Volume 97, Issue 3

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 few nm 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.

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 γ 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.

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 10 000 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.

The influence of the thermoelastic effect on the sound propagation in metals is discussed. A strict description of the effect based on a combined solution of the equations of elasticity and heat conductivity is presented. It is shown that acoustic relaxation resonance caused by the thermoelastic effect can be observed when metal single crystals of high purity are studied by pulse-echo spectroscopy at about 10 K.

Bulk metallic glasses (BMGs) form a new class of alloys which can be manufactured amorphously at a cooling rate of less than 100 K/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 processing of light-metal-based composites.

A new class of ordered structures designated as of the ABC 6 -type has been found in the ternary CuMnPt 6 alloy. It has a cubic symmetry with the unit cell consisting of 2 ⨯ 2 ⨯ 2 fcc unit cells, whose space group is Fm3m. A closely related structure with Cu replaced by Pt has also been found in the binary MnPt 7 alloy. Both alloys undergo a double-step order – disorder phase transition, ABC 6 -type – Cu 3 Au-type – fcc disorder. A partial phase diagram was constructed on the Pt-rich side of Pt –Mn. MnPt 7 is isostructural to CuPt 7 previously found by Schneider and Esch, though the latter does not show the double-step transition. Within the Bragg–Williams approximation, the order – disorder phenomena in the three alloy systems can be successfully reproduced. It has been found that an ordering energy of negative sign, a preference of unlike pairs, between second-nearest neighbours plays a decisive role in the formation of the ABC 6 -type structure. An increase in the relative magnitude of the ordering energy suppresses the double-step transition and the ABC 6 -type phase forms directly from the disordered phase.

Elastic diffuse neutron scattering of an Fe-21.9 at.% Al single crystal was measured at 878, 966 and 1073 K. 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-18.2 at.% Al aged at 598 K, gave no indication for B32 as ground state structure.

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 Al 3 M trialuminide compounds with a cubic L1 2 crystal structure, which are chemically and structurally analogous to Ni 3 Al 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 in Al with slow coarsening kinetics, and criterion (iv), which is discussed based on the binary phase diagrams.

Phase decomposition in rapidly solidified Co–Al – Fe B2 alloys during isothermal ageing at 923 K has been investigated by means of X-ray diffraction and transmission electron microscopy. At the early stage of ageing, A2 disordered particles precipitate inside B2 matrix grains, being aligned along the <100> direction of the matrix, and equilibrium A1 phase (α-Co) is also formed mostly on grain boundaries. With further ageing, the plate-like A2 particles increase in size. On the other hand, the A1 phase on grain boundaries increases in size and also spreads into the B2 grain like tree branches as so-called Widmanstätten precipitate. Finally, the A2 phase disappears. The A1 phase remains on grain boundaries and also inside the B2 grains as spherical particles. Using the composition gradient method, the metastable precipitation limit of the A2 phase, that is the metastable phase boundary B2/(A2 + B2), was experimentally determined at 923 K. It extends smoothly into the stable A2 + B2 two-phase field on the Co–Fe side of the ternary Co–Al –Fe system.

“Order – order” kinetics was studied by means of “in situ” and quasi-residual (REST) resistometry in bulk and thin films of L1 0 -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 L1 0 -ordered bulk FePd and FePt and nano-layered FePt have been carried out using Glauber dynamics with vacancy mechanism of atomic jumps. Multi-timescale “order – order” relaxations observed in the bulk were predominated in nanolayers by a reorientation of the initial z- variant L1 0 superstructure into a mixture of x- and y- variants.

An oil-in-water microemulsion of the system C 12 E 5 /water/ n-decane with a fixed surfactant-to-oil volume fraction ratio φ s /φ o of 0.85 is studied. This clear microemulsion is temperature-quenched from a clear one-phase region (26 °C) with spherical microemulsion droplets to an unstable region (13 °C) where the excess oil phase is in equilibrium with the microemulsion droplets. Using a high-resolution version of a laboratory small-angle X-ray scattering (SAXS) instrument, the evolution of the excess oil and its growth as a function of time is studied. Unlike coarsening in alloys and polymer blends, a maximum in the intensity at a finite scattering vector is not observed. This clearly indicates that the droplet of the excess oil is formed homogeneously and that there is no depletion zone around the formed droplets. The scattering curves are quantitatively analysed as a function of time. The microemulsion droplets are modeled as polydisperse core-shell ellipsoidal particles, using molecular constraints and the oil droplets are modeled as polydisperse spheres. The average radius of the oil droplets increases with a scaling exponent in agreement with that of coarsening of precipitates in alloys and phase separations in polymer blends, suggesting some degree of universality of coarsening processes.

Small-angle scattering (SAS) has long been used to study nucleation and growth of particles in solid or liquid matrices. In some special cases, nucleation of particles occurs on interfaces, e. g., grain boundaries in crystalline solids or membranes in a solvent. Clearly, the position of the particles is constrained in this case to a narrow region close to the interface, which leads to correlations between the particles. In the present paper, we derive simple expressions for the analysis of SAS data from particles located on planar interfaces, and compare the analytical approximations with computer simulations.

Data are presented on the kinetics of coarsening of γ ′ -type Ni 3 Ti precipitates (L1 2 crystal structure) in three binary Ni –Ti alloys containing 10.31, 11.84, and 13.72 at.% 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 X Ti ∝ t –1/ n for the depletion of the solute concentration of the matrix, X Ti . 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 ∝ r m , where n = m + 2. The shape of the scaled distribution of particle sizes (PSD) depends on n and the thermo-physical kinetic constants are independent of volume fraction. The data on kinetics are evaluated and compared for n = 2.375, determined from analyses of the PSDs, and for n = 3, which is the traditionally accepted value. The agreement between the data on kinetics and predictions of the TIDC theory is acceptable, and the TIDC theory is the only one capable of explaining the experimentally observed absence of an effect of volume fraction on the kinetics at larger volume fractions, and the shapes of the PSDs.

The texture evolution in equiaxed polycrystalline L1 0 -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 L1 0 -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 Σ2-boundaries and are associated with (90 ± 5)°-rotations about a common cube-axis between the neighboring grains. The emergence of Σ2-boundaries may play an important role for previously reported abnormalities in the grain growth kinetics.

Amorphous Cr-silicide films are prepared by electron-beam-deposition on a Cu-substrate at room temperature. The microstructures of as-grown films are investigated using transmission and analytical electron microscopy. The morphology of the amorphous films is dendritic. In addition, one-dimensional concentration modulations are observed. The modulated structure is revealed not only by TEM imaging but also by a series of satellite spots at the centre of the diffraction pattern. This concentration modulation is confirmed by analytical electron microscopy.

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.

The structure of Al 3 Sc 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 Al 3 Sc particles contain mainly Sc and small amounts of Zr, Si, and Cu. The addition of Zr limits the size of the Al 3 Sc precipitates to about 10– 30 nm, 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.

The late stages of precipitation in aluminum-rich Al –Ag alloys with silver contents from 3 at.% to 22 at.% were studied with transmission electron microscopy. Shapes, sizes, and arrangements of plates of the stable hexagonal Ag 2 Al phase forming on {111} planes of the f.c.c. matrix were analysed. The high-angle annular dark-field contrast in scanning transmission electron microscopy is calibrated. This calibration allows for the quantitative measurement of plate thicknesses from high-angle annular dark-field scanning transmission electron micrographs of Ag 2 Al plates inclined to the electron beam. Results from these measurements are in good agreement with direct bright-field micrographs of plates viewed edge-on. For samples with silver contents above 12 at.% a parallel lamellar arrangement of fine γ-plates and Al-rich matrix is found after extended heat treatments.

The composite model which has been created by Mughrabi, and developed by several other researchers, is analyzed with respect to high-temperature creep of Al and an Al– Mg alloy. Data for structural parameters such as local and global internal stresses, and dislocation densities have been obtained from X-ray line profile analyses and stress dip tests. Based on the composite model, quantitative relations have been derived which convert the internal stresses gained from X-ray profile analysis into those obtained from stress dip tests. This allowed for a detailed check of the composite model and its formalism describing local and global external and internal stresses, and local and global dislocation densities, during creep at high temperatures. While the local and global internal stresses show an excellent coincidence, the dislocation densities and subboundary volume fractions derived from experiment and modelling show certain deviations; however, these can be related to simplifications of equations used which are not part of the composite model.

The influence of damage and its correlation with the failure behaviour is discussed in the light of microstuctural aspects. The failure induced by microstructural mechanisms of the Al 2 O 3 -particle reinforced aluminium alloy 6061 under uniaxial tensile as well as under cyclic loading was investigated. The damage behaviour as a function of the strain ε can be correlated with stiffness E (ε). Under cyclic loading conditions the damage increases with the number of cycles and can be correlated with the strain amplitude ε a . Furthermore, the damage is associated with crack formation and propagation and the damage evolution depends sensitively on the kind of deformation. The crack formation was detected by metallographic investigations and calculated by numerical methods (FEM), taking the size and shape of particles into account. Generally, damage under mechanical loading is initiated by broken particles and agglomerations of particles. In case of the particulate-reinforced specimens the evolution of damage is quite different for monotonic and cyclic loading conditions but, nevertheless, in both cases the damage parameter is given by inhomogeneities characteristic for the MMC which act as seeds for crack initiation.

Magnesium micropowder was mixed and ball-milled with 3 vol.% graphite powder. The Mg + Graphite composite was deformed in compression at temperatures between room temperature and 300 °C at a constant crosshead speed, giving an initial strain rate between 1.4 · 10 –5 and 1.4 · 10 – 3 s –1 . The yield stress of the composite is higher than that of unreinforced magnesium prepared with the same technology. The yield stress and the maximum stress decrease rapidly with increasing temperature. At temperatures higher than room temperature, the true stress– true strain curves exhibit a work-hardening rate close to zero, which indicates a dynamic equilibrium between hardening and softening. Double cross-slip of screw components of c + a dislocations is considered as the main recovery mechanism.