Band 100, Heft 11

The atomistic modelling of materials with effective model potentials requires a reliable description of the breaking and making of interatomic bonds in different atomic environments. The bond-order potentials provide such a transferable description of atomic bonding while at the same time they are computationally efficient for application in large-scale atomistic simulations. We give an overview of the fundamentals of bond-order potentials and their derivation from the tight-binding electronic structure by linking the atomic structure to the electronic structure. We discuss the application of the structural energy difference theorem for studying trends in crystal phase stability and provide a brief summary of representative examples for modelling metals, hydrocarbons, and semiconductors with analytic and numerical bond-order potentials.

The investigation of segregation and embrittlement at grain boundaries in metals is a research area with a long tradition. In recent times, the feasibility of computational studies on the atomic scale gave a new impetus to the community. In this feature article we summarize our contributions to the understanding of segregation of interstitial impurities and embrittlement at grain boundaries in the bcc transition metals against the background of research activities in the field. We mainly discuss the benefits and limitations of ab-initio electronic structure calculations.

A brief review of modeling and simulation methods for a study of polymers at interfaces is provided. When studying truly multiscale problems as provided by realistic polymer systems, coarse graining is practically unavoidable. In this process, degrees of freedom on smaller scales are eliminated in favor of a model suitable for efficient study of the system behavior on larger length and time scales. We emphasize the need to distinguish between dynamic and static properties regarding the model validation. A model which accurately reproduces static properties may fail completely when it comes to the dynamic behavior of the system. Furthermore, we comment on the use of the Monte Carlo method in polymer science as compared to molecular dynamics simulations. Using the latter approach, we also discuss results of recent computer simulations on the properties of polymers close to solid substrates. This includes both generic features (as also observed in the case of simpler molecular models) as well as polymer specific properties. The predictive power of computer simulations is highlighted by providing experimental evidence for these observations. Some important implications of these results for an understanding of mechanical properties of thin polymer films and coatings are also worked out.

In this article we present a comparative analysis of different versions of the quasicontinuum method, which aim at a seamless transition from the atomistic to the continuum description of crystalline solids at zero temperature. All versions of this popular and powerful method exhibit the same building blocks, namely (i) a coarse-graining of fully atomistic resolution via kinematic constraints, (ii) an approximation of the energy/forces in coarse-grained regions via numerical quadrature and (iii) adaptive mesh refinement. The quasicontinuum versions are assessed in an example where a Lomer dislocation dipole is subject to shear deformation. In a second example, the fully nonlocal quasicontinuum method is used to simulate nanoindentation into an fcc single crystal. Compared with lattice statics good agreement is achieved with respect to significant details of the material behaviour for a small fraction of the computational costs.

Well–designed cementite–free bainitic steels are important in contributing to unique combinations of strength, toughness and cost. We examine here the relative effects of molybdenum and boron on the kinetics of transformation of austenite particularly into allotriomorphic ferrite and bainite. There are some surprising results on the role of boron, which is found in some circumstances to accelerate the transformation to allotriomorphic ferrite. This, and other features of transformation behaviour are assessed in the context of phase transformation mechanisms.

Thermodynamic calculations in the ZrO 2 –La 2 O 3 –Y 2 O 3 system were used to select compositions for experimental investigations. The selected compositions were investigated using X-ray diffraction, scanning electron microscopy combined with energy dispersive X-ray spectroscopy and differential thermal analysis after prolonged heat treatment at 1250, 1400 and 1600 °C. The X-ray diffraction investigations indicated stability of the La 2 O 2 –A + (La,Y) 2 O 3 –B + La 2 Zr 2 O 7 and (La,Y) 2 O 3 –B + La 2 Zr 2 O 7 + LaYO 3 -perovskite phase fields at 1250 °C, which is contrary to results found in the literature. The differential thermal analysis of the ZrO 2 -45.8La 2 O 3 -20.8Y 2 O 3 composition heat treated at 1250 °C showed no transformations up to 1500 °C. Two peaks were indicated at temperatures above 1450 °C: the first peak at 1530 °C could be attributed to the processes of Y 2 O 3 dissolution and disordering in the pyrochlore phase and the second peak at 1580 °C to transformation of perovskite to monoclinic B phase. The solubility of Y 2 O 3 in pyrochlore phase occurs by Y +3 substitution of both Zr +4 and La +3 ions.

Surface tension data of liquid Al–Cu binary alloys have been measured contactlessly using the technique of electromagnetic levitation. A digital CMOS-camera (400 fps) recorded image sequences of the oscillating liquid sample and surface tensions were determined from analysis of the frequency spectra. Measurements were performed for samples covering the entire range of composition and precise data were obtained in a broad temperature range. It was found that the surface tensions can be described as linear functions of temperature with a negative slope. Moreover, they monotonically decrease with an increase in the aluminium concentration. The observed behaviour with respect to both temperature and concentration is in agreement with thermodynamic model calculations using the subregular solution approximation.

Shaped metal deposition is an innovative technology, which creates near-net shaped components by weld deposition and saves time and material due to the absence of further machining. For Ti alloys this is a big advantage, since these alloys are difficult to shape and also very expensive. Ti alloys are very sensitive to the thermal history and shaped metal deposition introduces for each welding step an additional temperature gradient leading to different microstructures in different regions. Regions which were in the single β phase field during the last welding show a fine needle like microstructure, while regions within the α + β phase show coarser lamellae forming a Widmanstätten structure.

A model based on grain/interphase boundary sliding controlled flow that develops to a mesoscopic scale (defined to be equal to or more than a grain diameter) has been used to understand superplasticity in micrometer- and sub-micrometer grained intermetallics. The procedure for analyzing the experimental stress–strain rate data at a fixed initial grain size and temperature is so developed that the free energy of activation for the rate controlling deformation process, and the long range threshold stress which has to be overcome for boundary sliding to develop to a mesoscopic scale, σ 0disloc , are estimated directly from the experimental results. The analysis is validated using experimental data pertaining to a number of systems. Directions for further development of the model are indicated.

Ba(Cd 1/3 Nb 2/3 )O 3 powder with an octahedral shape stacked by small particles was successfully synthesized by the molten salt synthesis method at a low temperature of 850 °C, which was much lower than the columbite method, two-step solid-state reaction process. Thermogravimetry and differential thermal analysis curves combined with X-ray diffraction analysis determined the phase evolution of Ba(Cd 1/3 Nb 2/3 )O 3 powder at relative high temperature. Furthermore, Raman results showed that the Ba(Cd 1/3 Nb 2/3 )O 3 powder had a trigonal order structure. It was indicated that the “template formation mechanism” and crystal habit growth which obeyed the model of anion coordination polyhedral, play an important role in the formation and growth processes. The resulting ceramics exhibited favorable dielectric properties.

The strings of musical instruments are frequently made of stainless steel wires. Samples taken from consecutive steps of wire drawing showed the kinetics of deformation-induced martensite formation from metastable austenitic feedstock. Stacking fault and twin formation were described by transmission electron microscopy. The progress of martensitic transformation was accompanied by an increase in defect density and in micro-strains determined by synchrotron X-ray diffraction. The initial fall in the Young's modulus determined by tensile tests was accompanied by a drastic reduction in domain size and in grain diameter. Texture and phase transformation at the wire surface were measured by X-ray diffraction. The deformation-induced martensite formation has been described in its nucleation, growth and saturation stages. These three stages were defined by the correlation of the evolution of the microstructural features in the course of the wire drawing process.

Reverted austenite strongly influences the mechanical properties of maraging steels. Therefore, X-ray diffraction using high-energy synchrotron radiation was applied for in-situ studying of the reverse martensitic phase transformation under external load in a PH 13-8 Mo maraging steel. The volume fraction of austenite, the domain size of the crystallites, and the lattice parameters were determined as function of strain for differently aged samples. It is shown that the reverted austenite is not mechanically stable under external load. Scanning of the fractured sample along the axial direction shows that the volume fraction of the austenite and the domain sizes strongly depend on the distance from the point of fracture initiation.

The strain hardening of the Al-5.2Zn-1.2Mg alloy AA7030 in different conditions has been investigated. The dislocation related strengthening was described by the Voce equation and the quantitative effects of different superposition laws have been studied. All tempers, except the over-aged, suffer from plastic instability before the geometric Considère condition is reached. This was attributed primarily to the occurrence of shear failure due to dynamic strain ageing effects. For plastic strain levels below approximately 8 %, preceding the plastic instability, the results show that the saturation stress in the under and peak-aged tempers is independent of heat treatment, whereas the kinetics of structure formation differs. Using a linear superposition law in the under and peak aged tempers has negligible effects, whereas the use of Pythagorean addition for the over-aged temper significantly decreases the goodness-of-fit, although not to unacceptable levels. Furthermore, the naturally aged temper shows strong influence of dynamic precipitation.

Mechanical alloying of both steel strip and elemental Zn powder (with or without Fe powder) was performed in order to directly produce zinc based coatings on the surface of steel strip. Microstructures of coatings and milled powders were investigated using X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. Metastable phases, in particular Zn supersaturated with Fe were synthesized during ball milling. Iron supersaturation in FeZn alloy is almost twice as high (4.5 at.% Fe in Zn) on the milled steel strip as in the ball milled powder (2.8 at.%). The latter values remain lower than the maximum Fe solubility in Zn allowed by thermodynamics. Subsequent annealing at 300 °C forms a Γ 1 monophased coating which is not usually synthesized during batch galvanizing.

The Flory's approximate model, and the model of Prasad et al. have been used respectively to study size effects on the bulk and surface properties of Bi–Zn, Cu–Pb, K–Pb and K–Tl liquid alloys. Our results reveal that for the same value of order energy, W/T: the effect of size on the bulk properties is to reduce the tendencies toward phase separation in Bi–Zn and Cu–Pb while in K–Pb and K–Tl is to decrease the tendencies toward chemical ordering; size effect increases both the surface concentration and the surface tension of Bi–Zn and Cu–Pb while it decreases these same quantities in K–Pb and K–Tl.

The microstructure and mechanical properties of 0.15 at.% Hf and 0.2 at.% Ho doped Ni-33Al-28Cr-6Mo near-eutectic alloys prepared by conventional casting and suction casting were investigated. The results reveal that the addition of Hf and Ho results in the formation of Ni 2 AlHf and Ni 2 Al 3 Ho phases, respectively, along the NiAl and Cr(Mo) phase interface in intercellular regions. Compared with the conventional-cast alloy, the microstructure of suction-cast alloy is well optimized, characterized by the thin interlamellar spacing, high proportion of eutectic cell area and fine homogeneously distributed Ni 2 AlHf and Ni 2 Al 3 Ho phases. Furthermore, the room temperature mechanical properties of the suction-cast alloy improve significantly, which can be attributed to fine microstructure, uniform distribution of the Ni 2 AlHf and Ni 2 Al 3 Ho phases and solid solubility extension.

The failure of a rotating axial flow fan of a 123 MW electric power generator unit is analyzed. The fan was mounted on the generator-rotor at the turbine end. Initial investigations showed that three fan blades had fractured just about 11 h after resuming operation following the last overhaul, causing extensive damage to the unit. The failure of the blades was investigated using fractographic and microstructural characterization techniques as well as mechanical evaluations. Based on fractographic observations high cycle fatigue was identified as the failure mechanism. Formation of multiple primary cracks which coalesced during crack growth to form shallow and semi-elliptical crack geometry was indicative of the influence of additional bending stresses, probably resulting from excessive vibrations. Using fracture mechanics principles, the magnitude of the maximum stress at the time of final fracture of the first broken blade was estimated to be more than 3 times higher than the design operating stress. Furthermore, crack growth lifetime of the blades was evaluated using FRANC3D software. It was found that an operating period of just about one hour was required for small initial cracks to propagate to final failure under the action of excessive bending stresses.