Band 100, Heft 10

Recent advances in the thermodynamic description of reactions and phase transformations at interfaces between metals, semiconductors, oxides and the ambient have been reviewed. Unanticipated nanostructures, characterized by the presence of phases at interfaces and surfaces which are unstable as bulk phases, can be thermodynamically stabilized due to the dominance of energy contributions of interfaces and surfaces in the total Gibbs energy of the system. The basic principles and practical guidelines to construct realistic, practically and generally applicable thermodynamic model descriptions of microstructural evolutions at interfaces and surfaces have been outlined. To this end, expressions for the estimation of the involved interface and surface energies have been dealt with extensively as a function of, e. g., the film composition and the growth temperature. Model predictions on transformations at interfaces (surfaces) in nanosized systems have been compared with corresponding experimental observations for, in particular, ultrathin (< 5 nm) oxide overgrowths on metal surfaces, as well as the metal-induced crystallization of semi-conductors in contact with various metals.

This overview highlights recent results on the oxidation of Pd obtained on low-index and vicinal single crystal surfaces as well as epitaxial Pd nanoparticles on MgO(100). The emphasis is put on X-ray diffraction experiments, which were conducted in-situ during oxidation at elevated temperatures and near atmospheric pressure. On Pd(100) the formation of an ultrathin surface oxide layer with a superstructure was observed. This layer represents a considerable kinetic barrier against PdO bulk oxide formation. A similar surface oxide forms on the Pd(553) vicinal surface after oxygen-induced faceting into (332) and (111) oriented areas. In contrast to this, Pd nanoparticles on MgO(100) below a diameter of 10 nm undergo continuous oxidation without passivation layer formation, pointing to a much higher reactivity and a lowering of kinetic barriers. These differences in the oxidation behavior give evidence for a breakdown of classical oxidation theory on the nanoscale.

In this review, recent progress in understanding the short- and long-term oxidation behaviour of MCrAlY coatings and the instruments that can be employed to influence the oxidation behaviour of the MCrAlY's are discussed. Particular attention is given on the transient oxidation behaviour, the initial distribution of the Y in the oxide scale and how these initial oxide products influence the long-term oxide growth rate and adhesion of the oxide scale. It is shown that the growth kinetics and adhesion of the oxide scale are to a large extent determined by the amount and distribution of Y-oxide inclusions in the scale. Oxide scales with a regularly distributed and larger amount of small Y-oxide inclusions are faster growing, but perform better in thermal cycle tests. Transient oxide products like metastable aluminas (γ- or θ-Al 2 O 3 ) and spinels do not have a large influence on long-term oxidation behaviour except when the oxide scale is virtually free of Y-oxide inclusions due to Y depletion. Then, the nucleation of metastable aluminas at the onset of oxidation may be beneficial due to a slower-growing alumina scale with a larger grain size after transformation into α-Al 2 O 3 . Spinel oxides are considered detrimental for long-term oxidation performance, but their amount is usually limited on most types of MCrAlY coatings due to the rapid supply of Al along the numerous grain and phase boundaries in the fine-grained coatings. The key instruments that can be employed to manipulate the oxidation behaviour of MCrAlY coatings are: (i) the composition and size of the main phases in the coating (γ-Ni and β-NiAl), (ii) the initial distribution, chemical state (metallic or oxidized) and reservoir of the Y in the coating, (iii) the composition and roughness at the coating surface, and (iv) the oxidation conditions during post-deposition heat treatment (temperature, time, partial oxygen pressure).

Since the first reported publications about their possible application for corrosion protection, conducting polymers are discussed as promising candidates for a new generation of corrosion control coatings. However, although research has started about three decades ago and certainly about a thousand publications have been published on this topic, their technical application has not yet made its breakthrough. The reason for this is that most coatings based on conducting polymers fail so far under practical corrosion conditions. Reasons will be discussed in this paper, as well as future challenges. As will be shown, conducting redox polymers such as polypyrrole or polyaniline indeed posses a promising potential for intelligent corrosion protection, but up to now this potential has not been explored.

The contribution to this jubilee issue of IJMR concentrates on the analysis of laser surface treatments. In particular laser cladding processes using coaxial and side set-ups are evaluated to determine the optimal processing window that prioritizes the clad quality and the efficiency of the coating method. The microstructural features in these so-called processing maps are illustrated with Inverse Pole Figure (IPF) mapping, texture pole figures and Electron Backscatter Pattern (EBSP) techniques. The distribution of stresses inside Co-based laser coating is measured with the so-called three-dimensional X-ray Diffraction microscopy (3DXRD) and it is found very dispersive, i. e. stresses may change from one grain to another considerably. The mean value of observed hydrostatic stress in all grains at specific laser track depth is gradually changed from tensile state in the upper part of the coating to the compressive state in its lower part close to the substrate. On the other hand the average value of shear stress component does not show any substantial change with depth.

The present review addresses the state of the art of low-temperature gaseous surface engineering of (austenitic) stainless steel and is largely based on the authors' own work in the last 10 years. The main purpose of low temperature gaseous surface engineering of stainless steel is to develop a hardened case at the surface, while maintaining the superior corrosion performance. This can be achieved by dissolving colossal amounts of nitrogen and/or carbon without forming nitrides and/or carbides, thus developing so-called expanded austenite. The present work gives an overview over results obtained on homogeneous expanded austenite and covers the crystallography for nitrogen- and carbon-stabilised expanded austenite, the solubility for nitrogen and carbon, the diffusion of these interstitials as well as the stability of expanded austenite with respect to nitride and carbide formation. Subsequently, the fundamental understanding acquired on homogeneous samples is applied to understand the morphology, composition, and residual stress distribution in functionally graded material, as obtained by nitriding, carburising or nitrocarburising of austenitic stainless steel. Thereafter, aspects of modelling the compositional changes occurring during nitriding are addressed. The overview summarises with an outlook towards future scientific challenges.

Charles Crussard's discovery of what he called Recrystallization in situ in 1944 is compared with what Egon Orowan and Robert Cahn named Polygonization. The context of the previous research is recalled in order to show the novelty of these contributions. Progress in accurate deformation and observation techniques such as tensile machines and X-ray methods were important, as were theoretical models about plausible microscopic mechanisms. With Jacques Friedel and Barney Cullity, Ch. Crussard also wrote a pioneering article on grain boundaries, which contains an experimental study of the relative energies of both symmetrical and asymmetrical grain boundaries observed in aluminium together with the first atomistic calculation of energies of symmetrical low and high angle tilt boundaries in several metals. This less well-known contribution is also recalled.

In the processing of liquid materials various operations are performed, e. g., alloying, homogenisation of the composition, refining or inclusion removal. As was realised early on by Charles Crussard, electromagnetic effects can be used to process electrically conducting materials without any contact. When alternating magnetic fields are used, the material is directly heated and melted thanks to the induced electric currents. The induced electromagnetic forces are capable of shaping the free surface, levitating a liquid blob and stirring the liquid bulk or its surface. The latter principle may be applied to electrically conducting liquids such as metallic alloys, but also to poorly conducting materials such as oxides, glasses or plasmas. Those effects are used in many processes such as induction furnaces for liquid metals or glasses, refining ladles, electromagnetic levitation, continuous casting of aluminium or steel. In this paper, typical effects based on the use of electromagnetic systems are reviewed, as well as the corresponding applications.

This paper describes the contribution of Charles Crussard to sheet metal forming science. After recalling the state of this technique in the 1940s, when Crussard started his studies, we explain how the International Deep Drawing Research Group was launched, how he became the second president of this organisation and how much he influenced it. Some examples of his contributions to research are given, and finally a brief review of later progress in this domain will point out the development of some of his ideas.

In spite of frequent observations, and of some important consequences for industrial applications, the glide motion of dislocations in non-octahedral planes of fcc metals and alloys remains rather poorly known, and not sufficiently considered for the analysis of mechanical properties. Following the pioneering work of C. Crussard in 1945, the results of several independent studies are compared and shown to be fairly consistent. They indicate that non-octahedral slip occurs under fairly well known conditions, and that it can be described by a kink-pair mechanism acting on screw dislocations dissociated in intersecting {111} planes. It has a drastic effect on the intermediate-temperature mechanical properties of fcc crystals and aluminum-based alloys, as well as on texture formation under conditions of severe plastic deformation.

This article critically reviews progress in the understanding of strain hardening during the deformation stage II of pure face-centered cubic crystals since its discovery in the mid-1950s. A wealth of models attempted explaining why stage II exhibits a linear slope on the base of specific dislocation configurations and interactions associated with short- or long-range internal stresses. Slip trace observations and more recent investigations on dislocation avalanches led to the identification of intermittent elementary slip events that are now investigated in terms of dislocation mechanisms. Numerical estimates showed that the overcoming of junctions and locks accounts for most of the flow stress and the hardening rate, as assumed by the present forest models. A multiscale analysis of stage II is outlined with emphasis on the modeling of dislocation mean free paths by dislocation dynamics simulations.

After reviewing the seminal results of the work of Charles Crussard in the field of metal recrystallization, the remainder of this overview will be devoted to the origin and development of cube texture in the Fe-36 %Ni alloy. For this purpose, the macrotexture was characterized via X-ray and neutron diffraction. The microtexture and microstructure were analyzed using electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM), respectively. The deformation energy stored by the dislocations (the driving force for recrystallization) was calculated based on the measurement of the broadening of the neutron diffraction peaks. After cold-rolling, the cube grains store less deformation energy than the aggregate of grains having other orientations and also exhibit a cellular microstructure. These two parameters account to a large extent for the formation and rapid growth of cube nuclei.

Transient creep of many materials at high temperatures obeys Andrade's law, in which creep strain is proportional to the cube root of time. The present paper aims at revisiting in terms of criticality the different explanations proposed so far. In agreement with Mott's statistical theory, and using the concept of load shedding and mechanical cascades introduced in Cottrell's microscopic model, we show that Andrade creep is obtained assuming only long range back stresses and delayed obstacle overcoming, both of them involving a large number of interacting sites responsible for stress fluctuations. The time exponent is 1/3 if work hardening is linear, and larger otherwise. Andrade's creep appears as a power law approach of the linear (or χ) creep regime.

The cluster dynamics method is used to model at the atomic scale the kinetics of first order phase transformations. Clusters are embryos of the growing phase. Their formation kinetics from the solid solution are obtained by solving a set of master differential equations. This set is the continuity equation of the cluster size distribution function and is based on inter-cluster solute exchanges. These exchanges, absorption and emission of solute atoms, are controlled by the solute diffusivities and the cluster free energies. The model is applied here to the precipitation of Al 3 (Zr, Sc) dispersoids with the L1 2 structure in Al–rich Zr–Sc solid solutions. Several characteristic features are obtained and discussed: 1) In the ternary alloy, the fast (Sc) diffusing species always controls the nucleation, in contrast to classical thermodynamical descriptions. 2) A Zr–Sc thermodynamic coupling induces heterogeneous nucleation on Zr atoms. 3) A segregation on the dispersoids outer shell of the slow diffusing solute (Zr) occurs during the coarsening stage, slowing down their coarsening rate. Finally, some extensions and prospects of the method are considered.

The effect of low temperature recovery treatments on recrystallization kinetics during subsequent annealing was examined using a simple physically-based model. The results suggest that recovery may accelerate recrystallization in high stacking fault energy materials. Otherwise, recovery is expected to retard recrystallization by reducing the driving force for the nucleation and growth of recrystallized grains. These calculations help reconcile a number of apparently conflicting reports on the effect of recovery on recrystallization.

The concept of grain boundary, much studied for its relationship with dislocation motion in plastic deformation, is here developed and associated with that of disclination. After defining a disclination in terms of a Volterra process, we investigate the nature of the interplay between both, with application to two cases: i) in polycrystals the triple junctions result from the stress free merging of disclinations bordering the incident grain boundaries; the plastic properties of nanocrystals, which cannot be attributed directly to dislocations, are related to these disclinations; ii) in lamellar liquid crystalline phases grain boundaries are often the place not only of dislocations, but also of focal conic domains, a stunning disclination system described by G. Friedel one hundred years ago.

The present work attempts to pinpoint scaling constraints associated with plastic deformation of nanograined polycrsytals in relation to the analysis of the available data on their mechanical properties. In doing so, it is shown in particular that the systematic transfer to nanograined polycrystal metals of concepts applied with success to coarse-grained polycrystals counterpart has no solid basis. To uncover the physics of plastic flow in bulk nanograined polycrystal metals, simple arguments and recommendations are presented and discussed.

Thermoelectric power is the magnitude of an induced thermoelectric voltage in response to a temperature difference across a material. The thermoelectric power of metal alloys is strongly influenced by the presence of solute atoms. Measuring thermoelectric power is therefore an accurate technique for characterizing the microstructure evolution of alloys taking place during various phase transformations. The potentialities of this technique are illustrated through four examples: (i) the measurement of copper solubility limit in iron; (ii) the characterization of the strain ageing kinetics of extra-mild steels; (iii) the determination of precipitation kinetics of MoC in low alloyed steels; and (iv) the monitoring of ageing of neutron irradiated power plant steels.