Showing papers in "Zeitschrift Fur Metallkunde in 2006"

Criteria for developing castable, creep-resistant aluminum-based alloys - A review

Abstract: We describe four criteria for the selection of alloying elements capable of producing castable, precipitation-strengthened Al alloys with high-temperature stability and strength: these alloying elements must (i) be capable of forming a suitable strengthening phase, (ii) show low solid solubility in Al, (iii) low diffusivity in Al, and (iv) retain the ability for the alloy to be conventionally solidified. With regard to criterion (i), we consider those systems forming Al3M trialuminide compounds with a cubic L12 crystal structure, which are chemically and structurally analogous to Ni3Al in the Ni-based superalloys. Eight elements, clustered in the same region of the periodic table, fulfill criterion (i): the first Group 3 transition metal (Sc), the three Group 4 transition metals (Ti, Zr, Hf) and the four latest lanthanide elements (Er, Tm, Yb, Lu). Based on a review of the existing literature, these elements are assessed in terms of criteria (ii) and (iii), which satisfy the need for a dispersion...

Evolution of C-rich SiOC ceramics

Abstract: Carbon-rich Si–O–C polymer-derived ceramics (PDCs) were investigated by various spectroscopic techniques, in order to characterize the evolution of their predominantly amorphous microstructure upon thermal treatment up to 1450°C. Particular attention was addressed to modifications of the excess free carbon phase present in these materials. Surprisingly, the carbon clusters exhibited high stability above the pyrolysis temperature. Despite the high volume fraction of carbon, only a very limited carbothermal reduction process was detected. This study is divided into two parts: PartI deals with characterization tools that reveal a rather low lateral resolution and are hence termed here as integral spectroscopic techniques, i.e., solid-state NMR and Raman spectroscopy. In contrast, PartII illustrates the experimental results obtained from the very same materials characterized by spectroscopic and imaging techniques with high lateral resolution, i.e., electron energy-loss spectroscopy (EELS), high-resol...

Density and surface tension of liquid ternary Ni–Cu–Fe alloys

Abstract: Density and surface tension of liquid Ni-Cu-Fe alloys have been measured over a wide temperature range, including the undercooled regime. A non-contact technique was used, consisting of an electrom ...

Evolution of C-rich SiOC ceramics: Part II. Characterization by high lateral resolution techniques: electron energy-loss spectroscopy, high-resolution TEM and energy-filtered TEM

Abstract: In this Part II of the paper on the evolution of carbon-rich Si–O–C polymer-derived ceramics (PDCs), emphasis is placed on the strengths and limitations of transmission electron microscopy (TEM) techniques with high lateral resolution, i.e., electron energy-loss spectroscopy (EELS), high-resolution TEM (HRTEM) and energy-filtered TEM (EFTEM). Here, the identical SiOC materials, as described in Part I. EELS studies confirmed the progression of structural rearrangements within the SiOC matrix temperatures exceeding 1200°C. High-resolution TEM imaging showed that the SiOC matrices are indeed predominantly amorphous even upon high thermal treatment. Energy-filtered TEM analysis revealed, in contrast to the results obtained by Raman spectroscopy (Part I), that the excess free carbon phase undergoes a pronounced rearrangement within the amorphous microstructure. HRTEM characterization revealed the distribution of phases within the amorphous SiOC matrix; information that is not accessible by integral spe...

Between microscopic and mesoscopic descriptions of twin-twin interaction

Abstract: On a microscopic scale, deformation twinning is carried by the motion of twinning disconnections. A disconnection is an interfacial line defect characterized by a Burgers vector, a line vector, and a step vector. The Burgers vector (dislocation component of the disconnection) carries the deformation while the step vector (ledge component) carries the transformation from one twin variant to the other. On a mesoscopic scale, the deformation produced by twinning is a simple shear. A moving disclination dipole provides a mesoscopic model accounting for the twinning shear. Twin-twin interaction processes including the intersection of twins, the formation of structured twins, and the nucleation of cracks, may feature very complex mechanisms when analyzed on a microscopic scale. It turns out, however, that these mechanisms are controlled by the properties of large disconnection groups containing up to 10000 disconnections and more. These properties are sufficiently well approximated in the disclination dipole model. The disclination model for twin-twin interaction predicts orientation and volume fractions of secondary twins. The model also predicts the nucleation of cracks and crack growth. The disclination model was used to analyze the ductile-to-brittle transition of austenitic steel deformed at low temperature. The mesoscopic disclination model for twinning is successful because it accounts for the properties and mechanisms of disconnection groups.

Experimental observations on thecorrelation between microstructure andfracture of multiphase steels

Abstract: The aim of this study is to clarify the role of the microstructure in damage evolution. The influence of the transformation induced plasticity effect on the crack initiation and the impact of the different phases in development of cracks are important factors that control the fracture mechanisms. The fracture mechanisms of multiphase steels have been investigated on the basis of extensive light optical microscopy and scanning electron microscopy investigations. On the micro-scale, two failure modes appear: cleavage and ductile fracture, depending on the stress–strain conditions, the internal cleanness, the volume fraction of the retained austenite and the position of the austenite and the martensite grains. If a martensite or an austenite grain fails inside a bainitic island, the crack develops rapidly leading to cleavage fracture. If the failure starts inside the ferritic matrix due to void initiation at hard phases, the emerged void causes ductile damage to the surrounding ferrite.

Decomposition kinetics of expanded austenite with high nitrogen contents

Abstract: This paper addresses the decomposition kinetics of synthesized homogeneous expanded austenite formed by gaseous nitriding of stainless steel AISI 304L and AISI 316L with nitrogen contents up to 38 at.% nitrogen. Isochronal annealing experiments were carried out in both inert (N2) and reducing (H2) atmospheres. Differential thermal analysis (DTA) and thermogravimetry were applied for identification of the decomposition reactions and X-ray diffraction analysis was applied for phase analysis. CrN precipitated upon annealing; the activation energies are 187 kJ/mol and 128 kJ/mol for AISI 316L and AISI 304L, respectively. Isothermal stability plots for expanded austenite developed from AISI 304L and AISI 316 were obtained.

Experimental and theoretical characterization of Al3Sc precipitates in Al–Mg–Si–Cu–Sc–Zr alloys

Abstract: The structure of Al3Sc precipitates in Al–Mg–Si–Cu–Sc–Zr alloys annealed at 450°C was investigated using transmission electron microscopy and energy-dispersive X-ray spectroscopy. The Al3Sc particles contain mainly Sc and small amounts of Zr, Si, and Cu. The addition of Zr limits the size of the Al3Sc precipitates to about 10–30nm, and these precipitates are coherent with the matrix. Density functional energy calculations showed that exceptionally small energies are required to dissolve Zr and Cu in Sc and Al sublattices, respectively. On the contrary, Cu, Mg, and Si are difficult to be dissolved in the Sc sublattice.

Ripening of L12 Ni3Ti precipitates in the framework of the trans-interface diffusion-controlled theory of particle coarsening

Abstract: Data are presented on the kinetics of coarsening of γ′-type Ni3Ti precipitates (L12 crystal structure) in three binary Ni–Ti alloys containing 10.31, 11.84, and 13.72at.% Ti aged at 720°C for times up to 64 h. Data on the distributions of particle sizes (PSDs) are also presented. These data, as well as previously published data, are analyzed in light of a new theory of coarsening in which diffusion is controlled by transport through the non-sharp interface between the matrix and precipitate phases. The new theory, called the trans-interface diffusion-controlled (TIDC) theory of coarsening, predicts time (t)-dependent behavior of the type 〈r〉n ∝ t for the growth of precipitates of average radius 〈r〉 and XTi ∝ t−1/n for the depletion of the solute concentration of the matrix, XTi. The exponent n is intimately related to the width of the interface between the precipitate and matrix phases, δ, which is assumed to depend on the particle radius as δ ∝ m, where n = m + 2. The shape of the scaled distrib...

New aspects of bending rotation fatigue in ultra-fine-grained pseudo-elastic NiTi wires

Abstract: Bending rotation fatigue experiments are a standard evaluation procedure for pseudo-elastic NiTi wires used in the medical device industry. However, the non-linear mechanical behavior and the possible occurrence of localized deformation during bending complicate the physical interpretation of fatigue life data. In the present study, some related experimental and theoretical aspects are addressed: Optical measurements are used to characterize the local deformation of the wire specimens in a typical experimental setup, and a simple mechanical model of stresses and strains associated with bending rotation is employed to rationalize the impact of several experimental parameters on fatigue life data. Dissipated energies per cycle and unit length are calculated. How dissipation leads to an increase of both specimen temperature and maximum stresses and, therefore, considerably decreases fatigue lives is demonstrated.

Development of long-period ordered structures during crystallisation of amorphous Mg80Cu10Y10 and Mg83Ni9Y8

Abstract: The different transformations occurring during the crystallisation of amorphous Mg80Cu10Y10 and Mg83Ni9Y8 alloys have been elucidated. The formation of several unknown metastable phases was observed. The present work confirms that such phases exhibit long-period structures originated by the periodic alignment of (Y,TM)-layers (TM: transition metal). The long-period stacking-ordered phase, identified in the crystallised Ni-containing alloy, is stable at higher temperatures than that in the Cu-containing alloy.

Recent progress in the area of bulk metallic glasses

Abstract: Bulk metallic glasses (BMGs) form a new class of alloys which can be manufactured amorphously at a cooling rate of less than 100K/s and in dimensions of several centimeters. Because of their particular characteristics, these glassy materials have great potential for deployment as structural and functional materials. The mechanical, magnetic, and thermophysical properties of this new materials group and the advantages they promise for the processing and manufacturing of components are presented. The possibilities for deploying bulk metallic glasses in medical devices and lightweight construction are also set out. In these two areas, we discuss some results as to the biocompatibility of metallic glasses and their potential use in vascular intervention, as well as the properties of light-metal-based bulk metallic glasses in connection with their potential as a construction material. We also present ideas concerning the development of corrosion-resistant Mg-based glasses, and the manufacture and proc...

Dislocation micromechanisms under single slip conditions

Abstract: The microstructures formed in the early stage of deformation of Cu, Al, and TiAl in single slip and at low temperatures are analysed. While dislocations are highly mobile in Cu and Al, they are subject to a significant lattice friction in TiAl. Also, they are dissociated over a fewnm in Cu, not in Al and TiAl. These various properties are manifested in the self-organization of dislocations in bundles. The role of nanometric prismatic loops in producing specific debris is emphasized in the context of the so-called collinear interaction. Possible artefacts arising from microstructural relaxation after unloading are discussed and limitations of weak-beam analyses assessed.

Solidification curves for commercial Mg alloys obtained from heat-transfer modeled DTA experiments

Abstract: An improved method is presented for determination of solidification curves, i.e., solid fraction versus temperature, for commercial Mg alloys using heat-transfer modeled differential thermal analysis (DTA) curves. A better simulation of the measured DTA signal is attained through an independent measurement of the time constant as function of temperature for the applied equipment. This enables a better desmearing of the DTA signal. Challenging Mg alloys could be appropriately handled by redesigning the tantalum encapsulation. Due to high oxygen affinity and vapor pressure of the investigated magnesium alloys, this special adaptation of the DTA setup using sealed Ta capsules was indispensable for generation of reproducible and reliable data.

Characterisation of short-range order using dislocations

Abstract: The mobile dislocation pile-ups observed during in situ straining experiments have been used as probes to describe the short-range order (SRO) present in the phase of nickel-base superalloys. An experimental approach, based on the measurement of the dislocation positions, to characterise the SRO has been developed. This paper focuses on the main results we have obtained on this topic. A quantitative analysis allows to get the chemical forces associated with SRO, and also the SRO degree. For the first time, the energies associated with SRO are evaluated along a dislocation pile-up as a function of temperature. More generally, a description of the SRO is proposed: SRO is seen as a heterogeneous distribution at the micrometer scale of nanometric areas. Another characteristic has been evidenced: the reversible character of the SRO creation.

Thermodynamic assessment of the Mg–Nd system

Abstract: The Mg–Nd system was critically assessed by means of the CALPHAD technique. The solution phases (liquid, body-centered cubic, double hexagonal close-packed and hexagonal close-packed) were modeled with the Redlich–Kister equation. The intermetallic compounds Mg41Nd5, Mg3Nd and MgNd, which have a homogeneity range, were treated as the formulae (Mg, Nd)41(Mg, Nd)5, Mg3(Mg, Nd) and Mg(Mg, Nd) by a two-sublattice model with Mg and Nd or Mg on the first sublattice, and Mg and Nd on the second one, respectively. The compound Mg2Nd was treated as a stoichiometric compound. A set of self-consistent thermodynamic parameters of the Mg–Nd system was obtained.

Phase separation in Si-(B)-C-N polymer-derived ceramcis

Abstract: Details of phase separation in the microstructure of amorphous Si – (B) – C – N ceramics derived from polymers have been resolved using the recent results of structural investigations. The formation of an amorphous phase built of atomic compounds SiCi/4N(4 – i)/3 and consequently located along the composition line between SiC and Si3N4 in the ternary Si – C – N phase diagram demonstrates a generic feature of phase separation in all these materials. The amorphous carbon phase separates as a counterpart in the microstructure of Si – C – N ceramics, and in the case of Si – B – C – N ceramics such counterpart represents B – N – C domains of the composition (BN)cCy located along the tie line C – BN in the ternary B – C – N phase diagram. The effect of phase separation has been also pondered as a source of exceptional material properties.

Sigma phase formation and its effect on mechanical properties in the corrosion-resistant superalloy K44

Abstract: The sigma phase (σ) formation in long-term aged polycrystalline K44 superalloy has been studied as a function of aging temperature and time. The σ phase in K44 alloy forms mainly in dendrite cores with a few at interdendritic regions. The kinetics of σ formation can be described by the Johnson–Avrami–Mehl equation that yields activation energies of more than 130 kJ/mol in an early stage and less than 100 kJ/mol in steady state. Tensile experiments at room temperature and 1173 K and endurance experiments at 1173 K and 274 MPa were performed to test the effect of σ phases on these properties and no remarkable harmful effect is found. Moreover, the effect of long-term aging on the mechanical properties is realized mostly through the variation of γ′, not σ phase formation.

Re-optimization of the Mg-Sb system under topological constraints

Abstract: Non-existing inverted miscibility gaps in the liquid phase at high temperatures, reappearance of the liquid phase at low temperatures or reappearance of a solid phase at elevated temperatures, and other “approximation phantoms” sometimes result from thermodynamic assessments. A method for struggling against such artifacts is proposed. The method is based on introducing a sufficiently dense mesh of knots (not related to experimental points utilized in the optimization) and ensuring that specific inequality conditions (topological constraints) governing the appearance of the phase diagram are satisfied in these knots. It is shown that the syntax of the language describing conditions and experiments in the PARROT module of Thermo-Calc is suitable for performing such optimization. The workability of the approach proposed is exemplified by carrying out a re-optimization of the Mg–Sb system.

Kinetics studies of hydrogen reduction of Cu2O

Abstract: In present work, reduction kinetics of Cu2O by hydrogen gas was studied by thermogravimetric analyses (TGA). The TGA experiments were carried out both isothermally and non-isothermally on shallow powder beds. It was established that additions of Ni or NiO did not have any serious effect on the kinetics of reduction of Cu2O. The composition and microstructures of the reaction products were analyzed after each experiment by X-ray diffraction (XRD) as well as by scanning electron microscopy (SEM). The activation energy for the reaction was evaluated from isothermal as well as non-isothermal reduction experiments and was found to be in good agreement. The impact of the stability of the oxide on the activation energy for hydrogen reduction is also discussed.

Short-range order in Fe-21.9 at.% Al

Abstract: Elastic diffuse neutron scattering of an Fe-21.9at.% Al single crystal was measured at 878, 966 and 1073K. The separated short-range order scattering revealed the building elements of the B2 structure as the characteristic local atomic arrangement for all states investigated. Effective pair interaction parameters, obtained by the inverse Monte-Carlo method, were found to depend on temperature, thus prohibiting a determination of possible ground state structures from the high-temperature measurements. An additional X-ray study of single-crystalline Fe-18.2at.%Al aged at 598K, gave no indication for B32 as ground state structure.

Effects of an electric field applied during the solution heat treatment of the Al–Mg–Si–Cu alloy AA 6111 on the subsequent natural aging kinetics and tensile properties

Abstract: The effects of an external dc electric field of 0–5 kV/cm, applied at 475–550°C during solution heat treatment (SHT) to AA6111, on the subsequent natural aging kinetics and volume fraction of precipitated clusters were determined employing resistivity. The field increased the as-quenched resistivity and that during natural aging, the effect being significant from 0 to ≈0.5 kV/cm and increasing only slightly, if at all, thereafter. An Avrami-type analysis of the natural aging kinetics gave n = 0.59 ± 0.2 and k = (1.2 ± 0.8) · 10−2 min−1, relatively independent of SHT temperature and field strength. The field increased the volume fraction of precipitated clusters which occurred during natural aging. This was attributed to the increase in solubility during SHT produced by the field. Tensile properties equivalent to those obtained by the SHT at 550°C without a field were obtained at 500°C with a field of only 200 V/cm, representing a reduction of 50°C in the nominal SHT temperature.

The Pd-rich part of the Pd - B phase diagram

Abstract: The Pd–B system in the composition range of the solid solution PdBy has been the subject of a number of investigations as it provides an attractive model system for interstitial compounds. No consensus about the crystal structures and phase boundaries, particularly in the low-temperature regime, has been reached in the earlier works. In the light of very recent results on the crystal structure and composition ranges of the low-temperature phases, on the basis of X-ray, electron, and neutron diffraction analyses, a tentative phase diagram is presented which is compatible with practically all previous results.

Early stages of nucleation and growth of Guinier-Preston zones in Al-Zn-Mg and Al-Zn-Mg-Cu alloys

Abstract: The early stages of nucleation and growth of Guinier–Preston zones in Al–Zn–Mg and Al–Zn–Mg–Cu alloys are investigated by in situ synchrotron X-ray small-angle scattering. The in situ synchrotron measurements enable to follow the decomposition kinetics from about 2 minutes after quenching. Based on the measurement of the radii of gyration and integrated intensities, three distinct kinetic regimes are observed and discussed.

Texture evolution in equiaxed polycrystalline L10-ordered FePd during coarsening at 600°C

Abstract: The texture evolution in equiaxed polycrystalline L10-FePd during the coarsening regime of annealing at 600°C after cold-rolling has been investigated, using X-ray diffraction and scanning electron microscopy. A change in dominant texture was observed. Using the nomenclature established for texture description of face-centered cubic metals, the texture of the L10-FePd develops from a brass-type texture into a strong cube-type texture during coarsening. Also, a large density of special high-angle grain boundaries emerged during coarsening, which can be described as coherent R2-boundaries and are associated with (90±5)°-rotations about a common cube-axis between the neighboring grains. The emergence of R2-boundaries may play an important role for previously reported abnormalities in the grain growth kinetics.

Atomic migration and ordering phenomena in bulk and thin films of FePd and FePt

Abstract: “Order–order” kinetics was studied by means of “in situ” and quasi-residual (REST) resistometry in bulk and thin films of L10-ordered FePd and FePt intermetallics. Substantial effect of magnetic ordering on the activation energy for chemical ordering was revealed in FePd. A discontinuous change of ordering dynamics was detected in FePt between 800 and 830 K. The results are consistent with the data of Fe* diffusion in FePt multilayer examined by means of nuclear resonant scattering in grazing-incidence geometry (GINRS). Monte Carlo (MC) simulations of “order–order” processes in L10-ordered bulk FePd and FePt and nano-layered FePt have been carried out using Glauber dynamics with vacancy mechanism of atomic jumps. Multi-time-scale “order–order” relaxations observed in the bulk were predominated in nanolayers by a reorientation of the initial z-variant L10 superstructure into a mixture of x- and y-variants.

Diffusion of chromium in β-Ti under high pressure

Abstract: It is shown that Arrhenius plots of diffusion coefficients of Cr in β-Ti can be represented under each pressure by two straight lines which are fitted in the temperature regions above and below about 1390 K. The activation enthalpies for high temperature are 167 kJ/mol under 0.1 MPa and 191 kJ/mol under 2.1 GPa, and those for low temperature are 147 kJ/mol under 0.1 MPa and 153 kJ/mol under 2.1 GPa. The ratios of activation volumes for impurity diffusion to the molar matrix volume are in the range of 0.38 to 0.45. When the phonon-softening model is applied, the activation enthalpy for the impurity diffusion in the absence of any phonon-induced lattice instabilities is 226 kJ/mol under 0.1 MPa and increases with pressure. Its pressure dependence indicates that the activation volume for the diffusion with no assistance from phonons is 66% of the molar matrix volume, and this result agrees with the activation volume for the diffusion in “normal” bcc metals.

Formation, stability, and presence of magnesium nitride in magnesium recycling processes

Abstract: In this study an attempt has been made to identify analytical methods for detecting magnesium nitride, and document in which part of the magnesium recycling process this phase is concentra...

Phase decomposition and precipitation of metastable A2 phase in B2 ordered Co–Al–Fe alloys

Abstract: Phase decomposition in rapidly solidified Co–Al–Fe B2 alloys during isothermal ageing at 923K has been investigated by means of X-ray diffraction and transmission electron microscopy. At t...

Effect of surface roughening on increasingthe spectral selectivity of cermet solarselective absorbers

Abstract: The spectral selectivity of Cu–Al2O3 cermet solar absorbers was studied theoretically and experimentally in different layer designs. The results showed clearly that, by increasing the substrate temperature during film deposition, the desired random roughness features will be created on the surface of the cermet, which leads to higher selectivity. Theoretical simulation showed that a single cermet structure with the assistance of an anti-reflection layer, results in the best optical properties. Instead of such a structure, which is sensitive to fine thickness control, it was tried experimentally to increase the selectivity of a single cermet structure as much as possible using a roughness template layer. Surface roughening using such a layer design increased the solar absorptance from 0.840 to 0.956 while keeping the thermal emittance almost unchanged at about 0.028.