Crystallography of Fe2Al5 phase at the interface between solid Fe and liquid Al

In this study, the microstructures and crystallographic features of a η-Fe₂Al₅ phase formed on pure Fe hot-dipped in a pure Al melt at 750 °C were examined in order to understand the η phase layer formation having a saw-tooth morphology. A number of the columnar η grains (forming the η phase layer) grow towards the solid Fe (α-Fe) side along the [001] direction, resulting in a significant saw-tooth morphology at the interface between the η and α-Fe phases. The neighboring η grains have high-angle boundaries with a common [001] axis. In the η phase layer, the low-angle boundaries develop close to the liquid Al side, and their density becomes higher with longer dipping times, resulting in the development of a fine dislocation substructure in the η phase. In the α-Fe phase, fine substructure consisting of a high density of low-angle boundaries develops around the growth tips of the columnar η grains. These substructure developments are likely responsible for the α → η transformation strain. A possible mechanism for the formation of this η phase layer having a saw-tooth morphology will be discussed in terms of the stress field caused by the α → η transformation.     

Creep of Fe3Al-based alloys with vanadium and carbon additions

Creep experiments were conducted on Fe₃Al-based alloys with vanadium and carbon additions in the temperature range from 923 to 1023 K, corresponding to the occurrence of B2 lattice. The alloys contained (in atomic %) (i) 27.0 Al, 1.17 V, and 0.02 C and (ii) 27.0 Al, 1.13 V, and 0.73 C (Fe balance). The alloys were tested in the as-cast state and annealed at 1273 K for 50 h. Creep tests were performed in uniaxial compression at a constant load with stepwise loading. Stress exponents and activation energies for the creep rate were determined. The values of the stress exponent in low-carbon alloy correspond to a five-power-law creep. The activation energy is greater than the activation enthalpy of diffusion of both Fe and Al in Fe₃Al and is substantially greater than the activation enthalpy of diffusion of V in Fe₃Al. The creep rate is impeded in the high-carbon alloy by the presence of tiny carbide particles. Consequently, the creep resistance of the high-carbon alloy in the as-cast state is greater, especially for higher temperatures and lower stresses. The carbide particles coarsen during annealing at 1273 K and are unable to obstruct dislocation motion because the mean distance between them is too large. The high-carbon alloy is then creep-weaker due to the reduced amount of vanadium present in the matrix.     

Thermal expansion of semiconducting silicides β-FeSi2 and Mg2Si

The thermal expansion of β-FeSi₂ and Mg₂Si was investigated at high temperatures (ranging from 300 to 1173 K for β-FeSi₂ and from 293 to 873 K for Mg₂Si) using powder X-ray diffraction. The linear thermal expansion coefficients α_L for the three lattice parameters of β-FeSi₂ range from 10.6(2) to 11.8(4) × 10⁻⁶ K⁻¹, which indicates small anisotropy, which is in contrast to the large anisotropy reported previously. The volumetric thermal expansion coefficient α_V for β-FeSi₂ is relatively large among the transition-metal disilicides. α_L for Mg₂Si can be expressed by the linear expression of T: α_L = 11(1) × 10⁻⁶ + 6.9(2) × 10⁻⁹T K⁻¹. α_V for Mg₂Si is larger than that of the transition-metal disilicides, including β-FeSi₂. Based on a comparison of α_L among Mg₂Si, several metals and silicides, the candidates for electrode materials are discussed. In particular, temperature dependence and value of α_L for Ni is close to those for Mg₂Si, which suggests that Ni is a good candidate electrode material with respect to thermal expansion.     

In situ XRD studies on Al–Ni and Al–Ni–Sr alloys prepared by rapid solidification

In this paper the structure and stability of Al–17 wt.%Ni(Al–17Ni) and Al–17 wt.%Ni–2 wt.%Sr alloys prepared by rapid solidification was investigated by means of XRD techniques. Our work demonstrates that both alloys are crystalline and composed of fcc (Al–Ni) solid solution and orthorhombic Al₃Ni phases. The ternary alloy shows in addition the presence of small amount of tetragonal Al₄Sr phase. In situ XRD experiment demonstrates the stability of the solute solution up to 650 °C, Al₃Ni above 750 °C while Al₄Sr overcomes melting of the major phases at 800 °C. High-temperature structure analysis proved strong bindings between Al and Ni atoms in Al₃Ni phase, corroborating its covalent nature, linear and faster increase of the fcc volume with annealing temperature. The linear correlation between constituting atoms decreases with increase of the temperature.The work also documents the applicability of pair distribution function (PDF) analysis to the study of multiphase crystalline systems.     

Size dependent phase transformation in atomized TiAl powders

The paper focuses on the phase transformation of undercooled Ti–48Al (at.%) droplets atomized by high pressure gas. The microstructural evolution was analyzed using the transient nucleation theory in combination with the microstructural examinations. The primary phase, final phase volume fraction and the hardness are related to the droplets size to provide fundamental understanding. The competitive formation of the primary phases α and β are strongly controlled by the droplets size, and a critical diameter of about 25 μm was identified corresponding to an undercooling of 102 K. The final phase volume fraction of γ increased with the increase of powder size from 12% to 85%. The highest hardness with the value of 652 HV was obtained for powder of 50 μm in diameter.     

Solidification morphology and phase selection in drop-tube processed Ni–Fe–Si intermetallics

Drop-tube processing was used to rapidly solidify droplets of Ni_64.7Fe₁₀Si_25.3 and Ni_59.7Fe₁₅Si_25.3 alloys. In the larger droplets, and therefore at low cooling rates, only two phases, γ-Ni₃₁Si₁₂ and β₁-Ni₃Si were observed. Conversely, in the smaller droplets, and therefore at higher cooling rates, the metastable phase Ni₂₅Si₉ was also observed. The critical cooling rate for the formation of Ni₂₅Si₉ was estimated as 5 × 10³ K s⁻¹. SEM and TEM analysis reveals three typical microstructures: (I) a regular structure, comprising single-phase γ-Ni₃₁Si₁₂ and a eutectic structure between γ-Ni₃₁Si₁₂ and β₁-Ni₃Si; (II) a refined lamellar structure with a lamellar spacing <50 nm comprising γ-Ni₃₁Si₁₂ and β₁-Ni₃Si; (III) an anomalous structure with a matrix of Ni₂₅Si₉ and only a very small proportion of a second, and as yet unidentified, phase. These results indicate that there is an extended stability field for Ni₂₅Si₉ in the Ni-rich part of the Ni–Fe–Si ternary system in comparison to the Ni–Si binary system. With an increase of cooling rate, an increasing fraction of small droplets experience high undercoolings and, therefore, can be undercooled into the Ni₂₅Si₉ stability field forming droplets consisting of only the anomalous structure (III). The Fe atoms are found to occupy different substitutional sites in different phase, i.e. Fe substitutes for Ni in the γ phase and Si in the L1₂ (β₁) phase respectively.     

Characterization of complex planar faults in FeAl(B) alloys

Complex planar faults were observed by diffraction contrast transmission electron microscopy in a B2 FeAl based alloy containing Ni and B. The comparison of experimental images to simulated ones revealed the detailed structure of these faults that lie on {001} planes with both in-plane and out-of-plane components of the displacement vector. It was deduced that these defects form by segregation of (B-Al vacancies) complexes on a<100> dislocations, leading to various defect structures depending on the screw or edge character of the dislocation.     

Phase transformation of the CuAl2 intermetallic alloy during high-energy ball-milling

In the present investigation, we have studied the phase transformation of the CuAl₂ intermetallic alloy with tetragonal structure by high-energy ball-milling. The structural changes with milling time were followed by X-ray diffraction, differential scanning calorimetry and electron microscopy. It is found that CuAl₂-phase transforms to body centered cubic structure after a long periods of high-energy ball-milling time. According to SEM results, the milling powders change their mechanical behavior from brittle to ductile during the phase transformation. Rietveld analysis showed that around 29% of unit cells in the metastable cubic structure are occupied by Cu suggesting Al segregation in the material and explaining the change in the mechanical behavior of the alloy.     

High carrier concentration in Mg2Si1−xSbx (0 ≤ x ≤ 0.10) prepared by a combination of liquid-solid reaction,ball milling,and spark plasma sintering

Ternary compounds of Mg₂Si_1−xSb_x (0 ≤ x ≤ 0.10) are prepared by a combination of liquid-solid reaction, ball milling, and spark plasma sintering. The carrier concentration of Mg₂Si_1−xSb_x increases with the Sb content x and reaches 1.1 × 10²¹ cm⁻³ at x = 0.10, which is approximately ten times higher than that previously reported. The high carrier concentration is attributed to the facilitation of Sb-doping by ball milling and the suppression of Mg vacancy formation by short-time sintering. No decrease in the carrier concentration of Mg₂Si_0.90Sb_0.10 is observed after annealing at 773 K for 100 h in a semi-closed system, which suggests that the compound is stable at 773 K under a high partial pressure of Mg.     

Obtaining of a fine near-lamellar microstructure in TiAl alloys by Spark Plasma Sintering

This work presents a study of the Spark Plasma Sintering of a boron and tungsten containing alloy (Ti_49,92Al₄₈W₂B_0,08, called IRIS) as a function of the sintering temperature. Microstructures of sintered alloys are analyzed by scanning and transmission electron microscopies. Investigations mainly focus on a fine near-lamellar microstructure. Attention is paid to both characteristic dimensions of this microstructure and orientation relationships between various phases.The fine near-lamellar microstructure is formed by lamellar grains surrounded by extended γ zones containing β₀ precipitates. The size of lamellar grains ranges from 35 to 45 μm while the width of the borders remains between 5 and 10 μm. Effects of both boron addition and sintering temperature are studied. Orientation relationships between γ lamellae and γ grains in the borders, as well as between the γ matrix and the β₀ precipitates are investigated. As a result of these investigations, a formation mechanism of this microstructure is proposed and discussed. The origin of the grain growth limitation during the SPS processing is particularly analyzed.     

Characterization of silicide phases formed during pack siliconizing coating on the Nb-1Zr-0.1C alloy

The present paper describes the morphology, chemistry and crystallography of the phases observed in the silicide coatings produced by pack cementation technique on Nb based alloys. Cross-sectional microstructures examined by transmission electron microscopy and scanning electron microscopy techniques have shown that the coating has two silicide layers: NbSi₂ and Nb₅Si₃. NbSi₂ formed at the surface of the sample and Nb₅Si₃ formed in between the substrate (Nb alloy) and NbSi₂ coating layer. Electron diffraction analyses revealed that NbSi₂ has hexagonal crystal structure with lattice parameters as a = 0.48 nm and c = 0.66 nm and Nb₅Si₃ has tetragonal crystal structure with lattice parameters as a = 0.65 nm and c = 1.19 nm. Nb₅Si₃ showed fine equiaxed grains, whereas, NbSi₂ exhibited duplex morphology having columnar grain morphology near to the Nb₅Si₃ layer and large equiaxed grains at the surface of the coating sample. The presence of duplex morphology was explained by estimating diffusion of various species and it was shown that columnar morphology of grains could be attributed to outward diffusion of Nb and equiaxed grains to inward diffusion of Si. In the case of Nb₅Si₃, growth takes place due to single element Si diffusion, leading to development of single equiaxed grain morphology of the Nb₅Si₃ phase.     

Effect of Fe additions on microstructure and mechanical properties of a multi-component Nb–16Si–22Ti–2Hf–2Al–2Cr alloy at room and high temperatures

The phase constitutions, microstructural evolutions, and mechanical properties of Nb–16Si–22Ti–2Hf–2Al–2Cr–xFe alloys (where x = 1, 2, 4, 6 at.%, hereafter referred to as 1Fe, 2Fe, 4Fe and 6Fe alloys, respectively) prepared by arc-melting were investigated. It was observed that the nominal Fe content affected the solidification path of the multi-component alloy. The as-cast 1Fe alloy primarily consisted of a dendritic-like Nb_SS phase and (α+γ)-Nb₅Si₃ silicide, and the as-cast 2Fe and 4Fe alloys primarily consisted of an Nb_SS phase, (α+γ)-Nb₅Si₃ silicide and (Fe + Ti)-rich region. In addition to the Nb_SS phase, a multi-component Nb₄FeSi silicide was present in the as-cast 6Fe alloy. When heat-treated at 1350 °C for 100 h, the 1Fe and 6Fe alloys almost exhibited the same microstructures as the corresponding as-cast samples; for the 2Fe and 4Fe alloys, the (Fe + Ti)-rich region decomposed, and Nb₄FeSi silicide formed. The fracture toughness of the as-cast and heat-treated Nb–16Si–22Ti–2Hf–2Al–2Cr–xFe samples monolithically decreased with the nominal Fe contents. It is interesting that at room temperature, the strength of the heat-treated samples was improved by the Fe additions, whereas at 1250 °C and above, the strength decreased, suggesting the weakening role of the Nb₄FeSi silicide on the high-temperature strength. As the nominal Fe content increased from 1 at.% to 6 at.%, for example, the 0.2% yield strength increased from 1675 MPa to 1820 MPa at room temperature; also, the strength decreased from 183 MPa to 78 MPa at 1350 °C.     

The structural,elastic, electronic and thermodynamic properties of hexagonal η-Cu6−xNixSn5 (x = 0, 0.5, 1, 1.5 and 2) intermetallic compounds

Based on first-principles calculations, the effects of various Ni concentrations on the structural, elastic, electronic and thermodynamic properties of hexagonal η-Cu₆Sn₅ compound have been systematically investigated. The results demonstrate that higher Ni concentration in the η-Cu_6−xNi_xSn₅ (x = 0, 0.5, 1, 1.5 and 2) leads to thermodynamically stable compounds, and Ni atoms preferentially occupy Cu₂ + Cu_1c sites forming the η-Cu₄Ni₂Sn₅ compound. It is also found that the unit cell volume and lattice parameter of the ‘a’ axis decrease with increasing Ni concentration, which are consistent with the other experimental results. Furthermore, the polycrystalline elastic properties are obtained from single-crystal elastic constants. Our results indicate that the addition of Ni enhances the mechanical stability, brittleness, modulus and Debye temperatures of η-Cu₆Sn₅ compound. Analyzing the electronic structure and charge density distribution provides the explanation that Ni develops distinct bonding energy to Cu and Sn in the structure.     

Structure and magnetic properties of SmxCo72−xFe16Zr7B4Cu1 (x = 8, 12, 15) alloys

Sm_xCo_72−xFe₁₆Zr₇B₄Cu₁ (x = 8, 12, 15) alloys with low level of Sm and mixed addition of several elements (Fe, Zr, B, Cu) were prepared via arc melting and rapid quenching technique. The influences of Sm content and cooling rate on the microstructure and magnetic properties of alloys were investigated. The ribbons melt-spun at 40 m/s present a novel composite microstructure with nanocrystalline phases embedded in amorphous matrix. Such a unique structure endows the x = 12 ribbons a coercivity as high as ∼20,000 Oe. Ferromagnetism of the amorphous matrix, the pinning effect of amorphous phase during the movement of domain-walls and exchange interactions among adjacent grains may contribute to the high coercivity performance.     

Ordered intermetallic alloys: an assessment

The paper summarizes our present understanding, as established at a recent workshop, of two classes of intermetallic alloys: nickel and iron aluminides, which are currently used by industries; and advanced intermetallic alloys including silicides and Laves-phase alloys, which have a great potential to be developed as new high-temperature structural materials for future industrial use. The workshop emphasized close interaction and co-operation between basic research, applied research, and industrial development, and stressed discussion of critical scientific and technological issues. The current status of these intermetallic alloys was assessed, and the directions for future research and development, as well as emerging opportunities, were identified. The information presented in the text is summarized from the presentations at the workshop, and so no references are given to the published literature. However, an extensive bibliography is appended, in which further details may be found.     

High temperature oxidation of Ti–46Al–6Nb–0.5W–0.5Cr–0.3Si–0.1C alloy

A newly developed Ti–46Al–6Nb-0.5W-0.5Cr-0.3Si-0.1C alloy was oxidized isothermally and cyclically in air, and its high-temperature oxidation behavior was investigated. When the alloy was isothermally oxidized at 700 °C for 2000 h, the weight gain was only 0.15 mg/cm². The parabolic rate constant, k_p (mg²/cm⁴·h), measured from isothermal oxidation tests was 0.002 at 900 °C and 0.009 at 1000 °C. Such excellent isothermal oxidation resistance resulted from the formation of the dense, continuous Al₂O₃ layer between the outer TiO₂ layer and the inner (TiO₂-rich, Al₂O₃-deficient) layer. The alloy also displayed good cyclic oxidation resistance at 900 °C. Some noticeable scale spallation began to occur after 68 h at 1000 °C during the cyclic oxidation test.     

Physical properties of the GaPd2 intermetallic catalyst in bulk and nanoparticle morphology

Intermetallic compound GaPd₂ is a highly selective catalyst material for the semi-hydrogenation of acetylene. We have determined anisotropic electronic, thermal and magnetic properties of a GaPd₂ monocrystal along three orthogonal orthorhombic directions of the structure. By using ⁶⁹Ga and ⁷¹Ga NMR spectroscopy, we have determined the electric-field-gradient tensor at the Ga site in the unit cell and the Knight shift, which yields the electronic density of states (DOS) at the Fermi energy ε_F. The DOS at ε_F was determined independently also from the specific heat. To see the change of electronic properties of the GaPd₂ phase on going from the bulk material to the nanoparticles morphology, we have synthesized GaPd₂/SiO₂ supported nanoparticles and determined their electronic DOS at ε_F from the ⁷¹Ga NMR spin-lattice relaxation rate. The electronic DOS of the GaPd₂ was also studied theoretically from first principles. All results are compared to the chemically related compound GaPd. The active–site-isolation concept for an increased catalytic selectivity is discussed in relation to the GaPd₂ and GaPd structures.     

Microstructure evolution and room temperature fracture toughness of as-cast and directionally solidified novel NiAl-Cr(Fe) alloy

The microstructure evolution and room temperature fracture toughness of as-cast and directionally solidified NiAl-Cr(Fe) alloy were investigated using OM, SEM, EDS, DSC and three-point bending tests. From the as-cast microstructure and DSC result, NiAl-34Cr-4Fe (at.%) is a eutectic alloy which consists of eutectic cells in different sizes. The half-baked mesh-like structure is observed at the cell center, and the radial emanating thicker or longer Cr(Fe) phases embedded within NiAl matrix are observed near or at the cell boundary. In the directional solidification process, the solid-liquid interface morphology has an evolutionary process of planar to cellular, even dendritic interface with increasing the withdrawal rates, and the eutectic cell and the microstructure at the cell center refines gradually. From the transverse microstructure, the characteristic of eutectic cell is similar to that of eutectic cell in as-cast alloy. It can be seen from the longitudinal colony/cell center that the broken (short) Cr(Fe) rods are observed at 6 μm/s, and they evolve to granular Cr(Fe) phases when the withdrawal rate increases further. Moreover, regardless of vacuum induction melting (as-cast) and directional solidification, NiAl-34Cr-4Fe (at.%) eutectic alloy possesses a poor fracture toughness due to the inferior brittleness of both NiAl and Cr(Fe) phases. Meanwhile, the crack propagation and fracture surface are observed to better understand the fracture behavior.     

Phase equilibria in the Fe–Al–Nb system: Solidification behaviour,liquidus surface and isothermal sections

The Fe–Al–Nb phase diagram including isothermal sections at 1000, 1150, and 1300 °C as well as the liquidus surface and corresponding reaction scheme was studied experimentally by a combination of scanning electron microscopy (SEM), electron probe microanalysis (EPMA), X-ray diffraction (XRD), and differential thermal analysis (DTA). No genuinely ternary intermetallic phase exists in the system, but the two Fe–Nb phases NbFe₂ (C14-type Laves phase) and Fe₇Nb₆ (μ phase) have extended homogeneity ranges in the ternary system, where large amounts of Fe can be substituted by Al in both cases. The solubility of the third element was studied for all binary phases and the effect on the lattice parameters is discussed. From analysis of the as-cast microstructures and DTA experiments, the liquidus surface including all invariant reactions as well as the occurring solid state reactions were established. Three ternary eutectics, one eutectoid, and two peritectic reactions were found, and the list of invariant points is completed by seven U-type reactions.     

Microstructural evolution and mechanical properties of a β-solidifying γ-TiAl alloy densified by spark plasma sintering

This study deals with the densification of a pre-alloyed Ti–44Al–6Nb–1Mo–0.2Y–0.1B (at.%) powder by spark plasma sintering (SPS). The powder was produced by a plasma rotating electrode process (PREP), and then SPS densified at temperatures between 1200 and 1320 °C. At SPS temperatures below 1240 °C, the α₂-dominated dendritic structure in the PREP powder particles disappeared and the fully dense microstructure mainly consisting of γ and B2 grains formed during SPS, but several original powder particle boundaries (OPBs) still remained. While sintered above 1240 °C, OPBs vanished entirely and an uniform duplex microstructure emerged. Furthermore, fully-lamellar (FL) microstructure with mean colony size smaller than 20 μm was produced via β-homogenization annealing. This FL microstructure renders a good tensile elongation of 1.25% and yield strength of 665 MPa at room temperature. However, instability of α₂/γ lamellar structures was induced by final stabilization annealing, resulting in sharp reduction of both room-temperature ductility and high-temperature strength.