Horst Biermann

FE investigation of the effect of particle distribution on the uniaxial stress–strain behaviour of particulate reinforced metal-matrix composites

Abstract A multi-particle 2D finite element model of a 20% particulate reinforced metal-matrix composite was developed on a statistical basis taking into account the correlations between the position, size and orientation of the ceramic particles in the matrix. The stress–strain curves in tension and compression given by the clustered multi-particle model are compared with the curves obtained from one-particle unit cell simulations. It is shown that clustering of particles increases the plastic strain accumulated in the matrix leading to a higher strain hardening and thus to a higher flow stress. The size of the representative volume element (RVE) should be at least equal to the correlation length of the geometrically relevant correlation functions, which was ∼2.4 times larger than the average interparticle distance for the experimentally studied case. Reasonable agreement is obtained between computed residual strains and data available in the literature.

Application of in situ thermography for evaluating the high-cycle and very high-cycle fatigue behaviour of cast aluminium alloy AlSi7Mg (T6)

The present paper illustrates the application of infrared thermal measurements for the investigation of crack initiation point and crack propagation in the high-cycle and the very high-cycle fatigue range of cast AlSi7Mg alloy (A356). The influence of casting defects, their location, size and amount was studied both by fractography and thermography. Besides internal and surface fatigue crack initiation as a further crack initiation type multiple fatigue crack initiation was observed via in situ thermography which can be well correlated with the results from fractography obtained by SEM investigations. In addition, crack propagation was studied by the development of the temperature measured via thermography. Moreover, the frequency influence on high-cycle fatigue behaviour was investigated. The presented results demonstrate well that the combination of fractography and thermography can give a significant contribution to the knowledge of crack initiation and propagation in the VHCF regime.

Fracture behaviour of ultrafine-grained materials under static and cyclic loading

Abstract Fracture-mechanics experiments were carried out on ultrafine-grained (UFG) samples of copper, titanium and an Al alloy to obtain the fracture behaviour under static and cyclic loading. The UFG-materials investigated show crack resistance behaviour under static loading, which was confirmed by ductile fracture surfaces. Under cyclic load the crack growth rate was described well by the ESACRACK model.

Influence of reinforcement morphology and matrix strength of metal-matrix composites on the cyclic deformation and fatigue behaviour

Abstract The cyclic deformation behaviour of three metal–matrix composites, namely AA6061-T6 reinforced with 20 vol.% alumina particles and short-fibres, respectively, and pure aluminium reinforced with 20 vol.% short-fibres, has been investigated at temperatures between T =−100°C and T =300°C in total strain controlled symmetrical push–pull fatigue tests. The cyclic stress response exhibits initial cyclic hardening, subsequent saturation and cyclic softening, depending on the test parameters for temperatures lower than T =150°C. Initial cyclic hardening is less pronounced with increasing temperature and decreasing applied strain amplitude. Short-fibre reinforced composites — both with alloyed and unalloyed aluminium matrix — harden cyclically more than the particulate-reinforced composite. The comparison of the cyclic with monotonic stress–strain curves indicates that, depending on the testing conditions, both cyclic hardening and cyclic softening can occur.

Measurement of local elastic strains in the single-crystal nickel-based superalloy CMSX-6 by convergent-beam electron diffraction

Abstract High spatial resolution convergent beam electron diffraction (CBED) was used in this study as a powerful tool to investigate the local lattice parameters (and elastic strains) in the γ and γ′ phases of the single-crystal nickel-based superalloy CMSX-6 at room temperature, before and after high-temperature creep deformation in tension. With the aid of a newly implemented improved fast evaluation procedure, it was possible to determine the elastic strains in a large number of positions in the microstructure. The experimental results are in global agreement with earlier work but more specific with regard to details. Thus, it is confirmed that compressive elastic stresses exist in both horizontal and vertical γ channels in the initial state and that, after tensile creep at 980°C, in agreement with an earlier dislocation model, the elastic strains in the horizontal γ channels change from the compressive to a tensile state, acting perpendicular to the deformation axis, while the strain state in the vertical channels is only modified quantitatively. As a new result, it is noted that while the elastic strain in γ′ particles is initially homogeneous, an inhomogeneous distribution of the elastic strains in the γ′ phase is observed after high-temperature creep. The findings reported demonstrate the capability of the improved time-saving CBED evaluation procedure with respect to the reliable determination of local lattice parameters in fine-scale microstructures.

Case studies on the application of high-resolution electron channelling contrast imaging – investigation of defects and defect arrangements in metallic materials

In 1967, Coates discovered the electron channelling contrast of backscattered electrons (BSEs) in scanning electron microscopy, and by this the possibility to investigate arrangements of lattice defects in deformed microstructures of materials. Since that time, a straightforward development of the scanning electron microscopes as well as of the electron channelling contrast technique took place. Nowadays, the performance of scanning electron microscopes is high enough that the resolution of electron channelling contrast imaging (ECCI) micrographs is comparable with conventional bright field transmission electron microscopy (TEM) micrographs. In the first part of the present paper, a historical review on the development of the ECCI technique starting from its discovery more than 45 years ago up to the combination with other advanced methods of scanning electron microscopy like electron backscatter diffraction or high-resolution selected area channelling patterning in the last few years is given. Major important investigations using this technique for the visualization of individual lattice defects like stacking faults (SFs) and dislocations or dislocation arrangements are chronologically summarized. The second part demonstrates that nowadays, ECCI micrographs taken in high-resolution scanning electron microscopes can be called high-resolution ECCI (HR-ECCI). It is shown that the resolution of individual SFs and dislocations in the HR-ECCI micrographs is comparable to that of conventional TEM (about 15 nm defect image width). Furthermore, the paper is demonstrating that HR-ECCI micrographs can be obtained for various types of materials after different mechanical loadings and different grain sizes ranging from large grain size of 500 μm (cast steel) down to less than 2 μm (γ-TiAl).

Nanoindentation measurements on deformation-induced α’-martensite in a metastable austenitic high-alloy CrMnNi steel

The hardness of deformation-induced α’- martensite and parent austenitic matrix in high-alloy CrMnNi steel was investigated by nanoindentation measurements inside scanning electron microscope using picoindenter. After the indentation, the microstructure was investigated by electron backscattered diffraction measurements. The hardness values for α’-martensite are only 24% higher than those of austenite. Thus, the increase in strength during the formation of deformation-induced α’-martensite is rather caused by the small grain size of α’-nuclei resulting in a dynamic Hall–Petch effect than by its “intrinsic” hardness.

The influence of the free surface on the fracture of alumina particles in an Al–Al2O3 metal–matrix composite

Abstract The influence of the free surface on the load carried by elastic inclusions embedded in an elastic–plastic matrix was investigated by the finite element method. A 2D multi-particle model was developed based on the statistical evaluation of the microstructure of a commercial metal–matrix composite. The results of simulated tensile tests show that the average particle stresses are sensitive to the adopted boundary conditions. Inclusions near a free boundary carry in general smaller loads than the inclusions in the bulk. Damage predictions of the model are in good agreement with the experimental data obtained by X-ray microtomography.

Influence of graphite spherical size on fatigue behaviour and fracture toughness of ductile cast iron EN-GJS-400-18LT

Abstract The employment of ferritic nodular cast iron for components subjected to high stress requires a detailed fracture mechanics evaluation of crack growth under static and cyclic loading. During operation, for instance, in wind power plants, such component parts are subjected to loading of variable amplitude, which influences their lifetime considerably. For the evaluation of crack propagation and the remaining service life in this case, the calculation methods currently well-established in practice cannot be employed for cast iron with nodular graphite, since overloads lead to microstructure-related and material-specific load history effects in terms of crack growth acceleration. In this work, investigations of crack growth under constant and variable amplitude loading as well as static fracture toughness investigations and strain-controlled cyclic deformation experiments are presented.

EB Surface Alloying and Plasma Nitriding of Different Al Alloys

Within the last years, considerable progress was achieved in the research field of plasma nitriding of Al alloys. However, due to large property differences between the very hard AlN layer and the soft Al matrix material the load capacity of the nitride layer is limited. Electron beam (EB) surface alloying modifies the chemical composition of the area near the surface up to a certain depth. This, for instance, results in high hardness levels, and therefore this layer acts as support for the hard and wear-resistant thin AlN layer generated by plasma nitriding. In the present study, surface modifications produced by a combination of EB alloying with Fe based additives and plasma nitriding of wrought, cast and spray-formed Al alloys were investigated. After the EB treatment the layers were examined regarding their influence on the structure, the nitride layer growth mechanism, the effect of the EB layer for the support of the AlN layer and the resulting duplex layer properties, e.g. hardness and wear behaviour.