Christian Motz

Deformation behaviour of closed-cell aluminium foams in tension

Abstract The mechanical behaviour of commercially available ALPORAS aluminium foam with two different densities was studied under tension loading. In addition to the common stress–strain measurements, local deformation, notch-opening displacement and damage evolution were determined. The deformation characteristics deviated from those observed in aluminium foams under compression. No deformation bands or plastic instabilities could be observed in tension, which are very frequent in compression of metallic foams. Four regimes were evident in the stress–strain curves and deformation maps: the linear elastic regime, the plastic regime with no significant crack initiation and propagation, the regime of formation of a fracture process zone and, finally, the regime of fracture, where a main crack propagates through the specimen and leads to failure. The fracture strain was only a few per cent, with the higher-density foam showing a larger fracture strain, and the plastic Poissons ratio was about 0.35. The notched specimens showed increasing fracture strengths in terms of the net section stress with increasing notch depth. It is suggested that a change in stress state, caused by a non-vanishing Poissons ratio, in front of the notch tip creates an increase of the fracture strength similar to the behaviour in ductile bulk metals.

Characterization of the fracture toughness of micro-sized tungsten single crystal notched specimens

Fracture experiments using micrometer-sized notched cantilevers were conducted to investigate the possibility of determining fracture mechanical parameters for the semi-brittle material tungsten. The experiments were also used to improve the understanding of semi-brittle fracture processes for which single crystalline tungsten serves as a model material. Due to the large plastic zone in relation to the micrometer sample size, linear elastic fracture mechanics is inapplicable and elastic-plastic fracture mechanics has to be applied. Conditional fracture toughness values J Q were calculated from corrected force vs. displacement diagrams. Crack growth was accessible by direct observation of in-situ experiments as well as with the help of unloading compliances. As a further tool, fracture toughness can be determined via crack tip opening displacement. The micro samples behave more ductile and exhibit higher fracture toughness values compared to macro-sized single crystals and fail by stable crack propagation.

Yield stress influenced by the ratio of wire diameter to grain size – a competition between the effects of specimen microstructure and dimension in micro-sized polycrystalline copper wires

Polycrystalline copper wires with diameters of 25, 30 and 50 µm were annealed at temperatures between 200°C and 900°C, resulting in different microstructures with ratios of wire diameter to grain size between 1.1 and 15.6. The microstructure evolution and tensile behavior were studied systematically. In comparison with experimental data available in the literature, the results revealed that the tensile yield stresses of these micro-sized wires are influenced not only by the grain size but also by the ratio of wire diameter to grain size. This is clearly seen when comparing identical grain sizes but different wire diameters where thinner wires reveal smaller flow stress values. A model is proposed to explain the ‘smaller is softer’ phenomenon, taking into account the higher strengthening effect of grain boundaries compared to the free surface.

Fracture behaviour and fracture toughness of ductile closed-cell metallic foams

Standard fracture mechanics tests were carried out on two different types of aluminium foam, ALPORAS® foams and ALULIGHT® foams, with a variety of densities. Standard fracture toughness tests on compact tension (CT) specimens with widths from 50 mm to 300 mm and in situ tests in the scanning electron microscope were performed. Fracture toughness values in terms of the critical stress intensity factor, KIC, the critical J-integral, JIC, and the critical crack-tip opening displacement, COD5,i, were determined. To identify the fracture process, local deformation measurements were performed on the foam surfaces with a digital image processing system. From the deformation measurements it is evident that the deformation is strongly localised on different length scales. A relatively large fracture process zone, 6–8 cells in height, is developed, where only few of them are heavily deformed. On the cell wall level the deformation is again strongly localised to the thinnest parts of the cell wall, where cracks initiate and propagate. The crack propagates through the foam, building many secondary cracks and crack bridges. The comparison of K vs. Δa (crack extension), J vs. Δa and COD vs. Δa with the current fracture processes at the crack tip and the load–displacement response reveals that COD gives the most reliable measured values to characterise the fracture toughness.

Overview on established and novel FIB based miniaturized mechanical testing using in-situ SEM

Abstract Probing mechanical properties in the micrometer regime is of current interest in materials science. A focused ion beam microscope was employed to fabricate miniaturized specimens, while an indenter installed in a scanning electron microscope was utilized to actuate the samples and record the load and displacement data during the deformation. Examples for miniaturized compression, tension, bending, as well as newly developed bending fatigue and bending fracture experiments are presented, demonstrating the unique flexibility of in-situ mechanical testing in the scanning electron microscope at small length scales.

Dislocation storage in single slip-oriented Cu micro-tensile samples: new insights via X-ray microdiffraction

Synchrotron X-ray microdiffraction was used to characterize the deformation structure of single crystalline Cu micro-tensile specimens which were oriented for single slip. The 3-µm thick samples were strained in situ in a scanning electron microscope (SEM). Electron microscopy observations revealed glide steps at the surface indicating single slip. While the slip steps at the surface must have formed by the predominant activation of the primary glide system, analysis of Laue peak streaking directions revealed that, even at low strains, dislocations had been activated and stored on an unpredicted slip system. Furthermore, the µLaue scans showed that multiple slip takes over at a later state of deformation.

Tensile behaviour of micro-sized copper wires studied using a novel fibre tensile module

Abstract Tensile experiments on micro-sized polycrystalline copper wires with diameters ranging from 14 μm to 50 μm were performed using a recently developed fibre tensile module capable of high accuracy and flexibility. This module is able to fit into a scanning electron microscope for in-situ deformation studies. In this study the influence of the gauge length, wire diameter and grain size on the tensile properties is analysed. In-situ experiments performed in a scanning electron microscope revealed clearly that the occurrence of “pop-in” events in the load-displacement curves are related to slip. The results are compared to deformation studies of micro-sized copper samples in the literature and discussed taking in-situ scanning electron microscope observations into account.

The deformation-induced zone below large and shallow nanoindentations: A comparative study using EBSD and TEM

A comparison is made between the deformation-induced zone beneath nanoindentations obtained by Electron Backscatter Diffraction (EBSD) and Transmission Electron Microscopy (TEM). Since there are resolutional limitations associated with EBSD, especially at very small scan sizes, it is not known how accurately the deformed volume beneath the imprints can be characterized. To aid in answering this question, cross-sectional EBSD and TEM samples of nanoindentations were fabricated by means of a Focused Ion Beam (FIB) workstation, analyzed, and subsequently compared with each other. For large indentations as well as for shallow ones, agreement of the determined zones was found. The results of the EBSD and TEM experiments were also used to characterize the deformed volumes. In the EBSD maps of large indentations, strongly confined deformation patterns were found, while for the shallow indentations the observed patterns are more diffuse. The TEM micrographs and the Selected-Area Electron Diffraction (SAED) support these facts and give insight into the dislocation structure of the deformation zone.

Investigation of reversible plasticity in a micron-sized, single crystalline copper bending beam by X-ray μLaue diffraction

The observed mechanical behaviour of micron-sized samples raises fundamental questions about the influence of size on the underlying dislocation plasticity. In situ µLaue diffraction on a single crystalline copper bending beam was performed to study the feasibility of bending tests and their contribution to our understanding of size-dependent dislocation plasticity. Theoretical considerations lead to a minimum sample size where in situ µLaue experiments are useable. A critical size is evidenced below which, depending on Youngs modulus and maximum stress, the elastic and plastic contributions to the lattice curvature cannot be separated. The experiment shows the increase in geometrically necessary dislocations during plastic deformation followed by a decrease during unloading. This can be explained by the formation and dissolution of a dislocation pile-up at the neutral axis of the bending cantilever. The dissolution of the dislocation pile-up is caused by the back stress of the pile-up and a direct observation of the Bauschinger effect, which is consistent with the non-purely elastic mechanical behaviour when unloading the sample.

Novel temperature dependent tensile test of freestanding copper thin film structures

The temperature dependent mechanical properties of the metallization of electronic power devices are studied in tensile tests on micron-sized freestanding copper beams at temperatures up to 400 °C. The experiments are performed in situ in a scanning electron microscope. This allows studying the micromechanical processes during the deformation and failure of the sample at different temperatures.