Volume 94, Issue 10

Over a period of time, what started off as a phenomenological analysis for structural superplasticity in micro-grained materials has been developed into a mechanistic model and later extended to explain the quasi-viscous deformation of nanostructured materials. Regardless of the number of grain sizes, temperatures and stresses employed in the stress–grain size–temperature–strain rate space, it is now possible to account for the viscous deformation of structural superplastics and nanostructured materials in the optimal range (defined as the range from the slowest strain rate to the point of inflection on the sigmoidal log stress – log strain rate iso-thermal, iso-structural plot) with the help of 5 empirical constants. Of these, four constants can be calculated, in principle, ab initio .

Based on a generalization of a capillary equation for solids, we develop a method for measuring the absolute value of grain-boundary stress in polycrystalline samples having a large interface-to-volume ratio. The grain-boundary stress in nanocrystalline Pd is calculated from X-ray diffraction measurements of the average grain size and the residual-strain-free lattice spacings, yielding a value of 1.2 ± 0.1 N /m. The random distribution of crystallite orientations in the sample in conjunction with calorimetric data for the area-averaged interfacial energy and knowledge of the grain-boundary misorientation distribution function suggest that this value is characteristic of random high-angle grain boundaries in Pd.

Fatigue is due to accumulation of undesired defects by repeated loading cycles. Classical fatigue leads to the formation of localised strain and nucleation and propagation of cracks. In shape memory alloys, there are additional fatigue phenomena: 1. Thermal cycling plays a role mainly for the two-way effect; 2. Pseudo-elastic fatigue can precede classical fatigue. Pseudo-elastic fatigue consists in deterioration of the hysteresis and consequently the loss of damping ability and transformability. It is related to the localisation of the martensitic transformation reaction.

Experimental results are presented concerning the influence of primary intercrystalline defects, i. e., grain boundaries and triple junctions, on the properties of nanocrystalline nickel and its alloys. The properties considered are largely based on nanostructured materials relatively free of secondary defects introduced during synthesis, such as porosity, residual amorphous regions, lattice dislocations and extrinsic grain boundary dislocations. These property studies include thermal stability and grain growth, mechanical, electrical, corrosion and hydrogen transport.

The present paper reports on a combined analytical and structural study of nanocrystalline Fe 90 Zr 7 B 3 by means of positron annihilation, (analytical) high-resolution transmission electron microscopy (HRTEM), and X-ray diffraction. Particular focus is laid on the chemical nature of the intergranular amorphous matrix which occurs between the α-Fe nanocrystallites. Energy-dispersive X-ray measurements (EDX) with an electron nanobeam reveal an increased Zr content at the interface between the nanocrystallites and the intergranular amorphous phase. According to positron lifetime measurements, the intergranular amorphous phase and the interfaces between this phase and the nanocrystallites exhibit structural free volumes of the mean size slightly smaller than a lattice vacancy as in the amorphous precursor material. Coincident Doppler broadening measurements of the positron-electron annihilation photons show that the fraction of Zr in the neighborhood of the structural free volumes is higher in nanocrystalline Fe 90 Zr 7 B 3 than in the amorphous state indicating an enhanced Zr concentration in the interfaces. These results are in good agreement with the HRTEM/ EDX studies and demonstrate the potentials of the coincident Doppler broadening technique for a chemical characterization of structurally complex materials on an atomistic scale.

Ultrafine-grained (UFG) metals produced by techniques of severe plastic deformation, such as equal channel angular pressing (ECAP), exhibit extraordinary strength properties. However, in the as-ECAP-processed state, the heavily deformed microstructure of such UFG metals is rather unstable and is prone to undergo grain coarsening (recrystallization) at moderate temperatures. This microstructural instability is enhanced in the presence of modest mechanical stressing as, for example, in cyclic deformation. Thus, all measures to enhance the thermal stability are also considered as beneficial for the improvement of the mechanical stability. One main objective of the present work is to analyse the thermal and mechanical stability of ECAP-processed metals during specific annealing and cyclic deformation tests. As a by-product, some conclusions relating to the separate effects of dislocation density, grain size (in the UFG regime) and internal stresses on the (micro)yielding behaviour will be drawn. Another goal is to explore the potential of different annealing treatments with respect to the stabilization of the microstructure and the optimization of the mechanical properties of ECAP-processed UFG metals in terms of an optimal combination of strength and ductility. In order to demonstrate the potential and the limitations of this approach, experimental work performed on UFG copper, aluminium and α-brass produced by ECAP will be reported and discussed. The results presented indicate strongly that a heat treatment leading to a bimodal grain size distribution provides the best compromise between strength and ductility.

The synthesis of nanoparticles and nanopowders is in principle not difficult. Consequently, various methods on a laboratory and industrial scale have been developed in the last century. However, in the last decades it became obvious that the control of synthesis and processing is required to develop nanocrystalline materials with novel functional properties leading to specific applications. The potential of nanostructures was recognized through the pioneering work of Gleiter in the early 80’s [1]. In this review, we show that chemical vapour synthesis allows the production of nanostructured materials with tailored microstructures and properties.

It is widely accepted that for the very smallest grain sizes (typically below 20 – 30 nm), dislocations play no significant role in the deformation of nanocrystalline materials. However, the grain-boundary mechanisms responsible for the reported decrease in strength with decreasing grain size in this regime (the ‘inverse Hall–Petch effect’) remain unclear. Here, we demonstrate by molecular-dynamics simulation that, in the absence of both grain growth and any dislocations, nanocrystalline fcc metals deform via a mechanism involving an intricate interplay between grain-boundary sliding and grain-boundary diffusion. By quantitatively reproducing the well-known Coble-creep formula for coarse-grained materials, we show that the ‘inverse Hall–Petch effect’ arises from sliding-accommodated grain-boundary diffusion creep. Previous, apparently contradictory, suggestions that GB sliding, on the one hand, or GB-diffusion creep, on the other, are responsible for this behavior can thus be reconciled as originating from one and the same deformation mechanism. We discuss the reasons why we believe that these simulations also capture the room-temperature deformation behavior of nanocrystalline fcc metals in the absence of dislocation nucleation and microcracking.

Inert gas condensation of metallic and covalently bonded nanoparticles has been investigated using molecular-dynamics computer simulations. Using Ge as an example, the different phases of particle growth, nucleation, monomeric growth, and cluster aggregation, have been identified and the kinetics of each described. In addition, the evolutions of the morphologies of the different types of nanoparticles have been studied. It is shown that while covalently bonded nanoparticles tend toward a ramified structure, metallic nanoparticles remain compact, owing to deformation in the crystallized state. Finally, the strong influence of surface segregation on the structure of alloy nanoparticles is illustrated using a model system.

In this paper, suggestions from molecular dynamics on the plastic deformation mechanism of nanocrystalline (nc) fcc metals are discussed. Investigation of the local average stress in the grain boundaries during deformation highlights the role of the non-equilibrium grain boundary structure in both inter- and intra-deformation processes. The relevance of the mechanism suggested by computer simulations is discussed in terms of the inherent restrictions of the technique and experimental observations.

Under intense deformation of metallic multilayer samples, a nanometer-scale layer thickness and grain size develops during repeated cold-rolling. Along with the evolution of the highly refined microstructure, a nanoscale interfacial alloying occurs that can result in an amorphization reaction. The deformation of multilayers exhibits driven system behavior during alloying. As the length scale of the layer thickness converges to the length scale of the mixing zone during rolling, amorphization develops in appreciable volumes. The results from selected experiments demonstrate that the relative specific interfacial area is the key microstructural metric to describe the deformation-driven alloying.

The current research on grain boundary dynamics in metals is reviewed. The boundary dynamics, i. e., the boundary reaction to applied forces, is strongly dependent on the grain boundary character, which is commonly reduced to the orientation relationship between adjacent grains and impurity segregation at the boundary. The misorientation dependence of the motion of specific capillary driven grain boundaries, the effect of inclination on the mobility of planar boundaries and the dislocation absorption by moving boundaries are considered. The role of segregation behaviour and boundary crystallography for the orientation dependence of boundary mobility is addressed. The compensation effect for the motion of structurally different grain boundaries is discussed.

The effects of wet-milling mixtures of α-Fe 2 O 3 and cobalt hydroxide Co(OH) 2 over a range of Co/Fe ratios for 215 h have been investigated by neutron diffraction and Mössbauer spectroscopy. The starting materials were mixed according to the stoichiometric formula (Co x Fe 1 – x ) 3 O 4 for values of x = 0.037, 0.071, 0.133, 0.234 and 0.380 (i. e., from ≈ Co 0.1 Fe 2.9 O 4 to the cobalt spinel CoFe 2 O 4 ). These studies reveal the formation of a nanostructured, mixed Co–Fe spinel phase with non-stoichiometric composition (Co x Fe 1– x ) 3 – y O 4 ; the defect spinels have refined values x c = 0.04, 0.08 and 0.14 for the mixtures with the lowest Co content ( x = 0.037, 0.071 and 0.133) and defect concentrations in the range y ≈ 0.1 –0.2. Both the spinel phase and un-reacted α-Fe 2 O 3 are found to occur in the neutron diffraction patterns and Mössbauer spectra for the high Co content mixtures x = 0.234 and x = 0.380. Rietveld refinements of the neutron data indicate that the Co atoms predominantly occupy the octahedral B sites with vacancies also found to be located on the octahedral B sites. Analyses of the Mössbauer spectra of the milled samples confirm the existence of vacancy defects in the B sites and reveal that the vacancies cause similar effects to those of the Co ions, leading to a higher average charge state per iron atom.

Mechanical attrition, mechanical alloying and other methods of extreme plastic deformation (high pressure torsion, equal channel angular pressing) have been developed as versatile alternatives to other physical and chemical processing routes in preparing nanophase materials. Here several examples are discussed including the deformation-induced nanophase formation in powder particles, in thin-foil sandwich structures and at the surface of alloys exposed to friction-induced wear, leading to the formation of nanocrystals and, in some cases, amorphous nanostructures. This opens exciting perspectives in preparing nanostructured materials with a number of different interface types in terms of structure (crystalline/crystalline, crystalline/amorphous) as well as atomic bond (metal/metal, metal/semiconductor, metal/ ceramic etc.). It is expected that the study of nanostructure formation by mechanical means in the future not only opens new processing routes for a variety of advanced nanophase materials but also improves the understanding of technologically relevant deformation processes on a nanoscopic level.

A nanostructured surface layer was produced on an AISI 304 stainless steel plate by means of a surface mechanical attrition treatment (SMAT). Low-temperature plasma nitriding of the SMAT stainless steel sample was investigated in comparison with the coarse-grained sample by using structural analysis (X-ray diffraction, scanning electron microscopy, and transmission electron microscopy) as well as mechanical property measurements. It shows a much thicker nitrided layer is formed on the SMAT sample relative to that in the coarse-grained counterpart nitrided under the same condition. The nitrided layer in the SMAT sample is composed of nanostructured austenite expansion and martensite expansion with high supersaturations of nitrogen. The surface hardness and the hardened surface layer thickness, as well as the abrasive and the adhesive wear resistances of the nitrided SMAT sample are much enhanced relative to the nitrided coarse-grained sample.

In this paper, we review in view of their potential but also from the aspect of being potentially the most interesting examples of the non-intuitive composite effects, which provides an impression of the dynamic evolution and the future opportunities of non-intuitive composite effects, with many promising applications.

Triple junctions of grain boundaries, being line defects themselves, can contain other line defects – dislocations and disclinations. Geometrical properties of such line defects are considered in this paper. While some extrinsic defects can be brought into or generated in the junction, others are intrinsic to the triple junction, satisfying requirements of equilibrated structure.

The interface effect on Young’s modulus was investigated in electro-deposited fully-dense Ni –P alloys with a relatively constant phosphorus content (2– 3 wt%), but with different grain sizes ranging from 4 to 29 nm. Essentially the same Young’s modulus was observed for grain sizes ≥ 18 nm. A noticeable decrease in Young’s modulus was found at grain sizes ≤ 17 nm. The reduction in Young’s modulus was found to correlate well with the increase in all interface contributions. These observations agree with various studies on other fully-dense metals for grain sizes between 5 and 80 nm. Previously reported large decreases in the Young’s modulus were likely caused by the significant amount of porosity in the microstructure.

The peculiarities of the kinetics of phase formation in an ultra-thin (≈ 10 nm) crystalline layer, which is formed from two components on the surface of a substrate, have been studied theoretically. It has been shown that by varying the conditions during the growth of the layer (the temperature, the vapour pressure, the type of substrate) it is possible to control the phase composition of the layer.