Ni Antisites Research Articles

NiAl thin film growth on Ni(001) substrate using molecular dynamics simulations

We have studied thin film growth of NiAl on Nickel (001) substrate using molecular dynamics simulations (MD) based on the Embedded Atom Method (EAM) potential. An incidence energy of 0.06 eV at 800 K, 900 K and 1000 K was considered. After the deposition process, we have obtained a B2-NiAl structure film with different percentages; 32.6% for the temperature 1000 K, 30% for 900 K and 25% for 800 K. Our investigation has prompt us to analyze the crystalline structure. During the evolution of deposited film, we observe the formation of grains with different orientation, as well as the appearance of vacancies in Ni and Al sublattices and antisites.

Effects of Ni and Cu Antisite Substitution on the Phase Stability of CuGa2 from Liquid Ga/Cu–Ni Interfacial Reaction

Ga and Ga-based alloys have received significant attention for applications in the liquid state and also for their potential as a bonding material in microelectronic assemblies. This study investigates the phase stability of the CuGa2 phase as a product of the interfacial reaction between liquid Ga and Cu-10Ni substrates at room temperature. In the binary Ga-Cu system, CuGa2 is decomposed into liquid Ga and Cu9Ga4 as the temperature increases to around 260 °C, which prevents the widespread application of this alloy. In contrast to CuGa2 grown from a pure Cu substrate, CuGa2 from the Cu-10Ni substrate shows an increase in the decomposition temperature during heating from 25 to 300 °C. According to our first-principle calculations, there is only a minor difference in the total free energy between Ni solute at the Cu sublattice and the Ga sublattice in the tetragonal CuGa2 crystal structure. This result indicates that both of the sublattices can accommodate the dilute Ni solute with comparable probability. Regardless of the sublattice where the Ni impurities are located, the presence of diluted Ni in the matrix stabilizes the CuGa2 system by inducing some localized Ni 3d states at energy levels near the Fermi level. It is also shown that the formation of Cu antisite defects, which also stabilizes CuGa2, is preferable if the CuGa2 matrix is grown on a Ni-containing substrate.

MD simulations of primary damage formation in L12 Ni3Al intermetallics

Computer modelling by molecular dynamics (MD) was employed to study radiation damage created in collision cascades in L12 Ni3Al intermetallic compound. Either Al or Ni primary knock-on atoms (PKA) with PKA energy 5 keV ≤EPKA≤ 20 keV were initiated in the intermetallic crystals at ambient temperatures 100 K ≤T≤ 1200 K. At least 24 different cascades for each (EPKA, T, PKA type) set were simulated in order to mimic an isotropic spatial and random temporal distribution of PKAs. The total yield of nearly 1000 simulated cascades provides the largest yet reported sampling to get statistically reliable quantitative results. The obtained MD simulation data were analysed to get the number of Frenkel pairs, the fraction of point defects in point defect clusters, the fraction of Al and Ni vacancies, self-interstitial atoms (SIA) and anti-sites created in collision cascades as a function of (EPKA, T, PKA type). It was found that under identical simulation conditions, collision cascades in L12 Ni3Al initiated by Al PKAs generate more radiation damage, whereas collision cascades initiated by Ni PKAs produce more chemical disorder. Preferred formation of Ni SIAs in collision cascades in Ni3Al was observed under all ambient temperatures and PKA energies.

Effects of Point Defects on Properties of B2 NiAl: A First-principles Study

The basic properties, heats of formation, energies of formation, equilibrium concentration of point defects, elastic properties, and electronic structures for point defect structures of B2-NiAl crystal were analyzed by the density functional theory. Compared with B2-NiAl and other B2 intermetallic compounds, purity NiAl has better ductility and bonding strength. According to the calculated heats of formation, energies of formation, and equilibrium concentration of point defects, the Ni antisite and Ni vacancy are primary defects in B2-NiAl crystal. The calculated G/B0 and Cauchy pressure parameters C12-C44 values confirm that Ni vacancy, Ni antisite, and Al antisite can promote the brittleness of B2-NiAl, in which Ni vacancy is the primary defect, while Al vacancy with a low concentration can improve ductility of B2-NiAl. The density of state confirms that B2-NiAl intermetallic compounds are conductors, and point defects can promote the stability of the system expect Ni antisite defect.

Composition-dependent elastic properties in TiNi-Nb from first principle calculations

Abstract In this work, site-preference and elastic properties of TiNi-Nb austenite were investigated for various compositions, by using Exact Muffin-Tin Orbitals method in conjunction with Coherent Potential Approximation (EMTO-CPA). Moreover, composition-dependent elastic constants were calculated and used to explain the relation between composition and martensitic transition temperature. Our calculations suggested that Nb atoms prefer to occupy either Ni sites or Ti sites, depending strongly on composition. Variation of TiNi-Nb compositions is achieved through the formation of four sorts of defects: substituting Ti by Nb, substituting Ni by Nb, Ti anti-sites and Ni anti-sites. Our results suggested that c ′increases when Ni is substituted by Nb, while c 44 increases when Ni is substituted by Nb or Ni anti-sites form. Moreover, the maps of composition-dependent elastic constants were presented. TiNi-Nb with high c ′and c 44 could be obtained in Ni-deficient area. However, in Ni-excess area, only high c 44 could be obtained. Changing of martensitic transition temperature is sensitive to the variation of c 44 , rather than c ′. With the variation of c 44 , we explained the composition-dependent martensitic transition temperature for TiNi-Nb.

Defect Stability and Electronic Configuration of Off-Stoichiometric Ni-X-In (X = Mn, Fe and Co) Alloys: A First-Principles Study

Ni-Mn-In is a novel type of magnetic shape memory alloy, its shape memory effect has been realized through magnetic field induced reverse martensitic transformation. A variety of point defects would be generated during composition adjustment process, such as antisite defect, vacancy and exchange. The first–principles calculations within the framework of the density functional theory using the Vienna ab initio software package (VASP) have been used in this paper to investigate the defect formation energy and electronic configuration of the off-stoichiometric Ni-X-In (X= Mn, Fe and Co) alloys. The In antisite on the X sublattice (InX) and the Ni antisite on the X sublattice (NiX) have the lowest formation energies in the investigated series. The formation energy of the Ni vacancy is the lowest, while that of the in vacancy is the highest. It is confirmed that the in constituent plays a dominant role for stabilizing the austenitic phase.

Boron-substitution and defects in B2-type AlNi compound: Site-preference and influence on structural, thermodynamic and electronic properties

Using a combination of theoretical (first-principles total-energy and electronic structure calculations) as well as experimental (structural, thermodynamics) techniques, we systematically investigated the influence of B incorporation on the structural, electronic and thermodynamic properties of a series of technologically-important B-containing AlNi matrix. Special attention was paid to calculating the energy cost of placing B at various sites within the cubic unit cell. The most energetically favorable defects were identified to be, depending on initial stoichiometry, substitutional B at Al site (BAl), Ni vacancy (VNi), or Ni antisite (NiAl). We show that the induced variation in the lattice parameters can be correlated with the type and concentration of the involved defects: e.g. the surge of VNi defects leads to a stronger lattice-parameter reduction, that of NiAl ones to a relatively weaker reduction while that of BAl defects to a much weaker influence. Both electronic band structure calculations as well as thermodynamics measurements indicate that the 3d bands of Ni are fully occupied and magnetically unpolarized and that the resulting N(EF) is very small: all studied compounds are normal conductors with no trace of superconductivity or magnetic polarization.

Defect stabilities and magnetic properties of Ni-X-In (X= Mn, Fe and Co) alloys: a first-principle study

Ferromagnetic shape memory alloys (FSMAs) have received much attention as high performance sensor and actuator materials, since a large magnetic-field-induced strain by the rearrangement of twin variants in the martensitic phase was reported. Up to now, several FSMAs including Ni-Mn-Ga, Ni-Fe-Ga, Co-Ni-Ga, Ni-Mn-Al systems have been studied. Vast amount of knowledge accumulated at the properties of Ni-Mn-Ga Heusler alloys in the past decade can foresee the possibility of employing these alloys in device applications. However, the actuation output stress level of the Ni-Mn-Ga alloy is only less than 5 MPa, which represents a shortcoming of this alloy system. Recently, an unusual type of FSMAs Ni-Co-Mn-In Heusler alloy has been experimentally investigated. It shows magnetic-field-induced reverse martensitic transition (MFIRT), making it more attractive for practical application as magnetically driven actuator because it possesses a magnetostress level on the order of tens of MPa. An almost perfect shape memory effect associated with this phase transition is induced by a magnetic field and is called the metamagnetic shape memory effect. NiMnIn is the basic ternary alloy system of the NiMnInCo alloy, and possesses the same metamagnetic shape memory effect. Moreover, large magnetoresistance, large entropy change that generates giant reverse magnetocaloric effects (MCEs), giant Hall effect have been discovered in Ni-Mn-In alloys. Composition adjustment must be carried out around stoichiometric Ni2MnIn in order to obtain the appropriate martensitic transformation temperature and Curie temperature. Therefore, a variety of point defects would be generated in this process. In this paper, the defect formation energy and magnetic properties of the off-stoichiometric Ni-X-In (X= Mn, Fe and Co) alloys are systematically investigated by the first-principle calculations within the framework of the density functional theory through using the Vienna ab initio software package. The In and Ni antisites at the site of the X sublattice (InX and NiX) have the relatively low formation energies. For most cases of the site occupation, the excess atoms of the rich component directly occupy the site (s) of the deficient one (s), except for In-rich Ni-deficient composition. In the latter case, the defect pair (InX+XNi) is energetically more favorable. The formation energy of Ni vacancy is the lowest and that of In vacancy is the highest in the vacancy-type defects. It is confirmed that the In constituent is dominant for the stability of the parent phase. The value of the Ni magnetic moment sensitively depends on the distance between Ni and X atoms. The smaller the distance, the larger the Ni magnetic moment will be. For the anti-site type point defect, when the extra X atom occupies a Ni site, most of the free electrons gather around the extra X atom; while the extra X occupies an In position, the charges are regularly distributed between Ni and extra-X atoms. Moreover, with the increase of the X atomic number, the number of the valence electrons increases, and the bonding strength between the extra X and its neighboring Ni is also enhanced. The results are particularly useful in guiding composition design and developing new type of magnetic shape memory alloy.

A pseudo-binary interdiffusion study in the β-Ni(Pt)Al phase

Interdiffusion study is conducted in the Ni-rich part of the β-Ni(Pt)Al phase following the pseudo-binary approach. Interdiffusion coefficients over the whole composition range considered in this study increases with increase in Pt content, which is in line with the theoretical study predicting the decrease in vacancy formation and migration energy because of Pt addition. The trend of change in diffusion coefficient with the increase in Ni and Pt contents indicates that Pt preferably replaces Ni antisites.

Effects of Ni vacancy, Ni antisite, Cr and Pt on the third-order elastic constants and mechanical properties of NiAl

Effects of Ni vacancy, Ni antisite in Al sublattice, Cr in Al sublattice, Pt in Ni sublattice on the second-order elastic constants (SOECs) and third-order elastic constants (TOECs) of the B2 NiAl have been investigated using the first-principles methods. Lattice constant and the SOECs of NiAl are in good agreement with the previous results. The brittle/ductile transition map based on Pugh ratio G/B and Cauchy pressure Pc shows that Ni antisite, Cr, Pt and pressure can improve the ductility of NiAl, respectively. Ni vacancy and lower pressure can enhance the Vickers hardness Hv of NiAl. The density of states (DOS) and the charge density difference are also used to analysis the effects of vacancy, Ni antisite, Cr and Pt on the mechanical properties of NiAl, and the results are in consistent with the transition map.

Intrinsic point defects in thermoelectric half-Heusler alloys

The single-phase half-Heusler microstructure of ZrNiSn, (Zr0.5, Hf0.5)NiSn and Zr(Ni, Co0.2)Sn alloys was found to contain a high density of Heusler lattice bands 2–7 nm wide. These nanoscale Heusler lattice bands originated from clustering of Ni and/or Co antisites via occupation of structural vacancies by excess Ni or (Ni+Co) concentrations. The presence of these lattice point defects and their subsequent clustering could result in the reduction of thermal conductivity of the half-Heusler alloys.

First-principles study of point defects in Ni3Al

The energetics and structural properties of native, substitutional and interstitial defects in NiAl have been investigated by first-principles methods. In particular, we have determined the formation energies of composition conserving defects and established that the so-called penta defect, which consists of four vacancies on Ni sublattice and Ni antisite on the Al sublattice, is the main source of vacancies in NiAl. We show that this is due to the strong Ni-site preference of vacancies in NiAl. We have also calculated the site substitution behaviour of Cu, Pd, Pt, Si, Ti, Cr, V, Nb, Ta and Mo and their effect on the concentration expansion coefficient. We show the latter information can used for an indirect estimate of the site substitution behaviour of the alloying elements. The solution energy of carbon and its effect on the lattice constant of NiAl have been obtained in the dilute limit in the first-principles calculations. We have also determined the chemical and strain-induced carbon–carbon interactions in the interstitial positions of NiAl. These interactions have been subsequently used in the statistical thermodynamic simulations of carbon ordering in NiAl.

Microstructure evolution of nanoprecipitates in half-Heusler TiNiSn alloys

The microstructures of thermoelectric TiNiSn half-Heusler alloys have been studied in detail. For concentration ratios that are slightly rich in Ni, a high density of Heusler-phase nanosized precipitates tended to precipitate within a half-Heusler matrix. The morphology and average size of the Heusler nanoprecipitates were very sensitive to the Ni concentration ratio in the half-Heusler matrix of the alloys. Smaller Heusler nanoprecipitates with coherent ellipsoidal (<5nm) and disc (<10nm wide) morphologies tended to precipitate within the matrices of alloys with slightly elevated Ni concentration ratios (Ti:Ni:Sn=1.0:1.1:1.0). However, much larger coherent discs (<45nm wide and <5nm thick), semicoherent platelets (up to 1μm long and <30nm thick) and spheres (up to 1μm wide) were observed in the matrices of the alloys with larger Ni concentration ratios (Ti:Ni:Sn=1.0:1.2:1.0). Tetragonal structures were observed in the coherent Heusler nanoprecipitates. The formation of such structures was closely associated with the size, morphology and interface coherency of the nanoprecipitates. Moreover, most of the coherent Heusler nanoprecipitates were preferentially oriented parallel to the cubic {001}HH orientations. Interfacial defects between the Heusler and half-Heusler phases, as well as lattice point defects, Ni antisites and vacancies, were found to be closely related to the formation of the Heusler nanoprecipitates. A mechanism has been proposed in this study to describe the coarsening of the Heusler nanoprecipitates via the formation of lattice point defects and interfacial defects.

First-principles investigations of oxygen adsorption at TiNi surface and the TiO2/TiO–TiNi interface

Ab-intio study of the interaction of atomic and molecular oxygen with the TiNi(110) surface was performed using the projector augmented wave method with generalized gradient approximation for the exchange-correlation functional. The oxygen adsorption energies were calculated and the preferential adsorption sites of oxygen atom on the surface were determined. Our results confirmed the formation of a Ni-rich layer at the alloy–oxide interface. Atomic and electronic structure of both TiO/TiNi(110) and TiO2/TiNi(110) interfaces were analyzed. The formation energies (Ef) of point defects at the interfacial layers as well as in bulk TiNi, monoclinic TiO, and rutile TiO2 were estimated. It was shown that Ef of Ti–Ni swap defect has a lower energy than that for the Ni antisites at the TiO2(100)/TiNi(110) interface. In the case of TiO(100)/TiNi(110) interface, the formation energies of Ti–Ni swap and Ni-antisites defects are close to each other. Our results demonstrated that Ef of Ni-defect in TiO is twice less than that in TiO2. The increase in the formation energies of defects in TiO2 reveal the increase in diffusion barriers of Ni atoms in comparison with those in oxides with a lower oxygen content, which hampers Ni segregation.

Study of Nickel Segregation at the TiNi-Titanium Oxide Interface

Ab-intio investigations of atomic and molecular oxygen on TiNi(110) surface are performed by using the projector augmented wave method with generalized gradient approximation for the exchange-correlation functional. Our results confirm the formation of a Ni-rich interface TiO2(100)/TiNi(110), for which the formation energies (Hf) of point defects at the interfacial layers were estimated. It is shown that Hf of swap Ti-Ni defect has a lower energy than that for the Ni antisites at the interfacial layers. The formation energies of point defects in bulk TiNi, monoclinic TiO, and rutile TiO2 are also calculated. Our results demonstrate that Hf of Ni-antisites in TiO is twice less than that in TiO2. The formation of small Ni clusters is also discussed.

Molecular dynamics simulation of diffusion in a (110) B2-NiAl film

We report on the first direct molecular dynamics study of diffusion in B2-NiAl using one of the most reliable embedded-atom method potentials for this phase. The simulation is performed for near the stoichiometric composition at a temperature just below the melting temperature of the model. In the molecular dynamics simulation, an equilibrium point-defect concentration is generated and maintained by using a film sample with periodic boundary conditions only in two directions and free surfaces in the third direction. Two types of point defects – Ni vacancies and Ni antisites – are found in the bulk of the model. It is demonstrated that isolated Ni vacancies strongly dominate in concentration over all of their bound complexes with Ni antisites. Although we predict that some attractive interactions should occur between point defects to form bound Ni vacancy-Ni antisite pairs and bound Ni antisite-Ni vacancy-Ni antisite complexes, only about 2% of Ni vacancies and 1% of Ni antisites statistically randomly associate to form bound Ni vacancy-Ni antisite-Ni vacancy complexes (the so-called bound triple-defect complex). As a result, it is deduced that the triple-defect diffusion mechanism is not likely to be the dominant diffusion mechanism in the bulk of the model because this diffusion mechanism effectively requires that the Ni vacancies and Ni antisites must form bound triple-defect complexes. Furthermore, it is found that Ni atoms diffuse in the bulk of the model on average about 2.5 times faster than Al atoms. Therefore, we suggest that isolated Ni vacancies are likely to play a key role in atomic diffusion of both Ni and Al near the stoichiometric composition of B2-NiAl and, consequently, the most plausible and widely accepted candidate for dominant diffusion mechanism in B2-NiAl can be considered to be six-jump cycles of a Ni vacancy. Furthermore, we can suppose that additional next-nearest-neighbor jumps of a Ni vacancy may cause that Ni atoms diffuse faster than Al atoms.

In situ x-ray study of the oxidation of a vicinal NiAl(6,7,1) surface

We present an in situ surface x-ray diffraction study of the clean, oxidized and subsequently annealed surfaces of regularly stepped NiAl(6,7,1). Our results show that the UHV stable, clean surface is not faceted and consists of a regular array of (1,1,0) terraces and (0,1,1) steps. The topmost Al and Ni atoms on the terraces exhibit a rippled relaxation while the step atoms are relaxed towards the bulk. Preferential Al oxidation at 540 K and 6×10−6 mbar O2 leads to the formation of a 5 Å thin, disordered alumina layer and induces Al vacancies and Ni anti-sites in the Al-depleted interfacial region. The terrace-step structure of the clean surface is maintained, but strong inward relaxations of the interfacial atoms change the strain field around the steps. Massive (1,1,0) faceting with facets up to 50 times larger than the original terraces occurs after high-temperature annealing, during which the surface oxide develops a complex long-range ordering. These results can be understood by the change of interfacial strain, which removes the energy barrier for mass transport. In addition, unlike in the case of low-index (1,1,0) surfaces, we find the step-induced suppression of twin domain formation in the alumina film grown on NiAl(6,7,1). Our results show that the interplay between oxidation and strain can have dramatic effects on the morphology of vicinal surfaces.

Point defects in the NiAl(100) surface

The stability of various point defects in NiAl(100) has been investigated by first-principlescalculations. For Al-rich surfaces, Ni vacancies within the first Al layer are energeticallymost favourable. For Ni-rich surfaces, so-called double defects, consisting of both Ni-antisiteatom in the first Al layer and a Ni vacancy within the second Ni layer, form theconfiguration of lowest energy, superior to singular Ni antisites. An additional andsignificant energy gain is found in both cases by mutual lateral interaction of the defects,when they are arranged in the diagonal direction. Respective ordered configurations were found as the most stable structures. A 50:50 mixture of bothdefect types turns out to be even lower in energy than the ideal Al-terminated NiAl(100)surface, proving the latter to be metastable only. This is in line with the often reportedinability in experiments to prepare ideal NiAl(100) surfaces.

First-principles Investigations of Point Defect Behavior and Elastic Properties of TiNi Based Alloys

AbstractFirst-principles calculations by the use of a plane-wave pseudopotential method are performed to investigate intrinsic point defect behavior in TiNi. The results show that TiNi is an antisite type intermetallic compound. The calculated interaction energies between the point defects demonstrate that Ti antisites are attractive to each other whereas Ni antisites are mutually repulsive. The attraction between Ti antisites indicates that excess Ti in TiNi may agglomerate so that a Ti-rich phase can easily precipitate. The repulsion between Ni antisites implies that the excess Ni is of certain solubility in TiNi. This result explains well the asymmetric feature of TiNi field on the binary phase diagram. In order to understand the correlation between the composition dependent elastic modulus and martensitic transformation (MT) temperature, the elastic moduli critical to MT, i.e., c′ and c44, are calculated as a function of the composition of the off-stoichiometric TiNi and a series of ternary TiNi-X alloys, by the use of exact muffin-tin orbital method in combination with coherent potential approximation. It turns out that, generally speaking, the early transition metal (TM) alloying elements in the periodic table increase c′ but decrease c44; the middle ones increase both c′ and c44, whereas the late ones decrease c′ but increase c44. An examination of the theoretical composition dependent elastic modulus and the experimental MT temperature shows that the MT temperature is more sensitive to the variation of c44 than to that of c′.

Point defects and their interaction in TiNi from first-principles calculations

A first-principles plane-wave pseudopotential method based on density functional theory is used to investigate the concentrations of point defects and their interaction in TiNi. The calculations show that, in the thermal equilibrium state, TiNi develops antisite-type point defect configurations. In accordance with the sequence of the stability of the B2, B19, and B19(') phases, the concentrations of the point defects in stoichiometric TiNi at the identical temperature decrease from B2 to B19 to B19('). At high temperature, there are considerable amounts of vacancies in Ti-rich compounds. The calculated interaction energies show that the nearest-neighboring Ni antisite and Ti antisite are attractive to each other, whereas the second-nearest-neighboring Ni antisites repel each other and Ti antisites mutually attract, which indicates that a Ti-rich domain may appear in stoichiometric TiNi, and also explains the high solubility of excess Ni and low solubility of excess Ti in TiNi. The electronic structure mechanisms behind the interactions between the point defects are discussed based on charge density calculations. It was shown that the attractive interaction between Ti antisites is due to the accumulation of electron density between the Ti antisites, whereas the repulsion between Ni antisites is due to the depletion of electron density between them.