[en] We present the first measurement of the strain rate sensitivity of the ideal dislocation slip transmission through a coherent Σ3{111} copper twin boundary. For this purpose we have deformed 129 geometrically identical samples at different strain rates. The micron-sized samples are either single crystalline (87 pillars) or contain one vertical Σ3{111} twin boundary (42 pillars). The strain rate sensitivity of the ideal slip transmission event is 0.015 ± 0.009. This value is considerably lower than the strain rate sensitivity observed for nano-twinned bulk materials, which is addressed to multiple simultaneously activated deformation processes present in the latter case. The activation volume of the ideal slip transmission points towards a cross-slip like transmission process of dislocations through the twin boundary. Furthermore, the high number of geometrically identical samples is used to discuss the ability to identify the strength distribution function of micropillars.

[en] Flexible electronic devices call for copper and gold metal films to adhere well to polymer substrates. Measuring the interfacial adhesion of these material systems is often challenging, requiring the formulation of different techniques and models. Presented here is a strategy to induce well defined areas of delamination to measure the adhesion of copper films on polyimide substrates. The technique utilizes a stressed overlayer and tensile straining to cause buckle formation. The described method allows one to examine the effects of thin adhesion layers used to improve the adhesion of flexible systems. - Highlights: • Measuring the adhesion energies of ductile metal–polymer interfaces is difficult. • A Cu film would plastically deform under tensile strain without a Cr overlayer. • A Cr overlayer forces cracking and induces buckling between the crack fragments. • The adhesion energy of the metal–polymer interface can be measured

[en] Highlights: • Neither a change of crystal structure nor a change in composition influences the fracture toughness of NbCo₂ Laves phase. • The fracture toughness of NbCo₂ Laves phase obtained by micro-cantilever bending tests is 4.2 ± 0.2 MPa·m^1/2. • The fracture toughness values obtained at the micrometer scale are cross-checked by applying different sample geometries. • Diffusion couple technique and micromechanical testing were combined to study brittle intermetallic phases. Cubic and hexagonal NbCo₂ Laves phases are known to have composition dependent hardness and yield strength. However, it is unknown whether this dependence is also reflected in their fracture toughness values. In order to elucidate the fracture behavior, single-crystalline micro-cantilevers of the cubic and hexagonal NbCo₂ Laves phases having different compositions were fabricated in the diffusion layers grown by the diffusion couple technique. Micro-cantilever bending tests were performed to study the composition- and crystal-structure-dependence of the fracture toughness. To exclude the influence of micro-cantilever geometry, pentagonal and rectangular beams were tested and found to result in the same fracture toughness value. The present results reveal that neither a change of the crystal structure nor a change in chemical composition has a significant influence on the fracture toughness of NbCo₂ Laves phase.

[en] In situ micromechanical compression tests on Cu pillars were performed to evaluate the influence of twin boundaries on the mechanical behavior. The 1 µm sized Cu samples on a Si substrate prepared by focused ion beam milling were either single crystalline or contained 2–5 twin boundaries that were inclined to the compression direction. The strengths of the pillars vary, depending on the crystal orientation, associated twin boundary inclination and orientation of slip systems. Results show, that multiple slip systems are activated in each pillar. However, slip parallel to the twin boundaries prevails due to the long mean free path for dislocation movement

[en] The microstructural and mechanical characterization of an equiatomic YGdTbDyHo high entropy alloy with hexagonal close-packed structure was performed. The phase state and chemical homogeneity of the solid solution were analysed with respect to crystal structure, phase stability, and oxide formation. It was found that Y-rich precipitates form at grain boundaries and that the alloy is prone to oxidation, leading to a homogeneous distribution of ∼10 nm-sized oxides in the grain interiors. The plastic response at the sub-grain level was studied in terms of the activated slip systems, critical resolved shear stresses (CRSS), and strain hardening using micropillar compression tests. We observe plastic slip on the basal system, with a CRSS of 196 ± 14.7 MPa. Particle strengthening and strength dependence on sample size are discussed on the basis of dislocation particle interaction and mechanical size effects.

[en] Highlights: → In situ micro-compression testing of lamellar TiAl crystals in a SEM. → Mechanical twinning and dislocation glide are analyzed by TEM. → A model is developed to describe the twin induced deformation. → The size of the mechanical twins is correlated with the width of the TiAl lamellae. - Abstract: Titanium aluminides are the most promising intermetallics for use in aerospace and automotive applications. Consequently, it is of fundamental interest to explore the deformation mechanisms occurring in this class of materials. One model material which is extensively used for such studies are polysynthetically twinned (PST) TiAl crystals, which consist predominantly of parallel γ-TiAl and, fewer, α₂-Ti₃Al lamellae. In the present study, PST TiAl crystals with a nominal composition of Ti-50 at.% Al were machined by means of the focused ion beam (FIB) technique into miniaturized compression samples with a square cross-section of approximately 9 μm x 9 μm. Compression tests on the miniaturized samples were performed in situ inside a scanning electron microscope using a microindenter equipped with a diamond flat punch. After deformation, thin foils were cut from the micro-compression samples and thinned to electron transparency using a FIB machine in order to study the deformation structure by transmission electron microscopy (TEM). The TEM studies reveal mechanical twinning as the main deformation mechanism at strains of 5.4%, while at strains of 8.3% dislocation glide becomes increasingly important. The experimentally observed twins scale in size with the width of the γ-TiAl lamella. A kinematic and thermodynamic model is developed to describe the twin-related length change of the micro-compression sample at small strains as well as the relationship of an increase of twin width with increasing γ-TiAl lamella thickness. The developed twin model predicts a width of the twins in the range of a few nanometers, which is in agreement with experimental findings.

[en] Highlights: • The fracture toughness (K_IC) of sputtered Mo₂BC films is reported for the first time. • A correlation between deposition temperature, film microstructure, and fracture toughness is established. • The fracture toughness between 3.6 and 4.5 MPa√m renders Mo₂BC as a promising coating for wear resistant applications. • Residual stresses tremendously impact on the system toughness obtained by indentation-based techniques. The fracture behaviour and microstructure evolution of sputtered Mo₂BC films as a function of their deposition temperature is studied. Bipolar pulsed direct current magnetron sputtering was used to deposit Mo₂BC thin films onto Si (100) wafers at substrate temperatures ranging from 380 to 630 °C. Microstructural characterization by transmission electron microscopy revealed that increasing the deposition temperature induces larger and more elongated grains, and a higher degree of crystallinity, transitioning from a partially amorphous to a fully crystalline film. The intrinsic fracture toughness of the Mo₂BC films was studied by focussed ion beam milled micro-cantilever bending tests. A mild dependency of the intrinsic fracture toughness on the substrate deposition temperature was found. Fractograph analysis showed that the fracture behaviour was dominated by intergranular fracture or by fracture within the amorphous regions. Additionally, nanoindentation based fracture toughness measurements were used to probe the fracture behaviour of the Mo₂BC/Si system, where residual stresses define the ‘apparent’ fracture toughness of the system. Depending on the substrate deposition temperature either compressive or tensile residual stresses developed in the films. This causes a relative change in the system toughness by up to one order of magnitude. The fracture experiments clearly reveal that notched cantilevers provide intrinsic toughness values of a material, while nanoindentation probes the toughness of the entire coating-substrate system. The combination of both techniques provides valuable design information for enhancing fracture resistance of Mo₂BC films.

[en] CrN coatings were deposited on polycrystalline ferritic steel substrates at 350deg. C by magnetron sputtering using Cr targets in Ar + N₂ atmosphere. In order to simulate the thermal fatigue, the samples were repeatedly irradiated using a laser beam of 6mm in diameter. The thermal cycling was performed in the range of 50-650 deg. C with up to 100 000 cycles. Subsequently, the structures were characterized using high-energy synchrotron and high-temperature laboratory X-ray diffraction. The structures exhibit complex changes in the morphology and in residual stress state in the heated spot. The annealing results in the relaxation of compressive stresses in the coating and in the formation of high tensile stresses in the steel substrate. This effect decisively depends on the number of applied cycles. The reduction of compressive stress in the coating is caused by the annealing of point defects and by dimensional changes of the substrate due to its plastic deformation in the center of the irradiated spot. The plastic deformation of the substrate is also the probable reason for the ripples observed for samples cycled more than 3000 times. The presented approach allows a complex characterization of thermo-mechanical processes in coating-substrate composites and opens the possibility to understand phenomena related to the thermal fatigue of coated tools

[en] Coherent x-ray diffraction is used to investigate the mechanical properties of a single grain within a polycrystalline thin film in situ during a thermal cycle. Both the experimental approach and finite element simulation are described. Coherent diffraction from a single grain has been monitored in situ at different temperatures. This experiment offers unique perspectives for the study of the mechanical properties of nano-objects.

[en] Synchrotron X-ray diffraction analysis of three-dimensional residual stress fields in laser-pulsed uncoated and CrN-coated steels was performed by a combination of two complementary synchrotron approaches. In the uncoated steel, the measurements revealed crater-like tensile stress distributions close to thermal cracks. For the coated samples, a regular Gaussian distribution was observed for steel whereby a cracking occurs in CrN as a consequence of steel buckling. The data also revealed the mechanism by which hard coatings protect steel from thermal fatigue.