Interdiffusion in pseudobinary sections of Ni3Al–Co ternary system

Interdiffusion in Ni₃Al intermetallic alloyed by 0–19 at.% Co was studied in three pseudo-binary sections: c_Al=22, c_Co=3.5, and c_Co=16 (c in at.%). The experiments were carried out at temperatures between 1173 and 1473 K. An attempt was made to analyze the interdiffusion coefficient into intrinsic diffusivities of individual components. Since no detectable shift of tungsten inert markers was observed, it was concluded that the intrinsic diffusivities of element pairs with varying concentration (Al/Ni and Co/Ni) in respective pseudo-binary sections are approximately equal one to another. The interdiffusion in pseudo-binary alloys Al/Ni (c_Co=const.) runs slightly faster than that in pseudo-binary alloys Ni/Co (c_Al=const.). The relations between the measured interdiffusion coefficients in the Ni_3−xAlCo_x ordered γ′ phase agree well with literature data for the interdiffusion coefficients in both the ordered γ′ and γ′+Co phases and also for the disordered Al–Ni and Co–Ni solid solutions. They are qualitatively explained by mutual interaction between constituents and by their interaction with vacancies.     

Reaction-sintering of intermetallic alloys of the Ni–Al–Mo system

A powder metallurgy route has been used for producing binary and ternary alloys of the Ni–Al–Mo system. Elemental powder mixtures were compacted and, then, sintered in a dilatometer. In this way the dimensional changes involved with thermally induced transformations could be followed during continuous heating runs up to the sintering temperatures. Sintering was assisted by the formation of a liquid phase, promoted by the heat output coming from the intermetallic phase formation reactions. The amount of liquid phase and the efficiency of sintering was highly dependent on the heating rate. A threshold value for optimal densification was identified for some compositions. The effect of other processing parameters, such as pre-sintering compaction pressure and sintering atmosphere has been considered too. The characterisation of the final products was mainly based on X-ray diffraction analyses. The microstructural parameters and the phase composition of the sintered materials were evaluated. On the basis of these results it is possible to draw some conclusions concerning the main phenomena occurring during the sintering process.     

Thermal conductivity mapping of the Ni–Al system and the beta-NiAl phase in the Ni–Al–Cr system

Micron-scale-resolution thermal conductivity mapping on graded compositions created in diffusion-multiple samples can be used to rapidly establish composition-phase-property relationships and to reveal the effects of solid-solutioning, order-disorder transition, compositional point defect, and site preference on thermal conductivity.

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A novel ternary eutectic in the Nb–Al–Ni system

A novel ternary eutectic reaction L → Nb₂Al + Al₃Nb + AlNbNi occurs at the composition of Al–40.4Nb–2.42Ni and the temperature of 1553.6 °C. The third phase is an Al-rich AlNbNi phase with the composition Al–33.3Nb–12.3Ni. Ternary Nb–Al–Ni eutectic exhibits regular microstructure with the predominantly fibrous morphology.     

Phase equilibria of the long-period stacking ordered phase in the Mg–Ni–Y system

Phase equilibria in the Mg-rich Mg–Ni–Y system at 300, 400 and 500 °C have been experimentally investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), electron probe micro-analyzer (EPMA) and transmission electron microscopy (TEM). The results show that a long-period stacking ordered (LPSO) phase with 14H structure is thermodynamically stable in the Mg–Ni–Y system in a wide temperature range, but it dissolves varying from 492 to 559 °C depending on the alloy composition. The equilibrium 14H phase has a very limited solid solution range, and can be nearly regarded as a ternary stoichiometric compound with a formulae as Mg₉₁Ni₄Y₅. The isothermal sections of the Mg-rich Mg–Ni–Y system at 300, 400 and 500 °C have been finally established, and a eutectic reaction, Liquid ↔ α-Mg + 14H + Mg₂Ni, has been determined occurring at 492 °C with a liquid composition about Mg_84.8Ni_12.0Y_3.2.     

Phase equilibria in Ti–Ni–Pt ternary system

Phase equilibria in Ti–Ni–Pt ternary system have been experimentally determined through diffusion triple technique combined with alloy samples approach. Assisted with electron probe microanalysis (EPMA) and X-ray diffraction (XRD) techniques, isothermal sections at 1073 and 1173 K of this system were constructed and existence of ternary phase Ti₂(Ni,Pt)₃ was confirmed. In addition, binary compounds Ti₃Pt₅ and TiPt_3– were found to be stable at 1073 and 1173 K, and remarkable ternary solubility in some binary compounds was detected, e.g., solubility of Pt in TiNi can be up to about 36% (molar fraction) at 1073 K and 40% (molar fraction) at 1173 K. Furthermore, a ternary invariant transition reaction TiNi₃+Ti₃Pt₅→Ti₂(Ni,Pt)₃+TiPt₃₊ at a temperature between 1073 and 1173 K was deduced.     

Diffusion in the Ti–Al system

Many properties of industrial Ti–Al alloys, such as high-temperature stability of the lamellar structure and creep resistance, are determined by diffusion rates in the phases and along the interfaces. The knowledge of diffusion characteristics and fundamental understanding of diffusion mechanisms are of great importance to the research and design of industrial Ti–Al alloys. This paper gives an overview of recent progress in experimental and theoretical studies of diffusion behavior in the phases of the Ti–Al system. The experimental methods used in modern diffusion measurements are briefly described, and recent experimental results for Ti and Al diffusion in α-Ti(Al), β-Ti(Al), and intermetallic phases α₂-Ti₃Al and γ-TiAl, are summarized. The results for interdiffusion and impurity diffusion in these phases are also discussed in detail. The second part of the paper provides an overview of current understanding of point defects and diffusion mechanisms in Ti₃Al and TiAl. A statistical model of point-defect disorder in ordered compounds is presented and applied to Ti₃Al and TiAl using input data generated with embedded-atom potentials. Possible atomic mechanisms of diffusion in these compounds are analyzed in detail, and methods of diffusion calculations under different mechanisms are reviewed. The relative importance of different mechanisms in Ti₃Al and TiAl is evaluated by comparing their estimated activation energies. Prospective topics of further experimental and theoretical research in this area are outlined.     

Kinetics and mechanisms of reactive solid state dewetting in the system Ag–Ni–O

Thick silver films vapour deposited under ultra high vacuum onto high purity nickel foils, dewet the substrate when annealed at high temperature in air. No hole is observed in the silver film after an annealing at the same temperature in a pure argon atmosphere. SEM observations of the film surface and of cross-sections reveal that dewetting occurs when a nickel oxide layer is formed at the silver–nickel interface as a result of oxygen diffusion through the silver film [Phil. Mag. Lett. 80(2000)33]. The kinetics of the phenomenon has been studied for films 1.8 microns thick annealed at 1123 K. Four dewetting mechanisms have been identified and are described. In the case of short heat treatments (1 h at 1123 K in air) the main dewetting mechanism is observed to be the condensation at the silver–nickel oxide interface of vacancies into voids which grow towards the free surface of the silver film.     

Liquidus projection for the Mo-rich portion of the Mo–Si–B ternary system

The solidification behavior in the Mo-rich portion of the Mo–Si–B ternary system has been examined based on the microstructures of arc-cast alloys. Several solidification characteristics in the Mo-rich portion of the system have been identified using XRD, SEM and TEM. The liquidus projection in the Mo-rich portion includes six primary solidification reactions; five reactions originating from the Mo–Si and Mo–B binary sections (Mo, Mo₂B, βMoB, Mo₃Si and Mo₅Si₃) and one from the ternary-based (Mo₅SiB₂) T₂ phase. The liquidus surface in general descends from the high melting temperature Mo–B binary side to the lower melting temperature Mo–Si binary side. The solidification path of alloys with compositions in the T₂ phase region is always preceded by the peritectic reaction of βMoB+L⇒T₂. In addition, four Class II reactions (four-phase reactions) and one Class I reaction (invariant ternary eutectic reaction) have been identified. The extent of solidification segregation in alloys with compositions in the Mo(ss)+T₂ two-phase field is discussed as it pertains to materials processing.     

Enhanced growth of intermetallic phases in the Ni–Ti system by current effects

The effect of direct current upon interfacial reactions in the Ni–Ti system was investigated. Isothermal diffusion couple experiments were conducted under varying current densities to de-couple Joule heating from intrinsic effects of the current flux. Current densities of up to 2546 A cm⁻² were used in the temperature range of 625–850 °C. All of the intermetallic compounds (NiTi, Ni₃Ti and NiTi₂) present in the equilibrium phase diagram were identified in the product layer. In addition, β-Ti solid solutions formed in samples annealed above the α→β temperature, 765 °C. The growth of all product layers was found to be parabolic and the applied current was found to significantly increase the growth rate of the intermetallic layers. Using Wagner’s analysis the present results were compared to published results on current-free diffusion couples. The intrinsic growth rate constant of the NiTi₂ intermetallic was found to be 43 times higher under the influence of 2546 A cm⁻² than that obtained without a current at 650 °C. The effective activation energy for the formation of all phases was found to decrease with increasing current density. The effect was strong for all phases but the decrease was most marked for Ni₃Ti. In this case, the activation energy decreased from 292 kJ mol⁻¹ under the influence of a current density of 1527 A cm⁻² to 86 kJ mol⁻¹ when the current density was 2036 A cm⁻². The results are explained in terms of current induced changes in the growth mechanism arising from changes in the concentration of point defects or their mobility.     

Structure analysis of the constitutional phases in liquid phase sintered W–Mo–Ni–Fe heavy alloys

The crystal structures of constitutional phases in two different tungsten heavy alloys processed via different conditions were examined. The compositions of these two alloys were W–11.9Mo–17.0Ni–7.7Fe and W–29.6Mo–17.0Ni–7.7Fe (at.%), which were liquid phase sintered at 1500 °C for 5 or 240 min, and followed by either furnace-cooling or water-quenching. Increase in isothermal hold caused increased concentration of Mo but decreased concentration of W in the matrix phase, which did not affect the lattice parameter of the matrix phase to a significant extent. Quenching the specimen in water caused increase in the concentrations of both W and Mo in the matrix phase, and, consequently, increases in the lattice parameter of the matrix phase. A tungsten heavy alloy with a high alloying concentration of Mo was prone to induce the precipitation of an intermetallic phase during cooling, which was enhanced by increasing the isothermal hold at the liquid phase sintering temperature and decreasing the cooling rate. The structure of this intermetallic phase is analogous to that of MoNi, and can be designated as (W_xMo_1−x)(Fe_yNi_1−y). The composition of this intermetallic phase varied with the composition of the alloy and its cooling rate subsequent to sintering. For a furnace-cooling condition, the atomic ratio of W to Mo (x/1−x) in this intermetallic phase was about 0.47 times the atomic ratio of W to Mo of the original alloy composition. Such a proportional constant decreased to about 0.30 when the specimen was water-quenched.     

Phase equilibria in the system Al–Co–Si

The Al–Co–Si system was studied for three isothermal sections at 600 °C (equilibria with Si), 800 °C (alloys up to 50 at.% Co) and 900 °C (alloys with more than 50 at.% Co). A total number of seven ternary compounds were characterized in the ternary system and the homogeneity ranges of the various ternary solid solutions of binary Co–Al and Co–Si compounds were studied. X-ray powder diffraction and optical microscopy was used for initial sample characterization and electron probe microanalysis of the annealed samples was used to determine the phase compositions within the ternary system. Lattice parameters have been determined for all ternary compounds and the change of lattice parameters with the composition is given for the solid solution phases.     

Interdiffusion Behavior of Ni–Cr–Al–Y Coatings Deposited by Arc-Ion Plating

Ni–Cr–Al–Y coatings were deposited on the Ni3Al–base superalloys IC-6 and K17 by arc-ion plating. The results indicated that a small amount of substrate atoms, such as Co, Ti, Mo, etc., existed in the Ni–Cr–Al–Y coatings near the substrates, probably due to sputter and antisputter. For the alloy K17, the impeding effect of Al was not obvious, because the temperatures of the substrate and the coating were high (420–480°C) during the deposition process and interdiffusion was accelerated. However, for alloy IC-6 which contains Al, as well as a high concentration of Mo, the diffusion of Cr was impeded. Vacuum heat treatment at 1050°C drastically increased diffusivities and the presence of Al and Mo was not enough to prevent some Cr diffusion. Thus, the coating became more uniform and close to the desired composition.     

Reactive diffusion in the system vanadium–silicon

The diffusion-controlled growth of vanadium silicides (V₃Si, V₅Si₃, V₆Si₅, VSi₂) was studied on bulk V–Si diffusion couples annealed for 2–36 h at 1150–1390°C. The layer growth kinetics was parabolic for all of the silicides. Only at 1150 and 1200°C was an induction period observed before the formation of a continuous V₆Si₅ layer. The parabolic growth constants of the II kind for the exclusive growth of each silicide from the adjacent phases were calculated from the parabolic constants of the I kind measured on the V–Si diffusion couples. The rate constants of the II kind were in turn related to the diffusion properties of the silicides. As a result, the interdiffusion coefficient, taking into account the diffusion of both elements, was obtained for each phase. The resulting activation energies were 240 kJ mol⁻¹ for V₃Si, 250 kJ mol⁻¹ for V₅Si₃ and 190 kJ mol⁻¹ for VSi₂. The activation energies scale well with the melting point of the compounds. For V₆Si₅, the activation energy is strongly dependent on the set of thermodynamic data used in the calculation owing to the uncertainty in the decomposition temperature of this phase.     

Equilibrium phases at the Ir-rich corner of the Ir–Nb–Ni–Al quaternary system

The equilibrium phases of three Ir–Nb–Ni–Al quaternary alloys at the Ir-rich corner were investigated in some detail. The phases were determined by X-ray profile as well as microstructure observation. Three phases of fcc/L1₂-Ir₃Nb/B2-IrAl were found in two samples, and four phases of fcc/L1₂-Ir₃Nb/L1₂-Ni₃Al/B2-IrAl were characterized in one sample. The electron probe microanalysis (EPMA) technique was applied for analyzing the phase composition of this quaternary system. A partial quaternary phase diagram at the Ir-rich side was plotted according to the EPMA results.     

Study on reaction and diffusion in the Mo–Si system by ZrO2 marker experiments

The dominant diffusing element in the MoSi₂, Mo₅Si₃ and Mo₃Si was found to be Si from marker experiments using ZrO₂ particles. Based on these marker experiments, the formation mechanism of the Mo₅Si₃ and Mo₃Si layers by reaction and diffusion was also discussed.     

Phase equilibria of the Sn–Ag–Ni ternary system and interfacial reactions at the Sn–Ag/Ni joints

There are no previous phase equilibria studies of the Sn–Ag–Ni ternary system, even though the phase equilibria information is important for the electronic industry. The isothermal section of the Sn–Ag–Ni ternary system at 240 °C has been determined in this study both by experimental examination and thermodynamic calculation. Experimental results show no existence of ternary compounds in the Sn–Ag–Ni system, and all the constituent binary compounds have very limited solubilities of the ternary elements. The binary Ni₃Sn₂ phase is very stable and is in equilibrium with most of the phases, Ag₃Sn, ζ-Ag₄Sn, Ag, Ni₃Sn₄ and Ni₃Sn phases. A preliminary thermodynamic model of the ternary system is developed based on the models of the three binary constituent systems without introducing any ternary interaction parameters. This ternary thermodynamic model is used with a commercial software Pandat to calculate the Sn–Ag–Ni 240 °C isothermal section. The phase relationships determined by calculation are consistent with those determined experimentally. Besides phase equilibria determination, the interfacial reactions between the Sn–Ag alloys with Ni substrate are investigated at 240, 300 and 400 °C, respectively. It is found that the phase formations in the Sn–3.5wt%Ag/Ni couples are very similar to those in the Sn/Ni couples.