Reactive interdiffusion in the binary system Ni-Si: Morphology of the Ni3Si2 phase

In binary multiphase diffusion, it is generally admitted that interfaces between phases are necessarily plane. However, a few cases exist, as the binary diffusion couples Ni-Si, Mo-Si, and Fe-Al, for which an intermediate phase of each system grows with an irregular needlelike morphology. To characterize the nonplanar growth of Ni₃Si₂ in bulk samples, the authors studied the behavior of intermetallic compound formation by optical microscopy and x-ray microtomography, for different annealing times. They show that both the average height and the tip radius of curvature grow as the square root of time with two diffusion coefficients separated by orders of magnitude. Moreover, x-ray diffraction indicates that the needles are aligned along the crystallographicc-axis. These results could be consistently explained by an anisotropic diffusion model. This article was presented at the Multicomponent-Multiphase Diffusion Symposium in Honor of Mysore A. Dayananda, which was held during TMS 2006, 135th Annual Meeting and Exhibition, March 12–16, 2006, in San Antonio, TX. The symposium was organized by Yongho Sohn of University of Central Florida, Carelyn E. Campbell of National Institute of Standards and Technology, Richard D. Sisson, Jr., of Worcester Polytechnic Institute, and John E. Morral of Ohio State University.     

Interdiffusion kinetics in the Mo5SiB2 (T2) phase

Diffusion couples based on Mo₂B and Mo₅Si₃ were used to determine the diffusion kinetics of T₂ phase development and the relative diffusivities controlling the kinetics. Annealing the Mo₂B/Mo₅Si₃ diffusion couple above 1600 °C yielded an initial diffusion path sequence of Mo₂B/T₂/Mo₃Si/Mo₅Si₃, which was subsequently transformed to Mo₂B/T₂/(Mo₃Si+Mo₅Si₃)/Mo₅Si₃ by shrinkage of the Mo₃Si phase upon long-term annealing. The T₂ phase developed from Mo₂B and during the growth of the T₂ phase, Si and B atom movements were driven by the Si concentration gradient. The activation energy for Si interdiffusion in the T₂ phase was evaluated to be 355 kJ/mole compared with about 300 kJ/mole for Mo₅Si₃ in both Mo/Si and MoSi₂/T₂ diffusion couples. The larger activation energy is mainly responsible for the 10³ lower diffusivity for Si in the T₂ phase compared with Mo₅Si₃. The relatively slow diffusion in the T₂ phase is consistent with the enhanced creep resistance exhibited by T₂ phase microstructures. This article was presented at the Multicomponent-Multiphase Diffusion Symposium in Honor of Mysore A. Dayananda, which was held during TMS 2006, 135th Annual Meeting and Exhibition, March 12–16, 2006, in San Antonio, TX. The symposium was organized by Yongho Sohn of University of Central Florida, Carelyn E. Campbell of National Institute of Standards and Technology, Richard D. Sisson, Jr., of Worcester Polytechnic Institute, and John E. Morral of Ohio State University.     

Copper diffusion into silicon substrates through TaN and Ta/TaN multilayer barriers

A study of copper (Cu) diffusion into silicon substrates through Ta nitride (TaN) and tantalum (Ta/TaN) layers was investigated based on an experimental approach. TaN _x and Ta/TaN _x thin films were deposited by radiofrequency sputtering under argon (Ar) and Ar-nitrogen (N) plasma. The influence of the N₂ partial pressure on the microstructure and the electrical properties is reported. X-ray diffraction patterns showed that the increase of the N₂ partial pressure, from 2 to 10.7%, induces a change in the composition of the δTaN phase, from TaN to TaN_1.13, as well as an evolution of the dominant crystallographic orientation. This composition change is related to a drastic increase of the electrical resistivity over a N₂ partial pressure of 7.3%. The efficiency of TaN layers and Ta/TaN multilayer diffusion barriers was investigated after annealing at temperatures between 600 and 900 °C in vacuum. Secondary ion mass spectrometry profiles showed that Cu diffuses from the surface layer through the TaN barrier from 600 °C. Cu diffusion mechanisms are modified in the presence of a Ta sublayer. This article was presented at the Multicomponent-Multiphase Diffusion Symposium in Honor of Mysore A. Dayananda, which was held during TMS 2006, the 135th Annual Meeting and Exhibition, March 12–16, 2006, in San Antonio, TX. The symposium was organized by Yongho Sohn of the University of Central Florida, Carelyn E. Campbell of National Institute of Standards and Technology, Richard D. Sisson, Jr., of Worcester Polytechnic Institute, and John E. Morral of Ohio State University.     

First trial of pulse electric current sintering for high-temperature material Ir3Nb

Pulse electric current sintering (PECS) was tried for Ir-25 at.% Nb (Ir₃Nb), which has a high melting temperature of about 2435 °C. Ingot powder was made by crashing an ingot, and the ingot powders were sintered at temperatures between 1700 and 1900 °C up to 2 h under 40 MPa. Pure-metal powder was mixed to achieve a composition of Ir-25 at.% Nb, and the pure-metal powders were sintered at a temperature between 1700 and 1900 °C for up to 1 h. The microstructure and phase structures of sintered samples were investigated by scanning electron microscopy (SEM) and x-ray diffractometry. The sintering mechanism and problems for Ir₃Nb in PECS are discussed.     

Modeling of the dynamics of transient liquid films in ternary systems

The presence of a concentration gradient across a thin liquid film leads to the phenomenon of isothermal liquid film migration (LFM). In most cases, the concentration gradient is due to coherency stresses, and the migration of the liquid film results in the relaxation of these stresses. It is also possible to envisage cases in which LFM occurs as a result of chemical solubility differences between stable and metastable phases. Both situations are examined in this contribution with special emphasis on the effect of adding a second solute element to the liquid phase. The kinetics of LFM is examined as a function of the initial liquid concentration and the ratio of the solute diffusion coefficients in a model coherency-driven ternary system. The growth of the Γ₂ inside δ particles in the Zn-Fe-Al system is presented as a possible example of chemically driven LFM. This article was presented at the Multicomponent-Multiphase Diffusion Symposium in Honor of Mysore A. Dayananda, which was held during TMS 2006, the 135th Annual Meeting and Exhibition, March 12–16, 2006, in San Antonio, TX. The symposium was organized by Yongho Sohn of University of Central Florida, Carelyn E. Campbell of National Institute of Standards and Technology, Richard D. Sisson, Jr., of Worcester Polytechnic Institute, and John E. Morral of Ohio State University.     

Phase-field investigation of multicomponent diffusion in single-phase and two-phase diffusion couples

Interdiffusion in hypothetical ternary single-phase and two-phase diffusion couples are examined using a phase-field model by numerically solving the nonlinear Cahn-Hilliard and Ginzburg-Landau equations. For diffusion couples assembled with a regular single-phase solution, constant chemical mobilities were used to examine the development of concentration profiles including uphill diffusion and zero-flux plane. Zero-flux plane for a component was observed to develop for a diffusion couple at the composition that corresponds to the activity of that component in one of the terminal alloys. Experimental thermodynamic parameters and composition-dependent chemical mobilities were used to examine the morphological evolution of the interphase boundary in solid-to-solid, two-phase diffusion couples. Instability at the interphase boundary was introduced initially (t=0) by a small compositional fluctuation at the diffuse interface, and its evolution varied largely as a function of terminal alloys and related composition-dependent chemical mobility. This article was presented at the Multicomponent-Multiphase Diffusion Symposium in Honor of Mysore A. Dayananda, which was held during TMS 2006, the 135th Annual Meeting and Exhibition, March 12–16, 2006, in San Antonio, TX. The symposium was organized by Yongho Sohn of University of Central Florida, Carelyn E. Campbell of National Institute of Standards and Technology, Richard D. Sisson, Jr., of Worcester Polytechnic Institute, and John E. Morral of Ohio State University.     

Internal carburization and carbide precipitation in Fe-Ni-Cr alloy tubing retired from ethylene pyrolysis service

The events leading to the failure of an alloy grade HP Nb ethylene pyrolysis heater tubing were examined. X-ray maps indicated that a complex oxide coating, which inhibits carbon (C) diffusion, forms on the process side of the tubing during service. Phase equilibria studies predict that even without process C diffusion, metal carbides will precipitate out of the face centered cubic (FCC_Al) matrix. It was estimated that a 6 mm thick tube operating at 1100 °C would completely carburize in two years if the protective coating is damaged. This article was presented at the Multicomponent-Multiphase Diffusion Symposium in Honor of Mysore A. Dayananda, which was held during TMS 2006, 135th Annual Meeting and Exhibition, March 12–16, 2006, in San Antonio, TX. The symposium was organized by Yongho Sohn of University of Central Florida, Carelyn E. Campbell of National Institute of Standards and Technology, Richard D. Sisson, Jr. of Worcester Polytechnic Institute, and John E. Morral of Ohio State University.     

Analysis of interdiffusion data in multicomponent alloys to extract fundamental diffusion information

This paper reviews new procedures based around the random alloy model that have been established recently for analyzing chemical diffusion data in binary and ternary alloy systems. The authors show how atom-vacancy exchange frequency ratios, individual tracer correlation factors, and vacancy-wind factors can be extracted from the chemical diffusion data. Examples are taken from intrinsic diffusion data in the Ag-Cd and Ag-Cd-Zn alloy systems and from interdiffusion data in the Fe-Ni-Cr and Cu-Fe-Ni alloy systems. This article was presented at the Multicomponent-Multiphase Diffusion Symposium in Honor of Mysore A. Dayananda, which was held during TMS 2006, 135th Annual Meeting and Exhibition, March 12–16, 2006, in San Antonio, TX. The symposium was organized by Yongho Sohn of University of Central Florida, Carelyn E. Campbell of National Institute of Standards and Technology, Richard D. Sisson, Jr., of Worcester Polytechnic Institute, and John E. Morral of Ohio State University.     

Diffusional analysis of a multiphase oxide scale formed on a Mo-Mo3Si-Mo5-SiB2 alloy

Diffusional analyses were performed to understand the oxidation at 1300 °C of a multiphase Mo-13.2Si-13.2B (at.%) alloy. During oxidation, a protective glass scale formed with an intermediate layer of (Mo+glass) between the base alloy and external glass scale. Compositional profiles across the (Mo+glass) layer and the external glass scale were determined, and interdiffusion fluxes and effective interdiffusion coefficients for the various components were determined by using “MultiDiFlux” software. The motion of the (alloy/Mo+glass) and (Mo+glass/glass) interphase boundaries after passivation was examined. Additionally, vapor-solid diffusion experiments at 1300 °C were carried out with single-phase Mo₃Si and T2 specimens in addition to a multiphase Mo-10Si-10B (at.%) alloy. These specimens were exposed to vacuum to induce silicon loss resulting in the formation of a Mo layer. An average effective interdiffusion coefficient of Si in Mo at 1300 °C was estimated from the Mo₃Si-vapor couple to be in the order of 8×10⁻¹⁷ m²/s. This article was presented at the Multicomponent-Multiphase Diffusion Symposium in Honor of Mysore A. Dayananda, which was held during TMS 2006, 135th Annual Meeting and Exhibition, March 12–16, 2006, in San Antonio, TX. The symposium was organized by Yongho Sohn of University of Central Florida, Carelyn E. Campbell of National Institute of Standards and Technology, Richard D. Sisson, Jr., of Worcester Polytechnic Institute, and John E. Morral of Ohio State University.     

Water Vapor and Oxygen/Sulfur-Impurity Effects on Oxidation and Nitridation in Single- and Two-Phase Cr–Nb Alloys

The oxidation and nitridation behavior of Cr(Nb) solid solution and Cr2Nb Laves phase was studied individually and in combination at 1100°C and 950°C in dry air, humid air, and N₂–4H₂ for alloys containing high or low levels of oxygen and sulfur impurities. At 1100°C, humid-air exposure of alloys in which the Cr₂Nb phase was the matrix resulted in greatly increased subscale nitridation as compared with exposure in dry air. At 950°C, little difference between dry vs. humid-air exposures was observed; however, greatly increased subscale nitridation was observed in the Cr₂Nb-matrix eutectic alloy when the levels of oxygen and sulfur impurities in the alloys were reduced. In contrast, unalloyed Cr and Cr(Nb) solid-solution alloys were relatively insensitive to water vapor or impurity oxygen/sulfur effects under the conditions studied, although the addition of Nb as a solute in Cr did increase the extent of subscale nitridation. Possible synergistic effects between sulfur impurities and water vapor in the Cr₂Nb alloys and the possibility for similar effects in other intermetallic systems are discussed.     

Ir–Nb–Si ternary refractory superalloys with a three-phase fcc/L12/silicide structure for high-temperature applications

Ir–Nb binary alloys doped with silicon have been used in this work to attain a three-phase fcc/L1₂/silicide structure. Typical Ir–Nb binary alloys, including a hypoeutectic Ir–10Nb, an eutectic Ir–16Nb, and a hypereutectic Ir–25Nb, were used as alloy bases, and Ir was further replaced by 5 at% Si. With the addition of Si, the microstructures of the Ir–(10–25)Nb–5Si ternary alloys contained three phases: fcc, L1₂, and compounds of Ir and Si (referred to silicide hereafter). Compressive tests from room temperature to 1500 °C showed that the Ir–10Nb–5Si alloy, with a predominant fcc microstructure, always had the highest deformation hardening rate, strength, and ductility; on the other hand, the Ir–25Nb–5Si alloy showed the worst performance. With the silicide in the microstructures, the damage sustained by the Ir–Nb–Si alloys at both room and high temperatures was dominated by interface debonding, which occurred between the fcc and the silicide or the L1₂ and the silicide. It is believed that the interface debonding is an instinct failure mechanism of Ir-based alloys. Additionally, a strong solid-solution hardening effect of Si acting on the fcc phase was found to occur without loss of ductility. A principle in the composition and microstructure design is proposed in this paper for further development of Ir-based alloys with Si addition. This principle is to saturate the fcc phase with Si and other alloying elements so as to achieve maximum solid-solution hardening and tie-in fine silicides homogenously distributed within the fcc by elimination of the grain boundary concentration of silicides.     

The fracture resistance and crack-tip micromechanics of in-situ intermetallic composites

The Cr₂Nb intermetallic compound is attractive as a high-temperature material because of its high melting point, low creep rate, and good oxidation resistance. However, this material is also extremely brittle at ambient temperature. Consequently, substantial efforts have been made to develop Cr₂Nb-based in-situ composites containing a ductile phase with the aim of imparting fracture resistance while maintaining most of the high-temperature properties. This article presents an overview of the effects of matrix composition on the fracture resistance and crack-tip micromechanics for Cr₂Nb-containing in-situ composites based on the Nb-Cr-Ti system. Crack-tip strain measurements are presented to illustrate the important effects of titanium addition on the ductility of the solid-solution phase and the fracture process in the in-situ composites.     

Mechanical properties and oxidation characteristics of TiNiAl(Nb) intermetallics

The structural evolution, mechanical properties and the oxidation characteristics of Ti₅₀Ni_50−xAl_x alloys were investigated. The influence of additive Nb on the microstructure, mechanical properties as well as the high temperature oxidation resistance was examined. It is found that the substitution of Al for Ni can remarkably reinforce the TiNi-based intermetallics by precipitation of (Ti,Al)₂Ni phase and by solid-solution strengthening effects. It is also found that the addition of Nb has beneficial influence on the mechanical properties of TiNiAl intermetallics, and that the oxidation resistance of TiNiAl(Nb) alloys can be effectively improved by the Nb addition. The effect of Nb on the high temperature oxidation behavior of TiNiAl(Nb) alloys has been discussed.     

The Phase Competition and Stability of High-Entropy Alloys

Phase competition and stability of several typical high-entropy alloys (HEAs) were studied, and the effects of alloying additions and processing conditions on phase formation in these alloys were discussed. Alloying with chemically incompatible elements having a large difference in either the atomic size or enthalpy of mixing with constituting components in HEAs, e.g., Cu and Al in the FeCoNiCr alloy system, inevitably induced phase separation and stimulated formation of duplex solid-solution phases and even intermetallic compounds. The solid-solution phase in the as-cast FeCoNiCrMn HEA is extremely stable due to the good chemical compatibility among constituent components, but in the FeCoNiCrAl and (FeCoNiCrAl)₉₉Si₁ HEAs with the incompatible elements Al and Si, pretreatment and annealing processes could induce phase transitions and the formation of new phases, indicating that the as-cast solid-solution phases were destabilized by quenched-in chemical segregation, resulting from additions of the dissimilar elements.     

Interdiffusion in γ (face-centered cubic) Ni-Cr-X (X=Al, Si, Ge, or Pd) alloys at 900 °C

Interdiffusion in nickel (Ni)-chromium (Cr) (face-centered cubic γ phase) alloys with small additions of aluminum (Al), silicon (Si), germanium (Ge), or palladium (Pd) was investigated using solid-to-solid diffusion couples. Ni-Cr-X alloys having compositions of Ni-22at.% Cr, Ni-21at.%Cr-6.2at.%Al, Ni-22at.%Cr-4.0at.%Si, Ni-22at.%Cr-1.6at.%Ge, and Ni-22at.%Cr-1.6at.%Pd were manufactured by arc casting. The diffusion couples were assembled in an Invar steel jig, encapsulated in Ar after several hydrogen purges, and annealed at 900 °C in a three-zone tube furnace for 168 h. Experimental concentration profiles were determined from polished cross sections of these couples by using electron probe microanalysis with pure element standards. Interdiffusion fluxes of individual components were calculated directly from the experimental concentration profiles, and the moments of interdiffusion fluxes were examined to determine the average ternary interdiffusion coefficients. The effects of ternary alloying additions on the diffusional behavior of Ni-Cr-X alloys are presented in the light of the diffusional interactions and the formation of a protective Cr₂O₃ scale. This article was presented at the Multicomponent-Multiphase Diffusion Symposium in Honor of Mysore A. Dayananda, which was held during TMS 2006, the 135th Annual Meeting and Exhibition, March 12–16, 2006, in San Antonio, TX. The symposium was organized by Yongho Sohn of University of Central Florida, Carelyn E. Campbell of National Institute of Standards and Technology, Richard D. Sisson, Jr., of Worcester Polytechnic Institute, and John E. Morral of Ohio State University.     

On the redox kinetics and chemical diffusivity of wüstite in CO-CO2 mixtures

Intraphase redox kinetics were monitored across the wüstite, Fe_1–xO, phase of the iron-oxygen system in CO₂-CO and CO₂-CO-Ar mixtures at 1062 and 1106° C. Kinetic data expressed in terms of the chemical diffusion coefficient and the equilibration time as a function of composition strongly indicated the importance of a reaction at the gas-oxide interface in the overall reequilibration of a specimen. Both the chemical diffusion coefficient and the equilibration time were found to be inversely proportional to the deviation from stoichiometry. Surface rate constants calculated from initial linear oxidation data were found to decrease as the deviation from stoichiometry increased.     

Reactive growth of niobium silicides in bulk diffusion couples

The diffusion-controlled growth of niobium silicides (NbSi₂ and Nb₅Si₃) was studied in Nb/Si and Nb/NbSi₂ bulk diffusion couples annealed at 1200–1350 °C for 2–24 h. Both compounds were found to grow as parallel layers, according to the parabolic rate law. The concept of the integrated diffusion coefficient is used to describe the growth kinetics of the two silicides. The corresponding activation energy is 263 kJ/mol for Nb₅Si₃ and 304 kJ/mol for NbSi₂. The activation energy (in eV) scales as 0.98T_m(K)/1000 for Nb₅Si₃ and as 1.4T_m(K)/1000 for NbSi₂ in agreement with the general behavior observed for many transition metal silicides. The position of the Kirkendall plane inside the Nb₅Si₃ layer developed in Nb/NbSi₂ couples indicates that, in the present temperature range, the diffusion of Si in Nb₅Si₃ is considerably faster than that of Nb.     

Oxygen diffusion through Al-doped amorphous SiO2

Oxygen (O) diffusion through pure and aluminum (Al)-doped amorphous silica is investigated by using secondary ion mass spectrometry to profile the diffusion of an¹⁸O tracer. The oxides are formed by the thermal oxidation of polymer-derived SiCN and SiAlCN ceramics. The authors demonstrate that a small amount of Al dopant can significantly inhibit both the interstitial and network diffusion of O. The activation energy of O network diffusion for Al-doped silica is two times higher than that for pure silica. The results are discussed in terms of the modification of Al doping on the network structure of the otherwise pure silica. This article was presented at the Multicomponent-Multiphase Diffusion Symposium in Honor of Mysore A. Dayananda, which was held during TMS 2006, the 135th Annual Meeting and Exhibition, March 12–16, 2006, in San Antonio, TX. The symposium was organized by Yongho Sohn of University of Central Florida, Carelyn E. Campbell of National Institute of Standards and Technology, Richard D. Sisson, Jr., of Worcester Polytechnic Institute, and John E. Morral of Ohio State University.     

Defect structure and chemical diffusion in nickel sulfide β-Ni3S2

The chemical composition, defect structure, and diffusion in nickel sulfide -Ni₃S₂ have been investigated in H₂S-H₂ mixtures containing between 1 and 65% H₂S between 560 and 700°C. Gravimetric, density, and X-ray studies were carried out. In the thermodynamically stable compound the ratio of Ni/S varied between 1.3 and 1.75. The X-ray examination showed a step change in the lattice parameter at the Ni/S ratio 1.4. A linear dependence of the density values (between 5.5 and 6.2 g/cm³) on the composition was observed. On the basis of the chemical composition and density measurements the number of nickel and sulfur atoms in 1 cm³ were determined. It has been shown that the Ni₃S₂ phase is defected in both anion and cation sublattices and that its chemical formula may be described as follows: Ni_3±yS_2x, where y 2x. It has been found that in the mixture containing 10% H₂S the process of defect formation is determined by their diffusion in the sulfide. The temperature dependence of the diffusion coefficient is described by the equation D = 13.15 exp(-30,000/RT) cm²/sec. No dependence of D on the sulfur partial pressure was observed, but this may be due to the relatively large uncertainties in the measurement of the diffusion coefficients.     

Change in the magnetization of multilayer Fe/Si nanostructures during synthesis and subsequent heating

The magnetization of multilayer Fe/Si films made of nanolayers and fabricated by thermal evaporation in an ultrahigh vacuum is studied. The magnetization of (Fe/Si) _n films and its temperature gradient are found to depend on the Fe layer thickness. This dependence is shown to result from the formation of a chemical compound (nonmagnetic phase) at the Fe-Si interface during synthesis. The fraction of this phase accounts for up to 50% of the Fe layer thickness. The irreversible change in the magnetization of these nanostructures is analyzed, and a procedure is proposed for the estimation of the kinetic coefficients of the synthesis of the nonmagnetic phase (silicide) in the multilayer Fe/Si nanostructures at high temperatures. An Fe(1.2 nm)/Si(1.5 nm)/Fe(1.2 nm)/Si(1.5 nm)/Fe(1.2 nm)/Si(10 nm) film is used as an example in order to determine the activation energy E _a and the diffusion coefficient D ₀ of this process with this procedure.