Antistructure Bridge Mechanism Research Articles

Concentration of constitutional and thermal defects in UAl4

The point defect structure of intermetallic compound oI20 UAl4 is investigated using a combination of the statistical mechanical Wagner–Schottky model and first-principles calculations within a projector augmented wave pseudopotential method in conjunction with the generalized gradient approximation. The formation energies of eight point defects were calculated taking into account the four sublattices. The point defect concentrations are calculated as function of temperature and deviation from stoichiometry. Our results show that the aluminum antisite is the constitutional point defect on the Al-rich side. At this off-stoichiometric side the dominant thermal defect is an interbranch defect where four constitutional antisite aluminum atoms are replaced by five uranium vacancies. The point defect effective formation energies are obtained and these results allow us to identify the antistructure bridge mechanism as the most probable for the diffusion for Al atoms in the Al-rich UAl4 intermetallic compound.

Mechanism of Diffusion in Intermetallic Compounds

In a previous paper appearing in this journal (Tiwari and Mehrotra, Metall. Mater. Trans. A, vol. 43A, 2012, pp. 3654–62), the present authors advanced the hypothesis that the six-jump vacancy cycle, triple-defect mechanism, and antistructure bridge mechanism cannot explain the incidence of Kirkendall effect in B2 intermetallic compounds and hence their suitability as the operative mode of diffusion in these systems rests on dubious foundation. In the preceding contribution, Belova and Murch (BM) contested our hypothesis. This article is a reply to the BM’s critique of our paper.

Kirkendall Effect and Mechanism of Self-Diffusion in B2 Intermetallic Compounds

Single-phase interdiffusion studies in the B2 intermetallic compounds NiAl and FeAl have demonstrated the incidence of Kirkendall effect in these systems. The implication of this observation on the operating mode of self-diffusion in these systems is discussed. The direct exchange and ring diffusion mechanisms were excluded as the possible mode of substitutional diffusion in metals and alloys on account of the incidence of Kirkendall effect during single-phase interdiffusion studies. It is suggested that on similar phenomenological considerations, the six-jump vacancy cycle mechanism, triple-defect mechanism, and antistructure bridge mechanism are precluded as possible mode of diffusion in B2 intermetallic compounds.

Ni and Al diffusion in Ni-rich NiAl and the effect of Pt additions

First-principles density functional theory calculations are used to examine long-range Ni and short-range Al diffusion in Ni-rich NiAl. Since Ni diffusion in Ni-rich NiAl has been observed to have a lower activation energy than Ni diffusion in stoichiometric NiAl, another diffusion pathway, the antistructure bridge (ASB) mechanism, is believed to be activated once a threshold concentration of Ni antisite atoms has been reached. Our calculated activation energies for Ni diffusion via the ASB mechanism agree well with experimental data, confirming the ASB mechanism is likely operating in the Ni-rich region of the phase diagram, providing a means of long-range Ni diffusion. Since the ASB mechanism does not involve Al atoms, no long-range transport of Al occurs via this mechanism. Instead we looked at short-range Al transport via next-nearest-neighbor Al jumps into Al vacancies created during the ASB mechanism. The effect of Pt on these mechanisms was also examined; we predict that the presence of Pt enhances the rate of the ASB mechanism, which has implications for the lifetime of jet engine thermal barrier coatings.

Solute diffusion in the γ′ phase of Ni based alloys

We systematically investigate the diffusion mechanisms of 3d (Ti–Cu), 4d (Zr–Ag) and 5d (Hf–Au) transition metal solutes in γ′-Ni3Al phase using first-principles calculations. The results reveal that the diffusion of Ni-substituting solute is mainly controlled by the sublattice diffusion mechanism via Ni vacancies and the diffusion of Al-substituting solute is mostly governed by the formation of the anti-structure defects on the Ni sublattice with negligible contribution of the anti-structure bridge mechanism. The elements which occupy both the Al and Ni sites show a diffusion behavior similar to that of the Ni-substituting solutes. Our calculations show that larger atoms can move much faster than smaller atoms, which disprove the traditional view that larger atoms move slower than smaller atoms.

Atomistic simulations of diffusion mechanisms in off-stoichiometric Al-richNi3Al

This paper presents dynamics simulation results of diffusion in off-stoichiometric Al-richNi3Al (Ni73Al27) attemperature ranging from 1300 to 1550 K. The interatomic forces are described by the Finnis–Sinclairtype N-body potentials. Particular attention is devoted to the effect of the extra2% of Al atoms sitting on the Ni sublattice as antisite point defects(AlNi) on diffusion. Simulation results show that Ni atoms mainly diffuse through theNi sublattice at the temperatures investigated. Al atoms diffuse via both theintrasublattice and antistructure bridge (ASB) mechanisms. The contributionto Al diffusion from the ASB mechanism decreases at the lower temperature(T<1400 K). The presence of antisite point defects(AlNi) enhances both Aland Ni diffusion in Ni73Al27. The Ni–Al coupled diffusion effect is observed and understood at the atomic level for thefirst time.

Diffusion mechanisms in B2 NiAl phase studied by experiments on Kirkendall effect and interdiffusion under high pressures

The intrinsic diffusivities of both components and the activation volume for interdiffusion in the B2 type NiAl phase have been measured in the high temperature region from 1473–1773 K. The activation volume for interdiffusion in 40–49 at%Al is found to be almost constant value of 1.0 V 0 ( V 0: molar volume of alloys) at 1473–1773 K, suggesting that divacancies contribute to the diffusion. Near 43 at%Al, the diffusion mechanism for Al atoms is most likely the triple defect mechanism with the activation energy of 320 kJmol −1, while that of Ni atoms is probably the anti-structure bridge mechanism with the smaller activation energy of 240 kJmol −1. However, at 50.5 at%Al, Al atoms diffuse by the nearest neighbor mechanism with the smaller activation energy (260 kJmol −1), whereas Ni atoms diffuse probably by the next nearest neighbor jump related to the triple defect with the larger activation energy (360 kJmol −1).

Ni tracer diffusion in the B2-compound NiAl: influence of temperature and composition

The effect of composition and temperature on Ni bulk self-diffusion is investigated for nine different single crystalline NiAl alloys with well-defined compositions between 46.8 and 56.6 at.% Ni in the temperature range from 1050 to 1630K. The diffusion penetration profiles of Ni in NiAl were determined by applying two different techniques of profile detection. Radiotracer experiments have been carried out using the 63Ni tracer, a serial sectioning technique, and sensitive liquid scintillation counting for the high temperature measurements, while at lower temperatures the diffusion profiles were analyzed by secondary ion mass spectrometry (SIMS) using the highly enriched stable isotope 64Ni. In contrast to the literature data on Ni self-diffusion in NiAl alloys by Hancock and McDonnell [Phys. stat. sol. A4, (1971) 143], the present measurements show an unexpected concentration dependence of the Ni diffusion coefficients D with nearly constant diffusivities for stoichiometric and Al-rich alloys and increasing D values with increasing Ni content on the Ni-rich side of the NiAl composition range. The effective diffusion activation enthalpy Q is equal to about 3.0±0.07 eV for the Al-rich, stoichiometric, and slightly Ni-rich NiAl alloys, while for the compositions with larger Ni content a decrease of Q was observed with increasing Ni content, for example, Q=2.39 eV for the Ni 56.6Al 43.4 alloy. The present experimental results imply that mainly the same diffusion mechanism operates on both sides of stoichiometry in NiAl. This mechanism is identified with the triple defect mechanism. Its contribution is compositionally independent. The activation energy of Q=3.18 eV was calculated for the triple defect mechanism using empirical EAM potentials in agreement with the experimental data. The decrease of Q at large Ni concentrations on the Ni-rich side is explained by an additional contribution of the anti-structure bridge mechanism with the activation energy of Q=1.73 eV.

STUDY OF VACANCY MIGRATION IN NiAl BY MOLECULAR DYNAMICS

The molecular dynamics method is employed to simulate motions of monovacancies in the ordered intermetallic compound NiAl using Finnis-Sinclair N-body potentials.It is shown that the antistructure bridge mechanism is important at compositions deviated from stoichiometry, and near the stoichiometry, the six-jump cycle, the staight ［100］-type for Al vacancy and ［110］-type cycle for Ni vacancy, are energetically favorable. The calculated results, which are consistent with experimental measurement, clarify the recovery behavior of defects in NiAl quenching experiment.

Diffusional behavior in B2 intermetallic compounds

The Huntington-McCombie-Elcock (HME) and antistructure bridge (ASB) mechanisms proposed for B2 intermetallic compounds are reviewed. By using a thermodynamic argument, the HME mechanism is shown to be operative near the stoichiometric composition. On the other hand, the ASB mechanism is shown to be important at compositions away from stoichiometry. A combination of these two mechanisms is able to describe the compositional dependences of the experimentally determined self-diffusion coefficients for several B2 intermetallic compounds. By using conclusions drawn from these two mechanisms, it is further shown that the compositional dependences of the activation energies and pre-exponential factors are related to the partial molar enthalpies and entropies of the component elements. The frequently observed decreases in the activation energy and pre-exponential factor with deviations from stoichiometry are in accord with the corresponding decreases in the partial molar enthalpy and entropy. The extensive experimental data for NiAl are used to verify these relations.

On the composition dependencies of self-diffusion coefficients in B2 intermetallic compounds

It is shown that the Huntington-McCombie-Elcock (HME) mechanism proposed for B2 intermetallic compounds is able to account for the compositional dependencies of the self-diffusion coefficients in the compositional vicinity of stoichiometry using thermodynamic arguments in terms of point defects in the lattice. However, this mechanism becomes unimportant at large deviations from stoichiometry. A new mechanism, termed the anti-structure bridge (ASB) mechanism, is proposed. In contrast to the HME mechanism, which requires the diffusion of the component elements to be correlated, the ASB mechanism allows the diffusion of the component elements to be independent. The ASB mechanism is dominant at large deviations from stoichiometry. Using thermodynamic arguments in terms of specific point defects in this class of intermetallic compounds, it was shown that a combination of these two mechanisms is able to account for the compositional dependencies of the experimentally determined self-diffusion coefficients of five B2 intermetallic compounds reported in the literature. The compositional dependencies of the prefactors and activation energies for the self-diffusion coefficients can be properly accounted for by the variation of the partial entropy and partial enthalpy with deviations from stoichiometry.