Ni Antisites Research Articles

First-principles study of constitutional point defects in B2 NiAl using special quasirandom structures

We developed special quasirandom structures (SQSs) for substitutionally random pseudobinary A 1 − x B x C B2 alloys at compositions x = 0.25 and 0.5. The structures mimic the local pair and multisite correlation functions of the corresponding random simple-cubic alloy. Our SQSs were applied to study non-stoichiometric B2 NiAl alloys containing high concentrations of constitutional point defects. Direct first-principles calculations on the SQSs provide formation enthalpies, equilibrium lattice parameters and elastic constants of non-stoichiometric B2 NiAl alloys in satisfactory agreement with existing experimental data in the literature. Our calculations unambiguously show that Ni vacancies and Ni antisites are the stable constitutional point defects in Al-rich and Ni-rich B2 NiAl, respectively, up to large deviations from stoichiometry. Our SQS calculations also confirmed the experimentally observed structural instability of B2 NiAl at high Ni concentrations. Finally, we demonstrated that our SQSs can even give formation enthalpies of isolated defects in good agreement with 54-atom supercell calculations.

Atomic modelling of Co, Cr, Fe, antisite atoms and vacancies in B2–NiAl

The formation and preference site energies and volume changes of single defects and pairs of defects of ternary alloying elements in NiAl intermetallic compound were studied by the density functional theory. Exceptionally, a small formation energy of Co defect in Ni sublattice was found. The Co, Cr, Fe atoms and vacancies prefer to reside in the Ni sublattice. The formation energy for Co–Co nearest neighbor pair is the smallest. The Ni antisite in Al sublattice requires less energy than the Al antisite in Ni sublattice.

Site occupancies, point defect concentrations, and solid solution hardening in B2 (Ni,Fe)Al

A novel approach was used to characterize the site occupancies and point defect concentrations in the B2-ordered (Ni,Fe)Al single phase field over a wide range of composition and temperature. This approach combined atom location by channeling enhanced microanalysis (ALCHEMI) determinations of atomic site occupancies and vacancy concentration measurements. The triple defect structure was observed across the entire phase field. Qualitative thermodynamic predictions such as that Fe anti-site are more stable than Ni anti-sites were confirmed. However, in some portions of the phase field it was found that Ni anti-site concentrations were higher than expected from simplistic thermodynamic predictions of site preference. Such predictions are clearly inadequate if a quantitative determination of site occupancies is desired. The importance of this type of characterization in interpreting solid solution hardening (SSH) in ordered compounds was illustrated by considering three portions of the (Ni,Fe)Al phase field. In all three cases the solute atoms did not directly harden, but affected the hardness by altering the concentrations of other point defects.

Neutron spin-echo spectroscopy for diffusion in crystalline solids

Neutron spin-echo spectroscopy (NSE) offers unprecedented opportunities in the investigation of diffusion in crystalline systems due to its outstanding energy resolution. NSE not only enables measurements at lower diffusivities than the established techniques of neutron spectroscopy, but it also gives a very immediate access to the different time scales involved in the diffusion process. This is demonstrated in detail on the example of the binary alloy NiGa where the Ni atoms hop between regular sites on the Ni sublattice and anti-sites on the Ga sublattice. Experiments on two different NSE instruments are compared to measurements using neutron backscattering spectroscopy. The potential of NSE for the investigation of jump diffusion and experimental requirements are discussed.

Electronic structure and thermodynamics of defects inNiAl3

We present first-principles calculations of the structural and electronic properties of orthorhombic ${\mathrm{NiAl}}_{3},$ including the intrinsic point defects. The energies obtained from these calculations are employed in a statistical mechanics model to investigate the role of ${\mathrm{NiAl}}_{3}$ in the Ni-Al phase diagram. Our calculations show that the preferred constituent defect mechanisms for ${\mathrm{NiAl}}_{3}$ off stoichiometry are the $8d$ Ni antisites on the Ni-rich side and Ni vacancies on the Al-rich side. The calculated free energy is in agreement with the observation that ${\mathrm{NiAl}}_{3}$ is a stoichiometric compound.

Mean field theory of point defects in β-NiAl

We have used a mean field theory to study the equilibrium concentrations of vacancies and antisite defects in each sublattice of β-NiAl in the composition range between 45 and 55 at% Al and in the temperature range 800–2000 K. The interatomic potentials, which are inputs to the theory, are extracted from recently reported (continuous) pair potentials of Finnis–Sinclair type. The predominant defect in the stoichiometric alloy is a combination of an Ni-antisite defect and two vacancies on the Ni sublattice. The Al-rich alloys of composition (50+Δ) at% contain 2Δ% vacancies; since the alloys are almost perfectly ordered, these vacancies predominantly occupy the Ni sublattice. Similarly, the Ni-rich alloys of composition (50−Δ) at% contain Δ% Ni antisites. Both the vacancies on the Ni sublattice (in Al-rich alloys) and Ni-antisites (in Ni-rich alloys) show negligible temperature dependence, and hence owe their origin to the off-stoichiometry. In all the alloys, the Al-antisites have the lowest concentration (of the order 10 −6 even at 2000 K). We have critically compared our theoretical results with available experimental ones.

On the surface composition of intermetallic compounds: the case of MgNi 2

Basic factors governing surface segregation in “size-factor” intermetallics are introduced and applied to the case of Mg segregation in MgNi 2 thin films reported recently. The calculations, based on a Braggs–Williams-type approximation, are in agreement with the experimental results, and indicate a preferential population of surface Ni antisites by the larger Mg atoms from the nearby underlayers. The resultant increase in the equilibrium concentration of Mg with temperature is associated with entropy-driven enhancement of compositional disorder at the surface region of the intermetallic compound.

Re-ordering of disordered Ni3Al at low temperatures: contribution of mobile interstitials

The ordering efficiency of mobile interstitials in Ni3Al during long-range diffusion is investigated using a rate equation approach for the defect evolution. It is shown that experimentally observed disordering kinetics under electron irradiation below room temperature can be well explained by interstitial migration. It is proposed that the elementary ordering jump consists of an exchange of neighbouring Ni and Al antisites induced by the presence of a NiNi dumbbell interstitial. However, in ion-irradiated Ni3Al such ordering process is not efficient enough to produce the observed long-range order. Under these irradiation conditions a more effective re-ordering mechanism than interstitial-induced ordering, e.g. thermal spike ordering must be operative.