Ni is the major component of the multilayer.

Spectroscopic study of interfaces in Al/Ni periodic multilayers

Using electron-induced X-ray emission spectroscopy (XES), we have studied two Al/Ni periodic multilayers that differ only by their annealing temperature: as-deposited and annealed at 115C. Our aim is to show that XES can provide further details about the chemistry at the metal-metal interface, in addition to what is obtained by X-ray diffraction. The distribution of valence states exhibiting Al 3p and Ni 3d character is determined from the analysis of the Al Kbeta and Ni Lalpha emission bands respectively. The multilayer emission bands are compared to those of reference materials: pure Al and Ni metals as well as Al3Ni, Al3Ni2 and AlNi intermetallics. We provide evidence that, for temperatures up to 115C, Al3Ni is the major component of the multilayer.

An in-situ TEM investigation on microstructure evolution of Ni-25 at.% Al thin films

The microstructure evolution of Ni-25 at.% Al thin films prepared by magnetron co-sputtering is investigated by using in-situ transmission electron microscopy (TEM) study from room temperature to 750 °C. The diffraction rings of both Ni and Al are present in the selected area electron diffraction (SAED) of as-deposited films, and then the diffraction ring of Al disappears when the films are heated up to 300 °C. At 500 and 700 °C, the diffraction rings and spots of L1 2 intermetallic Ni 3Al phase are found, respectively, which means that the formation of ordered Ni 3Al phase starts from 500 °C. Moreover, it is found that the grain growth is relatively slow when the temperature is lower than 300 °C, while all of normal grain growth, abnormal grain growth and annealing twinning occur at higher temperatures.

Effect of O impurity on structure and mechanical properties of NiAl intermetallics: A first-principles study

We have investigated the effects of O on the structure and mechanical properties of NiAl intermetallics using a first-principles pseudopotential total-energy method based on the density functional theory with the generalized gradient approximation. We found that the impurity O atom can either replace Ni atom or go into the tetrahedron interstitial site depending on the surrounding environment. In both cases, O tends to form an Al 2O 3-like tetrahedron structure with its nearest Al or Ni atoms, leading to the formation of the stronger O–Al bond containing covalent component. We demonstrate that the presence of O will cause an increase of the brittleness and a decrease of the ductility of NiAl based on the calculated elastic constants and the empirical criterions for both substitutional and interstitial cases. Our calculations provide a way to suppress the negative effect of O by adding the alloying element with lower electronegativity than that of Al.

Bonding characteristics in NiAl intermetallics with O impurity: a first-principlescomputational tensile test

We have performed a first-principles computational tensile test on NiAl intermetallics withO impurity along the [001] crystalline direction on the (110) plane to investigate the tensilestrength and the bonding characteristics of the NiAl–O system. We show that the idealtensile strength is largely reduced due to the presence of O impurity in comparison withpure NiAl. The investigations of the atomic configuration and bond-length evolution showthat O prefers to bond with Al, forming an O–Al cluster finally with the break of O–Nibonds. The O–Ni bonds are demonstrated to be weaker than the O–Al bonds,and the reduced tensile strength originates from such weaker O–Ni bonds. Avoid-like structure forms after the break of the O–Ni and some Ni–Al bonds.Such a void-like structure can act as the initial nucleation or the propagationpath of the crack, and thus produce large effects on the mechanical properties ofNiAl.

Formation of a coplanar O–Al bonding cluster: the effect of O impurity on aΣ = 5 NiAl grain boundary from first-principles

The site occupancy, structure, and bonding properties of O in an NiAl grain boundary (GB)have been investigated by employing a first-principles total energy method based on densityfunctional theory with the generalized gradient approximation and ultrasoft pseudopotential. TheΣ5(310)/[001] tilt GB of NiAl has been chosen because (i) theΣ = 5 GB has been observed to be a higher fraction in NiAl experimentally, and (ii) theΣ5(310)/[001] is energetically favorable in comparison with theΣ5(210)/[001]. The NiAl GB is shown to favor the O segregation with a segregation energy of−1.75 eV, indicating that most of the O impurity will distribute in the NiAl GBthermodynamically. Moreover, O is shown to prefer occupying the interstitialsites rather than the substitutional sites in the GB according to the calculatedformation energies. The O–Al bond is energetically favorable as compared withthe O–Ni bond due to different electronegativity of Al and Ni in reference toO. Charge distribution and the density of states further indicate the intrinsicbonding properties of O–Al that contain obvious covalent characteristics. It isinteresting to find that O is coplanar with the surrounding Al atoms in bothinterstitial and substitutional cases with lower formation energies, forming strongercoplanar O–Al bonding clusters. Such stronger bonding clusters in the GB canembrittle the NiAl intermetallics and thus are not beneficial to the plasticity ofNiAl. Our results will provide a useful reference for improving the mechanicalproperties and for understanding the oxidation effect of the NiAl intermetallics.