Volume 100, Issue 4

The paper represents a comprehensive although rather brief overview of the scientific activity of the Institute of Physical Metallurgy and Metal Physics (IMM), RWTH Aachen University, in the field of grain boundary physics for the past twenty years under the leadership of Professor Günter Gottstein.

It will be shown that there is an analogy between surfactants in liquids which stabilize structures with large surface areas such as foams or microemulsions, and certain components in a crystalline solid, which stabilize defects such as grain boundaries and dislocations. These components are then called defactants. In the presence of defactants microstructural changes may occur because some of the defects compete more successfully for segregation of the defactants than others. Thus voids are formed at grain boundaries of Cu – Bi alloys because the formation energy of voids is provided by the interaction energy of Bi-atoms of the newly created surface. Or the beneficial role of rhenium on the high temperature creep of superalloys may be interpreted as a deficiency of defactants stabilizing dislocations and/or vacancies. These examples are discussed in the present work after a general treatment of the role of defactants.

Dynamic recovery in Stage III of strain hardening of pure fcc metals has been known for decades and has been associated with dislocation cell formation and refinement in scale with increasing plastic shear resistance when this exceeds a critical and temperature-dependent threshold level. Its rate controlling process had been attributed by Seeger 50 years ago to massive cross slip of screw dislocation groups circumventing Lomer-Cottrell type linear sessile dislocations blocking the primary slip system. Consideration of this cross slip-controlled mechanism has persisted in various modified forms in the literature up to the present, even though it is not consistent with the cellular dislocation structure of Stage III, and its evolution. Here a new mechanism is proposed in which dynamic recovery occurs through the systematic removal of short Lomer – Cottrell type sessile dislocation segments, i. e. LC locks, in the cell walls by statistical processes stimulated by the impingement of glide dislocations on cell walls when the applied stress exceeds a temperature-dependent threshold shear resistance τ III .

The microyielding behaviour of the monocrystalline nickel-base superalloy CMSX-6 has been studied experimentally in tensile tests up to small strains at 900 °C, 1000 °C and 1100 °C and by transmission electron microscopy of the deformed specimens. Inspection of the microyielding stress-strain curve with highly resolved plastic strain showed four distinct stages of deformation at the lowest temperature (900 °C) but not at the two higher temperatures. By considering the superposition of the external stress with the original coherency stresses and with the deformation-induced internal stresses and by inspection of the electron micrographs, these deformation stages could be assigned to successive dislocation glide processes in the horizontal and vertical -channels and, finally, in the ′-particles.

We introduce a new material point homogenization scheme – the ‘Relaxed Grain Cluster’ (RGC) – based on a cluster of grains and formulated in the framework of finite deformations. Two variants of this scheme, which allow for different degrees of relaxation, are compared to two variants derived from the infinitesimal-strain grain interaction model regarding the evolution of texture predicted for plane-strain compression of a commercial aluminum alloy. The RGC schemes give the closest match to experimental reference on both the -fiber and the -skeleton line. The intensity of the brass texture component is found to be rather sensitive to the homogenization scheme. However, the observed decrease in texture intensity as a function of the homogenization scheme for the Cu and S component on the -skeleton line can be correlated to the number of degrees of freedom in the cluster which are left unconstrained by the respective scheme. This is in line with the significant dependence of the Cu and S component intensity on boundary conditions reported in earlier studies.

We propose a new mechanism of recrystallization of cold-deformed metals. During the cold deformation, some of the high-angle grain boundaries are moved by the applied stresses, producing low-curvature bulges and leaving almost defect-free regions behind. The driving force for this motion is very high and can be comparable to the yield stress of the material. Upon subsequent heating without applied stresses, such moved boundaries can continue to move without encountering an energy barrier (which is typical of most nucleation models) under the much smaller driving force arising from the stored energy difference across the boundary.

The influence of Mn solute atoms on sub-grain boundary mobilities in Al has been determined by accurate electron backscatter diffraction analysis of the sub-grain sizes and misorientations during recovery annealing. High purity Al-0.1 and 0.3 wt.% Mn alloys were deformed by plane strain compression at room temperature to equivalent strains of 1.8 and annealed in the temperature range 150 – 300 °C. An original method of image analysis on sub-boundaries from electron backscatter diffraction maps was applied to quantify the sub-grain size distributions. The change in average sub-grain size with time at several temperatures was then used to estimate sub-grain boundary mobilities in both Al – Mn alloys. The activation energies for sub-grain mobility were found to be 48 and 52 and kJ mol – 1 for the 0.1 and 0.3 % Mn alloys respectively, with the higher Mn alloy exhibiting lower rates. The sub-boundary mobilities are higher than expected from previous similar work on deformed Al – Si crystals. The orientation dependence of sub-grain growth is also examined.

A new model of grain boundary segregation in systems with restricted solubility is proposed. The model is based on the following assumption: As soon as the solute concentration in the grain boundary achieves the bulk solubility limit, embryos of compounds appear in the grain boundary. The composition of these embryos corresponds closely with the nearest phase in the grain according to the phase diagram. The appropriate segregation isotherm taking into account these embryos demonstrates the concentration phase transition which shows itself as a jump in grain boundary concentration. The location and the height of this jump are discussed.

The faceting of the almost stationary tilt grain boundary with a misorientation angle of 84° in the Zn bicrystal has been investigated. The shape of the very slow migrating grain boundary has been studied in situ between 350 and 400 °C using polarized light. Two intersecting facets lie in the closely-packed planes of the constrained coincidence sites lattice with coincidence parameter = 15. In the as-grown sample flat grain boundary facets form the sharp first-order ridge with the break of the first derivative y/x of the grain boundary shape similar to those observed in our previous works on GB faceting in Zn. However, above 350 °C the sharp first-order ridge is substituted by the smooth grain boundary portion without a ∂ y /∂ x derivative break similar to that observed in twin grain boundaries in Mo. The transformation of the as-grown first-order grain boundary ridge into continuous one has been observed for the first time. The critical parameter α has been calculated using the grain boundary shape in the transition region. α increases from α = 1.7 ± 0.07 for 350 °C to α = 1.9 ± 0.07 for 385 °C approaching the α = 2 value predicted in the mean-field Andreev model for the continuous surface roughening.

We consider grain boundary diffusion in a polycrystal in which normal grain growth occurs simultaneously with diffusion. We assume that any point within the diffusion zone is swept a number of times by migrating grain boundaries, and develop a model which operates with the average concentrations of the diffusant in the planes parallel to diffusion front. We derive analytical expressions describing the diffusion profiles both for constant source and instantaneous source boundary conditions. Finally, we suggest a practical method for extracting the grain boundary diffusion coefficient from experimentally measured diffusion profiles. This method relies on microstructure characterization and requires the grain growth constant and grain growth exponent to be determined in the independent set of experiments.

Linear measures, including the average caliper, and mean width, are capable of correlating the geometric properties of polyhedral grains, including their boundary areas, volumes, and face curvatures. These measures also permit prediction of curvature-mediated grain growth rates and provide an important connection between kinetic behavior, grain size, and shape. Linear measures can be calculated exactly by using a topological representation of space-filling network grains via a special set of regular polyhedra, termed average N-hedra. These calculations yield exact comparisons of linear measures with polyhedral geometric properties and growth kinetics of well-annealed polycrystalline materials subjected to normal grain growth.

The von Neumann – Mullins relation has been extended to higher dimensions by MacPherson and Srolovitz. Their exact solution relates the rate of volume change of an individual grain in a 3-dimensional isotropic polycrystal to its mean width and total length of triple lines (assuming isotropic boundaries). The objective of this study is to verify that grains in a moving finite element grain growth model obey this law. Algorithms have been developed in order to calculate mean width of individual grains in digital microstructures for which the grain structure is discretized with both volumetric and surface meshes. Theoretical rate predictions were obtained from the measured mean widths and triple line lengths. Good agreement was found between growth rates measured in the simulations and the predictions of MacPherson – Srolovitz theory for the cases of an isolated shrinking sphere, individual grains in a digitally generated coarse polycrystal, and individual grains in a microstructure reconstructed from serial sectioning of stabilized cubic zirconia. Departures from this relationship appeared to be related to the grain shape.

The grain-boundary source and sink strengths for both vacancies and self-interstitials have been determined for columnar nanocrystalline structures of Mo using molecular-dynamics simulation. The microstructures, which are initialized to contain either an over-saturation or under-saturation of one particular point defect species (either vacancies or self-interstitials), are subjected to isothermal annealing at high homologous temperatures (T > 0.75T m ) while the point-defect concentrations are monitored throughout. We clearly observe an exponential approach of the point-defect concentrations to equilibrium, in agreement with classical rate theory. These observed approaches to equilibrium yield grain-boundary source/sink strengths that correlate with analytical solutions that depend solely on the grain size. Finally, we find that grain boundary sinks have an approximately equal affinity, or bias, to vacancies and self-interstitials.

A strongly textured sheet of magnesium alloy AZ31 has been subjected to tensile testing at temperatures between ambient and 300 °C. Structures have been examined by optical and transmission electron microscopy and also by atomic force microscopy to quantify surface displacements seen at grain boundaries. Plastic anisotropy varies strongly with test temperature as was observed previously by Agnew and Duygulu. The present findings do not support the view that crystallographic <c + a> becomes a major contributor to deformation at higher temperatures. Rather, the material behaviour reflects an increasing contribution from grain boundary sliding despite the relatively high strain rate (10 – 3 s – 1 ) used in the mechanical tests.

The principles of recrystallisation texture formation upon hot rolling and annealing of industrial aluminium alloy sheets are presented. It is demonstrated how texture control is applied during industrial processing of aluminium sheet so as to improve texture-related properties. This is illustrated by way of several examples, covering high-purity aluminium sheet for electrolytic capacitors, AlMnMg alloys for packaging and AlMgSi alloys for car body applications.

The potential for decreasing the texture intensity generated during the bulk deformation of Mg alloys was investigated using a combination of contraction twinning, double (secondary) twinning and static recrystallization. A large number of twins was induced by tensile deformation at room temperature. Their volume fraction and the variants selected during straining were found to be largely responsible for the changes evident in the deformation texture. Recrystallization of the twins generated a fine-grained microstructure, although no growth into the matrix grains was observed. In this way, annealing of the deformed samples did not lead to significant further texture changes.

A study of the microstructural effects on the void initiation and crack growth in aluminium alloyed TRIP (transformation induced plasticity) steels has been conducted. Understanding of this microstructural dependency of the crack development is necessary to increase the failure limit and the energy absorption rate of the material. Different microstructures in terms of phase volume fractions and spatial arrangement of these phases have been obtained by annealing. Additionally, different austenite stabilities, both geometrically as well as chemically based, have been obtained. First the macroscopic mechanical properties are linked to the microstructure. Furthermore, different characteristic prestraining levels are subsequently correlated to the microstructural damage evolution by observation of the samples with backscatter electron diffraction-based orientation microscopy in a high resolution scanning electron microscope. The experiments result in a prediction of the most critical microstructural parameters when high damage tolerance is needed.

In the present paper we use nanoindentation to investigate two shape memory alloys, austenitic Ti 50 Ni 48 Fe 2 and martensitic Ti 50 Ni 40 Cu 10 which both show two step martensitic transformations on cooling from the high temperature regime. No such two step features were observed during nanoindentation. Load controlled nanoindentation experiments were performed using a Berkovich indenter using maximum loads ranging from 0.5 to 85 mN resulting in maximum indentation depths between 30 and 1350 nm. For small maximum indentation loads below 5 mN, longer indentation times result in larger indentation depths (austenitic Ti 50 Ni 48 Fe 2 ). At loads of about 1 mN the indentation curves show an abrupt change in slope which we suggest to be related to an interaction of the deformation and failure of a thin oxide layer and the formation of stress induced martensite (austenitic Ti 50 Ni 48 Fe 2 ) or the de-twinning of martensite (martensitic Ti 50 Ni 40 Cu 10 ). Nanohardnesses and shape recovery ratios strongly increase with decreasing indentation depths below 400 nm. For larger indentation depths, constant values are observed (austenitic Ti 50 Ni 48 Fe 2 : nanohardness 2.45 GPa/recovery ratio 0.25, martensitic Ti 50 Ni 40 Cu 10 : nanohardness 1.6 GPa/recovery ratio 0.4).

Fatigue-induced damage accumulation was investigated in a third generation titanium aluminide alloy both at room temperature and at a temperature of 700 °C promoting oxidation. The digital image correlation technique was utilized for monitoring the evolution of local strain fields with cyclic deformation at both temperatures. With the aid of a newly adopted surface patterning technique, digital image correlation successfully detected the crack initiation sites prior to the actual formation of the cracks. Despite the oxidation at elevated temperatures, digital image correlation could detect the crack initiation sites at the early stages of the cyclic deformation, laying out the potential of this technique for monitoring the damage evolution in various metallic materials under severe service conditions.

The paper addresses the failure mode of ultrafine grained magnesium alloy AZ31 under superplastic deformation conditions. The ultrafine grain structure and the resulting exceptional superplastic properties were produced by Equal Channel Angular Pressing with an imposed hydrostatic pressure conducted at a relatively low temperature of 150 °C. The mechanism of the superplastic ductility was studied by examining the fracture surfaces and internal cavities in superplastically deformed specimens by scanning electron microscopy. The presence of Mg 17 Al 12 particles was identified as the controlling microstructural feature giving rise to porosity formation and limiting tensile ductility. Fine details of the failure process depending on the particle and precipitate structure as well as textural effects were described.

The importance of magnetic effects upon mechanical properties of ferromagnetic materials, discussed by Zener almost a half century ago, has been revisited in order to stimulate researchers' interest in this research subject. It is shown that effects of magnetism and magnetic transformation can affect creep deformation, tensile deformation and ductility in α-Fe-based solid solution binary alloys in the ferromagnetic state. The magnetic alloy strengthening observed in the ferromagnetic state has been quantitatively described and discussed by using a new parameter related to magnetic properties of solute atoms such as Co, Cr and solvent Fe. The prospect of future application of the magnetic strengthening is given for high temperature materials with excellent strength, ductility and thermal stability.