Antistructure Bridge Research Articles

Synthesis and Atomic Transport of CoSn3 NanoIMC by In Situ TEM.

In order to optimize the interfacial properties by adding Co to the bumps of copper pillars and to overcome the strong tendency of Co to oxidize, an intermetallic compound (IMC) "capsule" was developed for the purpose of transporting elements through the intermetallic compound. In this study, we present a comprehensive analysis of the transformation process of CoSn2 nanoparticles into CoSn3 at the nanoscale using in situ heating transmission electron microscopy (TEM). The experimental results reveal that CoSn2 nanoparticle growth occurs through polymerization, whereas CoSn3 nanoparticle formation relies on the reaction between CoSn2 and Sn. During the initial stages of the reaction, Co dissolves and diffuses into Sn, leading to the nucleation and growth of CoSn2 in Sn via Ostwald ripening. As the input energy increases, vacancies in CoSn2 drive a reaction at the Sn/CoSn2 interface, resulting in the generation of CoSn3. In this process, Sn nanoparticles enter the CoSn2 structure through the "Anti Structure Bridge (ASB) mechanism" to fill vacancies. Following the codeposition process, CoSn3 nanoparticles were successfully plated within the Sn layer of the Cu-pillar bumps. Upon reflow heating, the CoSn3 nanoparticles exhibited a preference for precipitating the vacant sites within the Sn layer. This process facilitated the release of Co atoms from CoSn2, enabling their diffusion throughout the entire Sn layer.

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.

Application of random walk concept to the cyclic diffusion mechanisms for self-diffusion in intermetallic compounds

Huntington–Elcock–McCombie (HEM) mechanism involving six consecutive and correlated jumps, a triple-defect mechanism (TDM) involving three correlated jumps and an anti-structure bridge (ASB) mechanism invoking the migration of an anti-structure atom are the three mechanisms currently in vogue to explain the self- and solute diffusion in intermetallic compounds. Among them, HEM and TDM are cyclic in nature. The HEM and TDM constitute the theme of the present article. The concept of random walk is applied to them and appropriate expressions for the diffusion coefficient are derived. These equations are then employed to estimate activation energies for self-diffusion via HEM and TDM processes and compared with the available experimental data on activation energy for self-diffusion in intermetallic compounds. The resulting activation energies do not favour HEM and TDM for the self-diffusion in intermetallic compounds. A comparison of the sum of experimentally determined activation energies for vacancy formation and migration with the activation energies for self-diffusion determined from radioactive tracer method favours the conventional monovacancy-mediated process for self-diffusion in intermetallic compounds.

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.

Interdiffusion in Intermetallics

Recently, Tiwari and Mehrotra asserted that the vacancy flux in interdiffusion is driven by a vacancy concentration gradient and also that interdiffusion in intermetallics cannot proceed by the six-jump cycle, triple defect, and antistructural bridge mechanisms. It is shown that those authors’ arguments are incorrect according to classical diffusion theory, by the numerous simulations that have been performed, and by analysis of these mechanisms.

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.

Atomistic simulation of point defects in L10‐type CuAu ordered alloy

AbstractThe configurations of point defects and their migration mechanisms in L10‐type CuAu ordered alloy have been investigated with the modified analytical embedded atom method. The results show that the antisite defects CuAu (especially) and AuCu are easier to form than Cu and Au monovacancies owing to their lower formation energies. For the divacancies, the Cu–Au divacancy is the most stable configuration among the three types of configurations (Cu–Cu, Au–Au, and Cu–Au divacancies). In five migration mechanisms of either a Cu or an Au monovacancy, the antistructural bridge migration is the most favorable due to its lowest migration energy and the one next‐nearest‐neighbor jump (1NNNJ) migration is the most difficult mechanism but can be achieved by six nearest‐neighbor cyclic jump (S[001]6NNCJ) or bent [001] six nearest‐neighbor cyclic jump (B[001]6NNCJ). For a Cu monovacancy, the favorable migration mechanisms are one nearest‐neighbor jump (1NNJ), B[001]6NNCJ, and S[001]6NNCJ successively; however, for an Au monovacancy, the favorable migration mechanisms are S[001]6NNCJ (or B[001]6NNCJ) and then 1NNJ.

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.

First-principles investigation of migration barriers and point defect complexes in B2–NiAl

We perform a systematic first-principles investigation of atomic hop mechanisms in B2–NiAl and discover a low barrier collective hop that can mediate Al diffusion through the anti-structural bridge mechanism. We also find an alternative hop sequence for the migration of a triple defect than that proposed previously. To shed light on the dominant hop mechanisms that mediate diffusion in B2–NiAl, we study point defects and defect clusters in B2–NiAl at high temperature by combining a cluster expansion with Monte Carlo simulations. Going beyond the mean field approximation, we find that the inclusion of interactions among the various point defects is crucial to predict the concentration of defect complexes, such as the triple defects of B2–NiAl. Interactions among point defects also introduce an important degree of short-range order between Al antisite defects and Ni antisite defects. We find an increasing probability between pairs of Al antisite atoms and Ni vacancies as the alloy concentration of B2–NiAl becomes both Al rich and Ni rich, suggesting that the anti-structural bridge mechanism should play an important role in facilitating Al transport.

Computer modeling of diffusion in Ni-richNi3Al and compositiondependence of diffusion in γ′ Ni3Al

Atomistic simulations of diffusion in off-stoichiometric Ni-richNi3Al (Ni77Al23) at temperatures ranging from 1300 to 1550 K and comprehensiveanalysis of the composition dependence of Ni and Al diffusion inγ′ Ni3Al of threecompositions, 73, 75 and 77 at.% Ni, are presented. The interatomic forces are described by Finnis–Sinclairtype N-body potentials. The simulations reveal that Ni diffusion is dominated by Ni vacancy mechanisms;Al diffusion is via both the intrasublattice and antistructure bridge (ASB) mechanism inNi77Al23 at the temperatures investigated. The presence of an extra 2% of antisitedefects enhances diffusion of both Ni and Al at off-stoichiometric Ni-enrichedNi3Al via the vacancy–antisite interaction. The single vacancy diffusivity of Ni and Al inNi3Al of three compositions, 73, 75 and 77 at.% Ni, at the above temperatures are corrected withthermal equilibrium concentration of point defects. The corrected Ni and Al diffusion dataare in good agreement with available experimental data. The Ni diffusivity decreases withthe Ni concentration, while Al diffusivity has a minimum at the stoichiometric compositionin the simulated temperature range.

Interdiffusion of Two L1<sub>0</sub>Phases without Long-Range Order Decrease: Experiments and Molecular Dynamics Simulations

The L10 ordered MPt(001) thin films (M = Fe or Co) are very interesting for perpendicular recording due to their magnetic anisotropy and magneto-optical behaviours. Epitaxial L10-ordered NiPt(001) / FePt(001) bi-layers were co-deposited on MgO(100) substrates by MBE. The L10 order parameter is high with the concentration modulation along the growth direction. Some FeNiPt2(001) thin films were obtained by interdiffusion of the bilayers. The long-range-order parameter is conserved after interdiffusion (S = 0.75 ± 0.06), which can be explained by different mechanisms: a second-neighbour jump, a six-jump cycle, an anti-structural bridge mechanism or an antisite-pair elimination and creation mechanism, a double vacancy or a triple defect diffusion mechanism. Quenched molecular dynamics calculations in the frame of the second moment approximation of the tight binding method have been performed to obtain the energetic paths of the different mechanisms. The secondneighbour vacancy jump, the simultaneous jumps of bivacancies and the triple defect mechanisms can be ruled out for energetic reasons.

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.

Atomistic simulations of diffusion mechanisms in stoichiometricNi3Al

The first molecular dynamics (MD) simulation of diffusion mechanisms in ordered stoichiometricNi3Al with the Finnis–Sinclair interatomic potential is reported in this paper. Results show thatNi atoms mainly diffuse through Ni sublattices via the vacancy mechanism; Al atomsdiffuse via both the antistructure bridge and the intrasublattice mechanism. Molecularstatics (MS) simulations are carried out to estimate the migration energy barriers for theelementary jumps and the different diffusion mechanisms with both the ‘embedded atommethod’ and Finnis–Sinclair interatomic potentials. It is proven that the six-jump cyclemechanism is not energetically favourable because of the high migration energy. Thus, MDand MS simulations yield the same results on the diffusion mechanism in stoichiometricNi3Al. Ni diffusion mechanisms obtained in this work agree with experimental results andtheoretical calculation.

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.

Tracer diffusion behavior of Ga as an Al-substituting element in Ti 3Al and TiAl intermetallic compounds

Tracer diffusion of 69Ga was measured in pure α-Ti, α 2-Ti 3Al (33 at% Al), and γ-TiAl (54 at% Al) over wide temperature intervals up to the upper limits of stability of the corresponding phases. The concentration profiles were determined by secondary ion mass spectroscopy (SIMS). The diffusion coefficients revealed a good Arrhenius behavior with the frequency factors D 0= 2.1 +0.7 −0.5 ×10 −3; 6.3 +9.7 −3.8 ×10 −5; and 4.4 +1.9 −1.3 ×10 −5 m 2 s −1 and the activation enthalpies Q=295.5±2.3; 315±10; and 293±4 kJ mol −1 for the Ga solute diffusion in α-Ti, Ti 3Al and TiAl, respectively. The systematics of Ga diffusion behavior in these materials is discussed in dependence on the topology of the Ti sublattice in the compounds. The experimental data of Ga diffusion suggest that Ga behaves as an Al-substituting element in titanium aluminides. The mechanisms of diffusion of Al-substituting solutes in ordered titanium aluminides are considered. It is shown that Ga diffuses in Ti 3Al most likely as anti-structure-like defects via the Ti sublattice by nearest neighbor jumps. The experimental data for the Ga diffusion in TiAl can be explained by a predominant contribution of anti-structure bridge jumps.

Self-diffusion in γ-TiAl: an experimental study and atomistic calculations

This paper presents the results of combined experimental and theoretical studies of Ti and Al self-diffusion in the intermetallic compound γ-TiAl. Ti self-diffusion coefficients were measured over a temperature range of 1184–1691 K using the radiotracer 44Ti and the serial sectioning method. The diffusion coefficients show a non-Arrhenius behaviour with significant upward deviations at high temperatures (above ∼1470 K). The pre-exponential factor and the activation energy of Ti self-diffusion determined in the low temperature range (<1470 K) are (D ∗ Ti ) 0≈1.5×10 −6 m 2/s and Q ∗ Ti ≈250 kJ/mol, respectively. By combining the obtained Ti self-diffusion coefficients with interdiffusion coefficients measured previously by Sprengel et al. (W. Sprengel, N. Opkawa, H. Nakajima, Intermetallics 4 (1996), 185) and using the Darken–Manning equation, Al self-diffusion coefficients in TiAl were evaluated. Within the scatter of the data points, Al self-diffusion follows the Arrhenius law with D ∗ Al 0≈2.1×10 −2 m 2/s and Q ∗ Al ≈360 kJ/mol. The energies of point defect formation and migration in TiAl were calculated by molecular statics with embedded-atom potentials. Several diffusion mechanisms in TiAl were considered, including migration of Ti and Al along their own sublattices by single vacancy jumps, 3-jump vacancy cycles and the anti-structural bridge mechanism. The activation energies of Ti and Al self-diffusion by different mechanisms were evaluated as functions of the alloy composition. The calculations predict that, in the compositions studied experimentally, Ti diffusion at low temperatures is dominated by the vacancy mechanism, while at higher temperatures the anti-structural bridge mechanism can essentially contribute to the overall diffusivity. Al diffusion occurs by the vacancy mechanism with some contribution of 3-jump cycles and anti-structural bridges. These predictions are in good agreement with the experimental results of this work.

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.

Percolation and the anti-structural bridge mechanism for diffusion in ordered alloys of the L12 type

The antistructural bridge mechanism is a subset of the vacancy mechanism in binary ordered alloys/intermetallic compounds and is an important contributor to diffusion when there is substantial antistructural disorder. In the present study the limits for the antistructural bridge mechanism are established for a number of jump possibilities for both components in A3B alloys taking the L12 structure. The general vacancy model used supposes that the sublattices exist a priori. Three vacancy exchange frequencies describe A jumps from the α-sublattice to the β-sublattice and its reverse, and intra α-sublattice jumps. Another three exchange frequencies describe B atom jumps from the β-sublattice to the α-sublattice and its reverse, and again intra α-sublattice jumps. The limits of the antistructural bridge mechanism are determined analytically by percolation arguments and assessed by computer simulation. The A component does not show percolation threshold behaviour within a useful range of composition. The B component show several types of percolation behaviour depending on which jumps are operative. A particularly interesting interstitialcy-like mechanism is identified in which A atoms diffuse by intra α sublattice jumps which assist B atom diffusion by intersublattice jumps. This is discussed in the context of Ni3Al.