Ni-Al System Research Articles

Electrolytic corrosion of materials of TiCrB2–AlN system in 3% NaCl solution

The corrosion resistance of titanium-chromium-diboride-based ceramic materials and cermets in 3% NaCl solution (to simulate sea water) is studied by the method of potentiodynamic polarization curves. The composition and microstructure of oxidized specimen surfaces are examined. Adding small amounts of AlN (5 to 10 vol %) to TiCrB2 provides an essential improvement of the corrosion resistance. Introduction of an NiAlCr metallic binder into the material composition results in a lower resistance to anodic oxidation.

Shape-Memory Effect and Pseudoelasticity in Fe–Mn-Based Alloys

Several Fe-based alloys are being considered as potential candidates for applications which require shape-memory behavior or superelastic properties. The possibility of using fabrication methods which are well known in the steel industry is very attractive and encourages a large amount of research in the field. In the present article, Fe–Mn-based alloys are mainly addressed. On the one hand, attention is paid to the shape-memory effect where the alloys contain (a) a maximum amount of Mn up to around 30 wt%, (b) several possible substitutional elements like Si, Cr, Ni, Co, and Nb and (c) some possible interstitial elements like C. On the other hand, superelastic alloys are analyzed, mainly the Fe–Mn–Al–Ni system discovered a few years ago. The most noticeable properties resulting from the martensitic transformations which are responsible for the mentioned properties, i.e., the fcc–hcp in the first case and the bcc–fcc in the latter are discussed. Selected potential applications are also analyzed.

Synthesis Mechanism and Strengthening Effects of Laminated NiAl by Reaction Annealing

NiAl with a laminated microstructure has been fabricated by reaction annealing of Ni-Al system at 1473 K (1200 °C). The laminated NiAl shows heterogeneity of chemical gradient and bimodal grain size distribution. The objective of this study is to investigate the synthesis mechanism and the strengthening effect of this laminated NiAl, therefore to promote further application of NiAl as a high-temperature structural material. Heat treatments at 1473 K (1200 °C) and subsequent characterization were utilized to study the synthesis mechanism. It shows that in original Al regions NiAl nuclei precipitate from Al(Ni) liquid phase and form fine-grained NiAl layers, whereas in original Ni regions NiAl nuclei precipitate from Ni(Al) saturated solution through diffusion and form coarse-grained NiAl layers. Moreover, heterogeneity of chemical gradient is generated through diffusion during annealing. The mechanical properties of laminated NiAl have also been studied via nanoindentation method. It shows that both chemical gradient and bimodal grain size distribution could strengthen the laminated NiAl.

Phase and structural stability in Ni-Al systems from first principles

We report on a comprehensive first-principles study of phase stability in the Ni-Al binary, both at zero Kelvin and at finite temperature. First-principles density functional theory calculations of the energies of enumerated orderings on fcc and the sublattices of B2 not only predict the stability of known phases, but also reveal the stability of a family of ordered phases that combine features of $\mathrm{L}{1}_{2}$ and $\mathrm{L}{1}_{0}$ in different ratios to adjust their overall composition. The calculations also confirm the stability of vacancy ordered B2 derivatives that are stable in the Al-rich half of the phase diagram. We introduce strain order parameters to systematically analyze instabilities with respect to the Bain path connecting the fcc and bcc lattices. Many unstable orderings on both fcc and bcc are predicted around compositions of ${x}_{\text{Ni}}=0.625$, where a martensitic phase transformation is known to occur. Cluster expansion techniques together with Monte Carlo simulations were used to calculate a finite-temperature-composition phase diagram of the Ni-Al binary. The calculated phase diagram together with an analysis of Bain instabilities reveals the importance of anharmonicity in determining the phase bounds between the B2 based $\ensuremath{\beta}$ phase and the $\mathrm{L}{1}_{2}$ based ${\ensuremath{\gamma}}^{\ensuremath{'}}$ phase, as well as properties related to martensitic transformations that are observed upon quenching Ni-rich $\ensuremath{\beta}$.

Nanosegregated amorphous AlBN2 alloy

We use ab initio molecular dynamics simulations to create an amorphous AlBN2 model and find that it consists of nanosegregated two-dimensional hexagonal BN-like and tetrahedral AlN-like domains. These domains are somewhat homogenously distributed in the network. There exist no chemical disorder and Al–B bonding. Amorphous AlBN2 is a semiconductor having a theoretical band gap energy of ∼2.24 eV, larger than that of amorphous AlN and BN systems. This amorphous nitride might find some applications as an electronic material.

Half-metallic and magnetic properties of AlN nanosheets doped with nonmagnetic metals: A first-principles study

We theoretically studied the structural, electronic, and magnetic properties in the two-dimensional (2D) AlN nanosheets (AlNNSs) doped with nonmagnetic (NM) atoms X(=Mg, Ca, Zn, and Sr), based on first-principles calculations. The structure relaxations show Mg and Zn atoms locate in the 2D AlNNS plane, while the Ca and Sr atoms lie out of it. The results based on GGA-PBE scheme show that all the doped AlN monolayers (ML) are half-metallic. Further, results within HSE06 scheme show that Mg-, Zn- and Sr-doped AlN ML remain half-metallic while Ca-doped case changes into magnetic semiconductor. Each dopant induces a total magnetic moment of 1.0μB per supercell which mainly stemmed from the spin-polarized holes resided on the three nearest-neighboring N atoms. Calculations illustrate that ferromagnetic (FM) states are energetically stable when two X atoms separate far away from each other, while anti-ferromagnetic (AFM) states are energetically favorable when two X atoms locate at adjacent lattice sites. The long-range FM coupling and AFM one are attributed to the strong p-d/p-p interaction and the virtual hopping mechanism, respectively. Remarkably, because of the participation of the delocalized Zn-d orbitals in Zn-doped case, strong p-d/p-p interactions gain supremacy in the competition between virtual hopping mechanism and p-d/p-p interaction, resulting in FM coupling even two Zn atoms locate at close crystal lattice. Calculations show the two-X-doped AlNNSs have FM states with Curie temperatures (Tc) higher than 600K, indicating that X-doped AlN systems are promising candidates for spintronic devices in the future.

Thermodiffusion in liquid binary alloys computed from molecular-dynamics simulation and the Green-Kubo formalism

In the presence of a temperature gradient, the components of a binary liquid tend to segregate. This phenomenon, generally referred to as thermodiffusion or the Soret effect, is usually quantified by the heat of transport. We report heat of transport values Qc∗ for NiAl and NiCu melts computed using molecular-dynamics simulation and the Green-Kubo formalism. Thermal conductivities are also reported. To develop a clear picture of the phenomena, we determined contributions to Qc∗ due to the convective and virial components of the heat current, which were then compared to the related terms in the partial enthalpy. It is shown that the contribution to Qc∗ from the convective component of the heat current is comparable to the average energy of the diffusing atoms, differing by an amount comparable to the activation energy for diffusion. The contribution to Qc∗ from the virial heat current is closely related to the pressure-volume term pcΩ in the partial enthalpy. It is established that the virial heat current plays a dominant role in determining the sign of the reduced heat of transport Qc∗′=Qc∗-hc. By comparing results obtained with different empirical potentials, a trend emerges. Specifically, it is found that the sign of the reduced heat of transport is correlated with the sign of the partial pressure associated with the low-mass component. It is also shown that two different empirical potentials for the NiAl system give vastly different results for Qc∗′. The results indicate that in developing a potential that might accurately predict Qc∗′, the distribution of the partial energy and partial pressure between the two components is critical. Based on these observations, it would appear that existing empirical potentials may not be able to generate reliable predictions for Qc∗′ without additional validation.

Electronic and magnetic properties in Mn‐doped IIIA‐nitride monolayers

Based on first‐principles calculations plus Hubbard U, we have studied the electronic structure and magnetic properties of Mn‐doped IIIA‐nitride monolayers. The substitution of Mn for Al or Ga atom induces a total magnetic moment of 4.00 µB per dopant, independent of the choice of functional. The doped AlN system is half‐metallic at GGA + U (<5 eV) level, but is a magnetic semiconductor at the HSE06 and GGA + U (≥5 eV) levels. The doped GaN system is a magnetic semiconductor at these two levels. The Mn‐doped InN converts into metal without magnetism, due to the robust p–d hybridization between the Mn‐3d and N‐2p states and the delocalized sd states. The Mn atoms have a clear clustering tendency with the FM state in the AlN and GaN hosts. The long‐ranged ferromagnetic coupling is observed along the ⋯N‐Al/Ga⋯ zigzag edge directions, which can be attributed to p–d hybridization mechanism. These results suggest that the Mn‐doped AlN and GaN monolayers are promising candidates for preparing spintronic devices at the nanoscale.

Origin of band-gap bowing in wurtzite AlN1−xPx alloys

The structural and electronic properties of AlN1−xPx semiconductors have been investigated from first principles within the density functional theory (DFT). An anomalously large bowing of calculated band-gaps was revealed in these materials. For low P contents x, in homogeneous atomic configurations of the alloy, this effect originates from the hybridised P 3p and N 2p bands located above the valence band maximum of the host AlN system. Such a band-gap bowing is strongly enhanced in clustered configurations of atoms in studied alloys as a result of broadening and incorporation of these states into the main valence band region.

Coating synthesis controlled by electron-beam heating

The methods of combined electron-beam treatment of parts made of steel with one- and two-layer coatings are studied experimentally. Ti-Ni, Ni-Al and Al-Ti systems were used as the examples in the experiments. The mathematical model is suggested for coating formation in the controlled regime of high temperature synthesis during high energy source motion along the preliminarily deposited layer of exothermic composition. The study takes into account the difference in thermophysical properties of the materials of coating and substrate, heat release from chemical reaction that leads to the coating properties formation and other factors. The realization of the synthesis depends on technological parameters. Various regimes of the treatment process are investigated numerically.

Phase formation in the combustion wave of SHS by diluting quartz equiatomic system Ni-Al

Research of «hardening» of the structure in the Ni-Al system equiatomic composition with the addition of quartz at a stop wave SHS combustion. Revealed several zones with characteristic microstructure, which allowed to establish a fol-lower - of phase formation during the formation of nickel aluminide initially heating the reaction cell in the process of SH-synthesis.

Layer-by-layer laser synthesis of Cu–Al–Ni intermetallic compounds and shape memory effect

We have studied conditions for the synthesis of intermetallic phases in the Cu–Al–Ni system by selective laser sintering/melting, in particular by heating a powder mixture to 300°C. The effects of laser synthesis and heating on the microstructure of the intermetallic phases in the samples obtained have been studied using electron microscopy, optical metallography, and X-ray diffraction analysis. The results demonstrate high sinterability of stoichiometric mixtures. Resistivity measurements indicate that the samples exhibit a shape memory effect. We discuss the feasibility of producing biomicroelectromechanical systems using layerby- layer synthesis.

Phase competition in ternary Ti–Ni–Al system

In this paper the reactive diffusion in Ti–Ni–Al system is discussed at 1173 K. The calculation method based on the binary approach is presented. The key kinetic parameter is Wagner integral diffusion coefficient. The experimental and simulation results of reactive diffusion between pure Ti and β-NiAl are compared at 1173 K after 100 h.

Structural, Electronic, Magnetic and Optical Properties of Ni,Ti/Al-based Heusler Alloys: A First-Principles Approach

AbstractIn this work, detailed first-principles calculations within the generalised gradient approximation (GGA) of electronic, structural, magnetic, and optical properties of Ni,Ti, and Al-based Heusler alloys are presented. The lattice parameter of C1bwith space groupF4̅3m(216) NiTiAl alloys is predicted and that of Ni2TiAl is in close agreement with available results. The band dispersion along the high symmetry pointsW→L→Γ→X→W→Kin Ni2TiAl and NiTiAl Heusler alloys are also reported. NiTiAl alloy has a direct band gap of 1.60 eV at Γ point as a result of strong hybridization between the d state of the lower and higher valence of both the Ti and Ni atoms. The calculated real part of the dielectric function confirmed the band gap of 1.60 eV in NiTiAl alloys. The present calculations revealed the paramagnetic state of NiTiAl. From the band structure calculations, Ni2TiAl with higher Fermi level exhibits metallic properties as in the case of both NiAl and Ni3Al binary systems.

Transformation of chemical and phase composition of the Al–Ni and Al–Ni–Cr laminated coatings after high-temperature heating

The study covers transformation of the structure, phase and chemical composition in the Al–Ni and Al–Ni–Cr coatings obtained by the diffusion interaction under the heat treatment of the impact bonded 12Kh1MF+NP2+AD1 and 12Kh1MF+Kh20N80+AD1 laminated composites. It was found that increasing the hold time at 1100 °C leads to the stabilization of chemical composition in the surface coating accompanied by the increase of nickel and decrease of aluminium proportion. It was shown that redistribution of aluminium within the diffusion coating during the temperature impact is slower in the Al–Ni–Cr system than in the binary Al–Ni system.

Fabrication of alloyed aluminum nickelides by metallothermy of metals oxides

Fabrication conditions of NiAl, NiAl–Cr, and NiAl–Cr–Mo–W alloys by joint aluminothermic reduction of initial metal oxides are investigated. Thermodynamic characteristics of accompanying reactions are determined. The temperature dependence of the change in the isobaric potential (ΔG°, kJ/mol) of reduction reactions of oxides points to the high formation probability of alloys. It is revealed by differential thermal analysis that the reduction of metal oxides enters the active phase after aluminum is melted at ~650°C and progresses according to the heterogeneous mechanism in a temperature range of 800–1100°C. The optimal composition of the initial charge, which provides the maximal yield of metals into alloys, is established. It is found experimentally that the yield of metals into alloys constitutes 85–92 wt %. Synthesis products are identified by the elemental and X-ray phase analyses as intermetallics of the Ni–Al system, which contain inclusions of chromium, molybdenum, and tungsten. It is shown that the concentration of inclusions varies in a range of 1.5–6.5 wt %. The microhardness of alloys is determined to vary from 3546 to 7436 MPa, depending on the content of alloying elements.

First-principles studies of multiple trapped impurity C by Ni vacancy and temperature effects in NiAl intermetallics

By using a first-principles pseudopotential method based on the density functional theory and Vienna ab initio Simulation Package (VASP), we investigate the multiple trapping of C by Ni vacancy (VNi) and its temperature effects in NiAl intermetallics. A single C atom is energetically and favorably situated at the Ni-rich octahedron interstitial site that surrounds Ni vacancy, which is shown via calculating the formation energy of C atom in NiAl with Ni vacancy system. Single C atom prefers to interact with neighboring Ni atom and Al atom to form a covalent bond. In NiAl intermetallics, C atoms prefer to be trapped in the Ni vacancy in the sequential way, thus easily forming the CnVNi (n=1, 2, 3, 4) clusters, in which the C4VNi clusters are most energetically favorable. It is interesting to find that when C atoms are trapped by Ni vacancy, all the C atoms themselves prefer to be combined with each other to form a bond, surrounding Ni vacancy. With the C atoms further added, both the charge density and the deformation charge prefer to bind with each other despite the Ni or Al environment and the intrinsic bonding properties of CC bond contain obvious covalent characteristics. Furthermore, using first-principles calculations combined with statistical model, we quantitatively predict point defect concentration as a function of temperature in NiAl intermetallics. It is concluded that the concentration of intrinsic Ni vacancies (VNi) will obviously increase as temperature increases. With the increase of temperature, the concentration of C atoms in the CnVNi cluster is higher than that at the intrinsic position. Besides, it indicates that most of C atoms in NiAl intermetallics are trapped by Ni vacancy, which is due to the larger binding energy of the CnVNi clusters and most of the C atoms are trapped directly by vacancies at room temperature or high temperature to form CnVNi clusters. Since the formation of CnVNi clusters is a process of heat releasing which will further increase the temperature of the NiAl system and produce more and more Ni vacancies, we can conclude that much more vacancies are created as a result of the presence of C impurity in NiAl intermetallics. However, the Ni vacancies exist in the form of CnVNi clusters from our calculation in a certain temperature range (less than 700 K). The existence of this kind of CnVNi cluster can effectively restrain the generations of cracks in the vacancies, which will produce some influences on the mechanical properties of NiAl intermetallic compound. Consequently, our results will provide a valuable reference for understanding the effects of C and vacancy on the mechanical properties of the NiAl intermetallics.

Revisiting the phenomenological model for effect of chemical ordering on diffusion

The recently developed phenomenological model (Helander T, Ågren J. 1999.) in two-sublattice (2SL) formalism for describing the effect of chemical ordering on diffusion in bcc-B2 ordered alloys was carefully re-analyzed and validated to be applicable in ordered bcc-B2, fcc-L10 and fcc-L12 phases. The model was then extended into one in four-sublattice (4SL) formalism in a thermodynamically consistent way. With the more physically based 4SL model, different ordered structures (e.g., fcc-L10 and fcc-L12) with respect to the same disordered phase (e.g., fcc-A1) can be simultaneously described. In order to evaluate the reasonable “endmembers” in 4SL model when the first-principles calculations and/or sufficient experimental diffusivities are not available, the “pair activation energy” was proposed in the present work to a first approximation. The suggested phenomenological model in 4SL formalism together with the concept of “pair activation energy” was finally applied to describe diffusion in the ordered fcc-L12 phase of the binary Ni–Al system. Several advantages of using the 4SL model over the 2SL model were also demonstrated.

Fabrication of alloyed aluminum nickelides by metallothermy of metals oxides

Fabrication conditions of NiAl, NiAl–Cr and NiAl–Cr–Mo–W alloys by joint aluminothermic reduction of initial metal oxides are investigated. Thermodynamic characteristics of accompanying reactions are determined. The temperature dependence of the isobaric potential change (ΔG0, kJ/mol) of reduction reactions of oxides point to high formation probability of alloys. It is revealed by differential thermal analysis that the reduction of metal oxides enters the active phase after aluminum is melted at ~650 °C and progresses according to the heterogeneous mechanism in a temperature range of 800–1100 °C. The optimal composition of the initial charge, which provides the maximal yield of metals into alloys, is established. It is found experimentally that the yield of metals into alloys constitutes 85–92 wt.%. Synthesis products are identified by the elemental and X-ray phase analyses as intermetallic compounds of the Ni–Al system, which contain inclusions of chromium, molybdenum, and tungsten. It is shown that the concentration of inclusions varies in a range of 1,5–6,5 wt.%. The microhardness of alloys is determined to vary from 3546 to 7436 MPa depending on the content of alloying elements.

Crystal structure of the Th2Ni10Al15 phase solved using electron diffraction tomography

Due course of study of the Th–Ni–Al system, new Th2Ni10Al15 phase was revealed. Despite the prolonged heat treatment– the studied alloy did not attained equilibrium and contained many intermetallic phases along with the phase of an interest. Thus, structure solution of this phase was performed by only viable option – electron crystallography, specifically electron diffraction tomography. Such structures (containing actinides, transition metals and Al) were never solved before using this method. Since it is possible that Th, as heavy scatterer, will smear the Fourier map, making it hard to find the positions of the Al atoms – case study was performed prior structure solution of the Th2Ni10Al15 phase. Finally, it was determined that the Th2Ni10Al15 structure has an orthorhombic unit cell with following lattice parameters a = 3.99 Å, b = 11.18 Å, c = 17.34 Å and Immm (No.71) space group. Its structural model contains 54 atoms in the unit cell, distributed over 10 unique atom positions. This structure can be described both as stacking of coordination polyhedra formed around heavy atoms and as layered structure with unique architecture.