Showing papers in "Zeitschrift Fur Metallkunde in 2004"

The Distribution of Internal Interfaces in Polycrystals

Abstract: Recent advances both in experimental instrumentation and computing power have made it possible to interrogate the distribution of internal interfaces in polycrystals and the three dimensional structure of the grain boundary network with an unprecedented level of detail. The purpose of this paper is to review techniques that can be used to study the mesoscopic crystallographic structure of grain boundary networks and to summarize current findings. Recent studies have shown that grain surfaces within dense polycrystals favor the same low energy planes that are found on equilibrium crystal shapes and growth forms of crystals in contact with another phase. In the materials for which comprehensive data exists, the distribution of grain boundaries is inversely correlated to the sum of the energies of the surfaces of the grains on either side of the boundary.

Assessment of thermodynamic parameters in the system ZrO2–Y2O3–Al2O3

Abstract: The thermodynamic parameters have been assessed for the systems of ZrO2–Y2O3, ZrO2 –Al2O3 and ZrO2–Y2O3–Al2O3 using the Calphad technique. The solid solutions are described by the compound energy formalism. A two-sublattice ionic liquid model is used to describe the liquid phase. The phase diagrams of the ZrO2–Y2O3 and ZrO2–Al2O3 and Y2O3–Al2O3 systems are in fairly good agreement with phase equilibrium data. Isothermal sections of the phase diagram of the ZrO2–Y2O3–Al2O3 system at subsolidus conditions are calculated. Introducing ternary interactions in the liquid phase results in a better fit to experimental data of the liquidus surface and isoplethal sections.

Mechanical model for the deformation of the wood cell wall

Abstract: The molecular mechanisms responsible for the deformation of wood, as well as the mechanical interaction of cell-wall components, such as cellulose, lignin and hemicelluloses, are not well understood. In a recently published experiment [1], we have shown that wood foils and single cells of compression wood of spruce (Picea abies [L.] Karst.) could deform permanently under tensile load via a stick and slip mechanism at the molecular level occurring during shear of the matrix between cellulose microfibrils. The shear originates from the fact that microfibrils are spiralling around the central lumen of the wood cell which can be considered as a hollow tube. During stretching, the microfibril angle between the cell axis and the direction of the cellulose was found to decrease in synchrotron X-ray diffraction experiments, giving rise to shear deformation of the matrix and to a recovery mechanism after irreversible deformation. The corresponding stick and slip mechanism is treated here in the framework ...

Intermetallic growth and Kirkendall effect manifestations in Cu/Sn and Au/Sn diffusion couples

Abstract: The Kirkendall-effect induced migration of inert markers during a diffusion-controlled growth of intermetallic compounds in the Cu/Sn and Au/Sn couples at 215 and 180°C were studied. It was shown that the behaviour of markers in the multiphase reaction zones can be rationalized in terms of the Kirkendall velocity construction. Observations on the microstructural features of the product intermetallic layers and the role of the Kirkendall effect in the morphogenesis of the interdiffusion systems are discussed. It was demonstrated that the velocity of markers in a product layer, the appearance of the Kirkendall plane(s), their location(s) and the morphological evolution of the reaction products can also be explained from a purely chemical point of view considering the diffusion-controlled interactions at the interphase interfaces. A representation of the reaction scheme and kinetics of intermetallic growth in each of the two systems is given.

Phases and phase equilibria in the Fe–Al–Zr system

Abstract: Isothermal sections at 800, 1000, and 1150 °C as well as a tentative partial liquidus surface of the ternary Fe–Al–Zr system were established by means of electron-probe microanalysis, X-ray diffraction, differential thermal analysis, and light-optical as well as scanning electron microscopy. The most prominent features of the ternary phase diagram are the extended homogeneity ranges of the Laves phases. By continuous substitution of Fe by Al, the structure changes three times starting from the cubic C15 structure of Fe2Zr to hexagonal C14 (λ1) back to cubic C15 (λ2) and again to hexagonal C14 (Al2Zr). The various Laves phase fields are separated by very small two-phase fields. Besides the Laves phases λ1 and λ2, three more ternary intermetallic phases were found, whose homogeneity ranges have been determined for the first time. In addition, new results concerning the homogeneity ranges of intermetallic phases in the binary subsystems Fe–Al and Al–Zr are reported. The solubilities of the third com...

Density and excess volume of liquid copper, nickel, iron, and their binary alloys

Abstract: The densities of liquid copper, nickel and iron and their binary alloys have been measured over a wide temperature range, including the undercooled regime. A non-contact technique was used, consisting of electromagnetic levitation and optical dilatometry. In all cases the density was a linear function of temperature. On the other hand, the concentration dependence was only linear for Fe–Ni, whereas Fe–Cu and Cu–Ni are characterized by a positive or negative excess volume respectively.

Grain boundary mobility – a brief review

Abstract: This brief review describes the major features of grain boundary mobility in metals and current limitations in our theoretical understanding. Although thermally activated exchange of atoms across boundaries provides a basic picture of grain boundary correct to within an order of magnitude, no refinement of this theory is available that accounts for the experimental characteristics of grain mobility as it depends on crystallographic type. An example is given of a simple extension involving density of kinks on the two surfaces that comprise a boundary to show that the resulting estimate of mobility is essentially different from the experimentally observed dependence.

Microstructural and thermodynamic study of γ-Ga2O3

Abstract: The metastable nanocrystalline y form of gallium oxide has been prepared and its microstructure and thermochemistry have been studied for the first time by employing X-ray and electron diffraction, high-resolution transmission electron microscopy, adiabatic and differential scanning calorimetry. The randomly oriented crystallites of maximum 5 nm in size have been observed. The sponge-like morphology of γ-Ga 2 O 3 particles may explain the high specific surface area, previously reported for this material. The defect spinel-type structure of γ-Ga 2 O 3 is similar to that of y and η-Al 2 O 3 . Up to 5.7 wt.% of water can be stored in γ-Ga 2 O 3 and subsequently released at elevated temperatures. Dry γ-Ga 2 O 3 specifically absorbs atmospheric water at room temperature. The transformation of γ-Ga 2 O 3 into stable β-Ga 2 O 3 occurs in two steps. In the range 650-800 K, γ'-Ga 2 O 3 is formed in the course of a reversible higher-order phase transition. The latter irreversibly transforms into β-form above 873 K. The enthalpy of this exothermic transformation is determined as -19.3 ± 0.4 kJ . mol -1 . The coefficients of the Gibbs energy equation for y and γ'-Ga 2 O 3 have been assessed.

Chemical reaction and stability of Ti3SiC2 in Cu during high-temperature processing of Cu/Ti3SiC2 composites

Abstract: Chemical reactions and stability of Ti3SiC2 in Cu during processing of Cu/Ti3SiC2 composites in the temperature range of 900-1070 degreesC were investigated using X-ray diffraction and scanning electron microscopy. The results indicated that Cu reacted with Ti3SiC2 above 900 degreesC, and the reaction products were closely related to the reaction temperature and the relative ratio of Cu and Ti3SiC2. At low Ti3SiC2 content or temperatures below 1000 degreesC, Cu(Si) solid solution and TiCx were formed, whereas at high reaction temperature and high Ti3SiC2 content, Cu-Si intermetallic compounds like Cu5Si, Cu15Si4 and (Cu, Si) eta' as well as TiCx were observed. The transporting process for the reaction was investigated and described. It was found that de-intercalation of Si from Ti3SiC2 dominated the reaction, which dissolved in copper to form Cu(Si) solid solution or Cu-Si intermetallic compounds such as Cu5Si, Cu15Si4 and (Cu, Si) eta'. TiCx was always present as the decomposition product of Ti3SiC2.

Grain boundary self-diffusion in α-iron of different purity: effect of dislocation enhanced diffusion

Abstract: Grain boundary (GB) self-diffusion in iron of 99.9938 and 99.97 wt.% purity was studied in B-type kinetics in the temperature range of 772 to 1050 K. For identical temperatures, the measured values of the product δ × Dgb are systematically lower for the less pure material than for the more pure one (here δ and Dgb are the GB diffusion width and GB diffusion coefficient, respectively). The GB diffusivities exhibit distinct non-linear Arrhenius temperature dependencies for both materials. This curvature is apparent only and is attributed to the change of tracer leakage from the GBs due to enhanced diffusion along the dislocation pipes. A model is suggested to describe GB diffusion in such conditions. According to the model, the pertinent Arrhenius parameters of δ × Dgb for the grain boundaries are 8.45 × 10–13 m3/s and 166kJ/mol for the purer material and 3.42 × 10–11 m3/s and 198 kJ/mol for the less pure material. The present results confirm the recently observed significant effect of dislocation ...

Thermodynamic modeling of the Ni-S system

Abstract: A thorough review and critical evaluation of phase equilibria and thermodynamic data of the nickel–sulfur binary system at 1 bar pressure has been made over the entire composition range for temperatures from 25°C to above the liquidus. The Gibbs energies of nine phases have been modeled, and optimized model parameters have been obtained which reproduce all data simultaneously within experimental error limits. For the liquid phase, the recently extended modified quasichemical model is applied. Two-sublattice models within the framework of the Compound Energy Formalism are used for the high-temperature heazlewoodite and monosulfide pyrrhotite solutions. The Gibbs energies of six stoichiometric compounds are also modeled.

Measurement of the surface tension of liquid Ga, Bi, Sn, In and Pb by the constrained drop method

Abstract: The effect of the droplet size on the accuracy of surface tension measurement by the sessile drop method is discussed for liquid metals through a simulation by using the Laplace equation. It is found that with increasing size of the droplet, a higher accuracy of the measured value of the surface tension can be obtained. In order to make a large droplet of liquid metals, the constrained drop method with a special crucible shape was applied to measure the surface tension of liquid Ga, Sn, Bi, In, and Pb. The uncertainty of the measured surface tension was within 1%. The temperature dependences of the surface tension of liquid Ga, Sn, Bi, In, and Pb were obtained in the present experiment as follows: Ga: σGa = 737 −0.062T mN/m (823 ≤ T≤ 993K)Sn: σSn = 579 −0.062T mN/m (723≤ T≤ 993K)Bi: σBi = 417 −0.070T mN/m (773 ≤ T≤ 873K)In: σIn = 600−0.082T mN/m (673 ≤ T≤ 993K)Pb: σPb = 499 −0.089T mN/m (757 ≤ T≤ 907K)

Challenges and interesting observations associated with feedback-controlled nanoindentation

Abstract: Accurate feedback control of the nanoindentation process is particularly challenging from the perspectives of many orders of magnitude change in contact stiffness and the use of the ramp/hold/ramp protocol. The conventional proportional-integral-derivative feedback control algorithm is not well suited for such an application. Here we provide a description and present performance data for a newly developed digital control algorithm that augments the familiar proportional-integral-derivative routine with an adaptive feedforward control having inputs related to open-loop device physics and encountered contact mechanics. This novel approach results in reproducing even a steep demand ramp with minimal feedback error. Additionally, we discuss interesting observations made with respect to the varied displacement-controlled nanoindentation responses of materials. Metals, in particular, are found to expose especially rich nanomechanical phenomena when the force-displacement curves are measured under displacement control. The findings of this study nicely illustrate the decided scientific advantages of displacement control over load control.

Characterization of the fcc/bcc orientation relationship by EBSD using pole figures and variants

Abstract: The orientation relationship (OR) between fcc and bcc lattices are described by crystallographic fundamentals using the example of Kurdjumov–Sachs (K–S) and Nishiyama–Wassermann (N–W). Complete pole figures containing all variants will be used to distinguish even between slightly different ORs. EBSD on iron meteorites and duplex steel has been used to analyse a large number of crystal orientations with regard to a high statistical significance and a high probability to capture all variants in a single measurement. It is shown that the use of fixed OR models like K–S, N–W, Bain, Pitsch, or Greninger–Troiano does not satisfacturally reflect the observed experimental pole distributions. It is not convenient to use high-indexed lattice planes and directions to describe the small deviations from the given models. The Euler subspace representation offers a readily comprehensible tool to get an idea about the characteristic of the experimentally detected OR.

Deformation behaviour of strontium titanate between room temperature and 1800 K under ambient pressure

Abstract: SrTiO3 was plastically deformed in compression along different crystal axes between room temperature and 1800 K. Data obtained from deformation experiments as well as from light optical and electron optical studies are presented. Irrespective of the compression axis orientation, the material exhibits a strongly pronounced ductile-to-brittle-to-ductile behaviour. For the compression axis the slip system {110} is active both at low and high temperatures. For compression axes other than the {110} system is active only at low temperatures whereas at high temperatures the {110} system dominates. This behaviour is discussed in terms of dislocation core structures.

Microstructural evolution and its effect on the mechanical properties of Cu-Ag microcomposites

Abstract: The microstructure and the mechanical properties of Cu–Ag alloys with 7 and 24 wt.% Ag are investigated. The microstructure of the alloys is mostly determined by the silver content. That of Cu-24 wt.% Ag alloys consists of a Cu-rich solid solution and the eutectic. Otherwise, the microstructure of Cu-7 wt.% Ag alloys consists of primarily solidified dendrites of a Cu-rich solid solution and small Ag-rich particles. The composition strongly influences the work-hardening rate. In order to achieve an ultimate tensile strength of 1 GPa, a logarithmic cold-deformation strain, η, of about 3.7 is required (η = 1n A0/A) for the 7 wt.% Ag alloy, whereas for Cu-24 wt.% Ag alloys η = 3.1 is sufficient. In as-cast alloys with 7 wt.% Ag a strong segregation is observed, which, consequently, leads to a strong decrease of the age-hardening effect. Therefore, the Cu-7 wt.% Ag alloy has to be homogenised before aging. The application of Cu–Ag alloys with a Ag-content below 8 wt.%, i. e. an the maximum solubility ...

Thermodynamic reassessment of the Al – V system

Abstract: An optimized set of thermodynamic functions to describe the Al – V system has been obtained by a computer-operated least squares method applied to the experimental phase diagram and thermodynamic data available from the literature. Special attention is paid to the modeling of the intermediate phase Al8V5. Based on the crystal structure data, Al8V5 is described by a sublattice model Al6/13(Al, V)2/13 (Al, V)3/13 V2/13 in a final treatment. The boldface Al and V mean the normal atoms (i. e., major species) in the sublattices. In order to provide reasonable starting values for the final modeling, three treatments are performed. In the first treatment, Al8V5 is assumed to be a stoichiometric compound; in the second and third ones, it is described by sublattice models Al6/13(Al, V)2/13 V5/13 and Al8/13(Al, V)3/13 V2/13, respectively. This step-by-step modeling procedure provides reliable estimates and useful starting values for the parameters at each of the higher levels. The other phases (liquid, fcc...

Sigma-phase in duplex-stainless steels

Abstract: The ferritic-austenitic duplex-steels have a very complex precipitation and transformation behaviour, which requires professional treatment. Especially the precipitation of the {sigma}-phase causes considerable changes with regard to the mechanical as well as the corrosive properties, which are to be considered during the treatment of the duplex-steels. (orig.)

Grain boundary characterization and grain size measurement in an ultrafine grained steel

Abstract: Ultrafine ferrite grains in a plain C–Mn steel (0.3 mass% C) were produced by large-strain warm compression and subsequent annealing treatment in a temperature range between 773 K and 1003 K. The samples were investigated by means of high-resolution electron back-scatter diffraction. The resulting microstructures showed very fine ferrite grains and homogeneously distributed cementite particles. The majority of the grain boundaries (55–70 %) were classified as high-angle ones (≥ 15° misorientation). When considering only these high-angle grain boundaries, the average grain size changed from 0.9 mm at a deformation temperature of 773 K to 2.2 mm at a deformation temperature of 1003 K. For the same range the average subgrain sizes increased from 0.6 mm to 1.5 mm. The basic result of this study is that the grain size characterization of polycrystalline microstructures with ultrafine grains requires the use of the high-resolution electron back-scatter diffraction method in conjunction with a careful a...

Some control mechanisms of spatial solidification in light alloys

Abstract: Two unconventional approaches of controlling the size and distribution of primary particles in aluminium and magnesium alloys are suggested. The first approach uses the ultrasonic cavitation treatment for multiplication of solidification sites for primary crystals by activating catalytic impurities that are present in the melt. The second way, on the contrary, involves elimination of active solidification nuclei by melt overheating with subsequent nucleation of primary particles at a higher undercooling. Both approaches result in refinement of primary crystals.

Lead-free solder materials: experimental enthalpies of mixing in the Ag–Cu–Sn and Cu–Ni–Sn ternary systems

Abstract: The partial and integral enthalpies of mixing of molten, ternary Ag–Cu–Sn alloys were determined at 500, 700 and 900 °C using a Calvet-type microcalorimeter. Five sections in a compositional range from pure Sn to about 40 at.% Sn were investigated. The data were fitted using a Redlich–Kister–Muggiano polynomial, where the binary interaction parameters were taken from the literature. Additionally, the temperature dependence of the ternary interaction parameters was described analytically by a linear function. In the ternary Cu–Ni–Sn system, the enthalpies of mixing were determined at 1250 °C in five sections, starting from pure Sn to about 40 at.% Sn. Again, the ternary interaction parameters were fitted using the substitutional Redlich–Kister–Muggiano model. For both ternary systems and all mentioned temperatures, isoenthalpy curves were constructed for the integral molar enthalpy of mixing.

Calorimetric study of Zn13La

Abstract: The thermodynamic properties of Zn13La were investigated by calorimetry. The standard entropy of formation at 298 K, ΔfS°298, was determined from measuring the heat capacities, Cp, from near absolute zero (2 K) to 300 K by the relaxation method. The standard enthalpy of formation at 298 K, ΔfH°298, was determined by solution calorimetry in hydrochloric acid solution. The standard Gibbs energy of formation at 298 K, ΔfG°298, was determined from these data. The coefficient, γ, of the electronic term contributing to the heat capacity of Zn13La was similar to the composition average of the values of pure Zn and La, indicating that the 5d- and 4f-electron states for La are localized in the vicinity of the Fermi energy level.

Physical metallurgy of high Nb-containing TiAl alloys

Abstract: Intermetallic titanium aluminides exhibit attractive thermo-physical properties, which give them the potential for extensive use as lightweight structural components. Novel design concepts are based on alloys with the general composition (in at.%) Ti-45Al-(5–10) Nb, which were subjected to precipitation hardening. Optimized compositions have been identified that are capable of carrying stresses in excess of 700 MPa at service temperatures of 700°C and have superior creep properties. The alloys exhibit at room temperature yield stresses in excess of 1GPa combined with plastic tensile elongations of about 2%. Wrought alloys of this type can be an attractive alternative to the nickel-base superalloys in certain ranges of stress and temperature. The future and promise of these new TiAl alloys lies in innovative processing methods designed to achieve better performance.

Model of growth kinetics of nitrided layer in the binary Fe–N system

Abstract: The mathematical description of the kinetics of a layer development is subjectal to a continuous evolution connected with a growing amount of empirical data. Commencing with attempts to describe selected stages of this process, attempts are made at present to mathematically represent phenomena occurring in a gas atmosphere, on the boundary of gas-metal, and in a nitrided material, together with a description of mutual dependencies between these phenomena. The model proposed in this paper, which describes the growth of the zones of a layer nitrided in conditions of a diffusion quasi-equilibrium, is a simple and convenient tool allowing for both the determination of nitrogen diffusion coefficients in the layer's monophase zones and for forecasting the growth of the layer's zones as a function of the time of the process and the nitrogen potential.

Phase equilibria in iron-rich Fe - Al - V ternary alloy system

Abstract: Phase separation and phase diagram in the Fe-rich corner of the Fe–Al–V alloy system were investigated by means of transmission electron microscopy, scanning electron microscopy and energy-dispersive X-ray spectroscopy. Phase separation into disordered A2 and ordered L21 phases was found in a wide composition range in the Fe-rich corner at 923 K, 973 K and 1023 K, and their tie-lines were almost parallel to the direction connecting the Fe corner with the center of the Gibbs triangle. In addition to the A2 + L21 phase separation, phase separation into B2 and L21 phases was also observed at 1023 K at the side of Fe–Al binary system.

Experimental investigation of the equilibrium composition of titanium carbonitride and analysis using thermodynamic modelling

Abstract: A new experimental investigation of the composition of titanium carbonitride (TiCxNy(cF8)) is presented. The C and N contents in equilibrium with graphite and N2 gas of pressures of 1 bar and 100 bar have been determined at 1430°C and 1850°C. Special effort has been made to verify that equilibrium is reached. Using the new experiments a revised thermodynamic description of the Ti–C–N system is presented, including also an assessment of the molar volumes in this system. Calculated phase equilibria are presented and compared with experimental information, and the agreement is discussed. Further, the differences in measured results are discussed.

Diffusion in intermetallic compounds: The ordered Cu3Au rule, its history

Abstract: The rule states that in compounds of the form AmBn, with m/n greater than 2, the atoms of species A should diffuse considerably faster than those of B. The essay is divided into several parts, the first of which considers original expressions of the Cu3Au rule, that were neglected by the scientific community, then almost entirely concerned with compounds of the simple form AB. The second part is a description of the circumstances that led to the formulation of the rule as it became known today, together with a scrutiny of those structures in which the rule should apply. The third section constitutes a discussion of the validity of the rule as exemplified in experimental studies. The essay ends with a sketchy map of possible avenues for future investigations.

Microstructure evolution in immiscible alloys during rapid directional solidification

Abstract: A model has been developed by taking into account the common action of the nucleation, the diffusional growth, the collisions and coagulations of the minority phase droplets and the spatial phase segregation to describe the microstructure evolution in a rapidly directionally solidified immiscible alloy. Numerical solutions have been performed to investigate the microstructure evolution in a directionally solidified hypermonotectic Al - Pb alloy. It has been shown that an undercooled region appears in front of the solid/liquid interface and the liquid-liquid decomposition takes place there. If the solidification velocity is high enough, a steady state of solidification can be established and a fine dispersion of minority phase particles in a hypermonotectic alloy can be obtained.

Efficiency of drag mechanisms for inhibition of grain growth in nanocrystalline materials

Abstract: An attempt is made to assess the efficiency of drag effects by different structural elements of a polycrystal on grain growth. The rate of grain area change is chosen as a measure of stability of a grain structure, and the inhibition of grain growth is pairwise evaluated among all drag effects considered. In aluminium, at temperatures of about 200 °C, triple junction drag was found to be most effective. The derived hierarchy of drag efficiency can be used as an effective engineering tool to assess and compare the role of chemistry and crystal defects on the microstructural stability of nanocrystalline and fine-grained materials.

Thermodynamic properties of liquid Bi–Sn alloys

Abstract: The thermodynamic properties of liquid Bi–Sn alloys were measured over the whole composition range by an electromotive force (emf) method using a liquid electrolyte. The basic thermodynamic functions were calculated from the corresponding excess functions by using the Gibbs–Duhem integration at 723 K. The liquid Bi–Sn system was classified as a subregular solution and an asymmetrical curve of enthalpy of mixing vs. composition was found.