Intermetallic compounds or ordered alloys (intermetallics) have recently attracted much attention as materials for high-temperature applications. Diffusion is fundamental and ubiquitous in the art and science dealing with solid materials at elevated temperatures. A knowledge of the diffusion behaviour of intermetallics is therefore of interest for the production of these materials and for their use in technological applications. Whereas diffusion in many pure metals and dilute alloys is thoroughly investigated and reasonably well understood, systematic diffusion studies for intermetallic compounds are relatively scarce. An atomistic understanding of diffusion in intermetallics in terms of defect structure and diffusion mechanisms is obviously more complex than for metallic elements.
This overview is devoted to binary intermetallics. The reader is reminded of the more frequent structures (B2-, L1₂-, D0₃-, D0₁₉-, L1₀- and cubic Laves-type structures) of intermetallics. Some structural implications for diffusion in these materials including the Cu₃Au rule are considered. Various diffusion phenomena in binary systems like self-diffusion, diffusion of foreign atoms, single-phase and multiphase interdiffusion are described and illustrated by experimental examples from our laboratory. Some limitations of the Darken-Manning equation, which is sometimes used to correlate tracer diffusion of components and interdiffusion, are discussed.
Self-diffusion of components—the most basic diffusion process in solids—is reviewed for intermetallics with the above mentioned structures. Relevant factors for self-diffusion like the crystal structure, the state of order and disorder, the temperature and composition dependence are illustrated. In a few cases diffusion of selected foreign elements will also be considered. Particular attention will be devoted to our present understanding of diffusion in terms of defect structure and atomic mechanisms. This review also includes a summary of recent investigations on diffusion in the D0₃-type intermetallic compound Fe₃Si, Ti self-diffusion in intermetallic compounds of the Ti–Al system and single-phase and multiphase interdiffusion in various intermetallics including the cubic Laves-phase Co₂Nb.

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Gibbs free energies of tetragonal and monoclinic phases at various temperatures are calculated for 8, 10 and 12 mol% CeO₂–ZrO₂ by referring to the revised CeO₂–ZrO₂ phase diagram given by Tani et al. and utilizing the Lukas program. The equilibrium temperatures T₀, between t and m phases are obtained as 999, 838 and 697 K for 8, 10 and 12 mol% CeO₂–ZrO₂, respectively, being much lower than the T₀ of ZrO₂.

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The M_s temperature in 8 mol% CeO₂–ZrO₂ with a mean grain size of 1.38 μm is calculated by using the calculated results of the Gibbs energies of both tetragonal and monoclinic phases in our previous paper. The required parameters for M_s calculation are obtained by experimental measurements or through estimation from some available data. The calculated M_s temperature is in good agreement with the experimental one and the difference of them is less than 5 degrees, showing that the approach given in this article is suitable for the prediction of M_s in zirconia ceramics.

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Nanogranular amorphous+fcc-Al phases were found to be formed in rapidly solidified Al₉₄V₄M₂ (M=Fe or Co) and Al₉₃V₅Ni₂ alloys. The grain size of the amorphous phase is 5 to 30 nm and of the fcc-Al phase 7 to 30 nm. Besides, the mixed structure consisting of nanoscale icosahedral (I) particles surrounded by an fcc-Al phase was formed in the rapidly solidified Al–V–M, Al–Cr–M and Al–Mn–M (M=Fe, Co or Ni) alloys examined in the present study. The particle size of the I-phase is about 15 to 70 nm and the interparticle spacing occupied by the fcc-Al phase is about 10 to 30 nm. The formation tendency of the nonequilibrium amorphous and I phases is greater for the V-containing alloy, followed by the Cr- and then the Mn-containing alloy. All these nonequilibrium phase alloys exhibit good bending ductility and the tensile fracture strength (σ_f) reaches about 1400 MPa for the amorphous+Al phases and about 1250 MPa for the I+Al phases. The σ_f values increase in the order of V>Cr>Mn and Fe>Co>Ni when the alloys with the same solute concentrations are compared. The high strength is attributed to the formation of the nanogranular amorphous+fcc-Al and I+fcc-Al phases. The high formation tendency of the amorphous phase for the V-containing alloy is presumably because the V element causes the formation of the stoichiometric I-phase with the lowest solute concentration and has the lowest diffusivity in the Al phase and more multiple peritectic reactions. The formation of the amorphous phase and the achievement of the high σ_f values for the Al-rich alloys without the lanthanide element are important as the basic information to devlop a new high specific strength material.

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High rate deposition of Al (99.999% in purity) from dual sources of W-baskets was carried out onto NaCl crystals which were cleaved in air and heated at temperatures of 523 to 643 K in an ordinary high vacuum (2×10⁻³ Pa). From a transmission electron microscopic observation, it was found that when deposited in thickness of 80 to 200 nm at a substrate temperature of 623 K the films grew continuously with island structure in (100) orientation. Moreover, (100) flat single-crystal films thicker than 200 nm often developed without containing islands and they usually included numerous dislocations. Most dislocations were come out from the films during heating to a high temperature above 873 K in the electron microscope.

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The microscopic process of the martensite transformation in a small model crystal with the Lennard-Jones potential is studied by introducing two modifications into the standard molecular dynamical simulation. One is the removal of the kinetic energy at proper intervals corresponding to the latent heat during the martensitic transformation. The other is to follow visually the evolution of numerical results. The visual observation of the nucleation of the martensite in the atomistic scale allows us to follow the process of the gradual evolution of the sharp boundaries from the diffused displacements created around the atom, which is given a finite displacement.

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It has been observed that the sulphides undergo weathering process to produce the metal sulphates at a very slow rate. Therefore, higher conversion to sulphates may be achieved by creating aggressive conditions at higher temperature similar to that of weathering process by using a mixture of steam and oxygen. The results obtained by this method shows a conversion of 82.32% copper sulphate from a chalcopyrite concentrate at 773 K in presence of 10% Fe₂O₃. The role of different additives like ferric oxide, ferrous sulphate and sodium sulphate on sulphation of chalcopyrite with steam and oxygen was investigated. The copper sulphate formation without any additive was found to be 71.9% whereas the values were 84.4 and 86.5% with 10% Na₂SO₄ and 20% FeSO₄ respectively at 773 K. The kinetics of the sulphation with additives has also been studied in the fixed bed in the temperature range 673–773 K in absence of external mass transfer effect due to particle size, flow rate, etc. The reaction was found to be topochemical in nature. The activation energy values using the Arrhenius equation plots were obtained as 29.2 kJ/mol with 10% Fe₂O₃, 23.57 kJ/mol with 10% Na₂SO₄ and 21.03 kJ/mol with 20% FeSO₄ in the above temperature range. The sulphated products were characterized by XRD and SEM studies.

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An investigation was made on the refinement of microstructures and improvement of mechanical properties in intermetallic TiAl binary alloys containing 44, 49 and 54 at% aluminum produced by the rheocasting in which the solidifying alloy was vigorously agitated at high rotation speeds of 15 to 70 rev/s of a stirrer immersed in the alloys in argon gas atmosphere. In the microstructure of the rheocast Ti-44at%Al alloy such a lamellar structure as seen in the alloy solidified without stirring was destroyed completely, and an extremely refined microstructure composed of dispersed agglomeration aggregates was formed. The mean sizes of agglomeration aggregates and individual crystal grains of the Ti-44%Al alloy rheocast at a rotation speed of 70 s⁻¹ were 6.4±3.7 μm and 2.2±1.3 μm in major axis and 2.0±1.1 μm and 1.0±0.6 μm in minor axis, respectively. However, oval lamellar grains were seen in the microstructure of the rheocast Ti-49at%Al alloy and lamellar substructure was observed in the oval primary crystal grains in the microstructure of the rheocast Ti-54at%Al alloy. In the microanalysis of the Ti-44%Al alloy rheocast at 70 s⁻¹ the uniform distribution of Al and Ti elements could be achieved by destruction of the lamellar structure with the mechanical stirring, although lamellar segregation was observed in the alloy solidified without stirring. The room temperature elongations of the Ti-44%Al alloy rheocast at 15 and 70 s⁻¹ were 2.7% and 3.3%, respectively, being higher than the elongation of the alloy solidified without stirring, i.e. 2.0%. The room temperature tensile strengths of Ti-44%Al alloy rheocast at 15 and 70 s⁻¹ were 359 and 468 MPa, respectively. They were also higher than the tensile strength of the alloy solidified without stirring, i.e. 245 MPa. The elevated temperature elongation and tensile strength of the rheocast Ti-44%Al alloys were improved as compared with those of the alloy solidified without stirring, although a fluctuation existed. The tensile strengths of the alloys rheocast at 70 s⁻¹ and solidified without stirring were 538 and 363 MPa at 1173 K, and 439 and 389 MPa at 1273 K, respectively.

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The effect of Si precipitation on the bonding damage at the Al-1 mass%Si film/transistor substrate interface has been investigated. Wedge shaped Si precipitates (1–3 μm high) grew on the PSG/Si substrate during the annealing process before soldering and wire bonding. The deformed layer in the Al-1 mass%Si film, after bonding, was almost to the top of the wedge shaped Si precipitates and hence this may cause damage in the PSG/Si system. The morphologies of the Si precipitates were greatly influenced by the substrate structure; there was lateral shaped Si precipitation with less than 0.5 μm height when substrate was Si. This may be due to solid phase epitaxial growth of Si precipitates on Si. It was also found that bonding damage decreased substantially by replacing the PSG/Si substrate with Si substrate.

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Enhancing reliability of thick Al wire bonds in high power transistor modules is very important. In order to estimate bond reliability, microstructural analyses of the bonding interface between Al wires and Al–Si electrode films have been investigated. The bonding interface between Al wire and Al–Si electrode film were composed of three kinds of microstructures, i.e. coincidence boundaries, layers of very fine grain size, and layers with very high dislocation density. These boundaries were assumed to be very strong and hence very stable in the reliability tests. After power cycle tests, the fine grain layers and the layers with a very high dislocation density disappeared almost completely and form a good quality bonding interface. Very thin amorphous layers 10–200 nm thick were found to be produced partly at the interface. This may be formed through a process in which amorphous phases in the very fine grain size layer were pushed out to the interface by grain growth during the power cycle tests.

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A ferromagnetic Co-based amorphous phase with a wide supercooled liquid region before crystallization was found to be formed in rapidly solidified Co₇₀M₃Al₅Ga₂P₁₅B₄C₁ (M=Cr or V) and Co₆₇Cr₃Fe₃Al₅Ga₂P₁₅B₄C₁ alloys. The glass transition temperature (T_g) and crystallization temperature (T_x) are in the ranges of 717 to 722 K and 766 to 768 K, respectively, and the temperature interval of the supercooled liquid region defined by the difference between T_g and T_x is 45 to 51 K. This is believed to be the first evidence on the appearance of Co-based amorphous alloys with the distinct glass transition and a wide supercooled liquid region. The crystallization takes place through a single stage resulting from the precipitation of four kinds of phases Co, Co₂P, Co₄B and Co₃C. The crystallization process implies the necessity of a long-range rearrangement of constituent elements in the supercooled liquid. The necessity seems to be closely related to the increase in the thermal stability of the supercooled liquid through the retardation of the precipitation of the four crystalline phases. These Co-based amorphous alloys exhibit soft ferromagnetism with Curie temperatures ranging from 365 to 420 K. The saturation magnetization, coercivity and permeability at 1 kHz are in the range of 0.27 to 0.34 T, 4.5 to 8.0 A/m and 4600 to 5800, respectively. From the combination of the soft magnetic properties and a wide supercooled liquid region, we can expect future development of Co-based amorphous alloys as a new soft magnetic material with large glass-forming ability and good viscous deformability.

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The deformation behavior in the supercooled liquid region for the amorphous Zr₅₅Al₁₀Ni₅Cu₃₀ alloy was examined as a function of temperature, applied stress and deformation rate. The deformation occurs through the Newtonian viscous flow with the strain rate sensitivity (m value) of approximately 1.0. By utilizing the ideal superplastic flow, the extrusion with an extrusion ratio of 3.6 for the cast amorphous Zr–Al–Ni–Cu cylinder was easily made in the condition where the extrusion temperature was in the range of 703 to 743 K and the extrusion velocity was 0.67 to 22 μm/s. The extruded alloy also keeps an amorphous phase and no appreciable difference in the mechanical properties is seen between the cast and extruded amorphous alloys. However, a distinct endothermic reaction is observed around the extrusion temperature on the differential scanning calorimetric curve of the extruded alloy. The endothermic reaction is due to the structural change from the relaxed disordered atomic configurations developed by extrusion to internally equilibrium disordered atomic configurations. The evidence on the modification in the disordered structure during the severe deformation of the supercooled liquid is important for future development of basic science and engineering utilization of the supercooled liquid.

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