Christoph Kirchlechner

Advanced nanomechanics in the TEM: Effects of thermal annealing on FIB prepared Cu samples

The effect of focused ion beam (FIB) fabrication on the mechanical properties of miniaturized mechanical tests has recently been realized, but is not well documented. In this study, the effect of post thermal annealing on the plastic properties of FIB fabricated micro- and nanometer-sized Cu samples was studied by means of advanced analytic and in situ transmission electron microscopy. In situ heating experiments on thin films and pillars revealed a reduction of the initially high dislocation density, but never a recovery of the bulk dislocation density. Aberration-corrected atomic imaging documented the recovery of a pristine crystalline surface structure upon annealing, while electron energy-loss spectroscopy showed that the remaining contamination layer consisted of amorphous carbon. These structural observations were combined with the mechanical data from in situ tests of annealed micro- and nanometer-sized tensile and compression samples. The thermal annealing in the micron regime mainly influences the initial yield point, as it reduces the number of suited dislocation sources, while the flow behavior is mostly unaffected. For the submicron samples, the annealed material sustains significantly higher stresses throughout the deformation. This is explained by the high stresses required for surface-mediated dislocation nucleation of the annealed material at the nanoscale. In the present case, the FIB affected the surface near defects and facilitated dislocation nucleation, thereby lowering the material strength.

Deformation-Induced Martensite: A New Paradigm for Exceptional Steels

Martensite steel is induced from pearlitic steel by a newly discovered method, which is completely different from the traditional route of quenching austenitic steel. Both experimental and theoretical studies demonstrate that Fe-C martensite forms by severe deformation at room temperature. The new mechanism identified here opens a paths to material-design strategies based on deformation-driven nanoscale phase transformations.

Sample Preparation by Metallography and Focused Ion Beam for Nanomechanical Testing

Abstract Mechanical size effects in micron and submicron scale sample testing are of immense interest in materials science. In this work, we report on a combination of structured chemical etching and focused ion beam fabrication to allow site specific and time efficient fabrication of miniaturized specimens for mechanical testing. Further, we demonstrate the applicability of these samples for quantitative in situ experiments in the scanning and transmission electron microscopes.

Adhesion measurement of a buried Cr interlayer on polyimide

A fundamental knowledge and understanding of the adhesion behaviour of metal–polymer systems is important as interface failure leads to a complete breakdown of flexible devices. A combination of in situ atomic force microscopy for studying topological changes and in situ synchrotron based stress measurements both during film tensile testing were used to estimate the adhesion energy of a thin bilayer film. The film systems consisted of 50–200 nm Cu with a 10 nm Cr adhesion layer on 50 μm thick polyimide. If the Cu film thickness is decreased to 50 nm the Cr interlayer starts dominating the system behaviour. An apparent transition from plastic to predominantly brittle deformation behaviour of the Cu can be observed. Then, compressive stresses in the transverse direction are high enough to cause delamination and buckling of the Cr interlayer from the substrate. This opens a new route to induce buckling of a brittle interlayer between a ductile film and a compliant substrate which is used to determine the interfacial adhesion energy.

The effect of size on the strength of FCC metals at elevated temperatures: annealed copper

Abstract As the length scale of sample dimensions is reduced to the micron and sub-micron scales, the strength of various materials has been observed to increase with decreasing size, a fact commonly referred to as the ‘sample size effect’. In this work, the influence of temperature on the sample size effect in copper is investigated using in situ microcompression testing at 25, 200 and 400 °C in the SEM on vacuum-annealed copper structures, and the resulting deformed structures were analysed using X-ray μLaue diffraction and scanning electron microscopy. For pillars with sizes between 0.4 and 4 μm, the size effect was measured to be constant with temperature, within the measurement precision, up to half of the melting point of copper. It is expected that the size effect will remain constant with temperature until diffusion-controlled dislocation motion becomes significant at higher temperatures and/or lower strain rates. Furthermore, the annealing treatment of the copper micropillars produced structures which yielded at stresses three times greater than their un-annealed, FIB-machined counterparts.

In-situ observation of stress-induced stochastic twin boundary motion in off stoichiometric NiMnGa single crystals

In-situ X-ray microdiffraction is used to illuminate the physics of non-uniform stochastic motion of type II twin boundaries in NiMnGa twinned crystals during external stress field loading. Asymmetry between tensile and compressive loading and a large hysteresis loop were found. The formation of local strained regions precedes each boundary movement. The location of strained regions adjusts to the position of the twin boundary. Abrupt motion of the boundary correlates with corresponding spikes at the load/displacement curve.

Maintaining strength in supersaturated copper–chromium thin films annealed at 0.5 of the melting temperature of Cu

The thermal stability of evaporated copper–chromium alloy films was studied by correlating hardness trends from nanoindentation to nanostructural–compositional changes from transmission electron microscopy. In particular, the hardness evolution with ageing time at ambient and elevated temperatures of two compositions, dilute (Cu96Cr4) and chromium-rich (Cu67Cr33) solutions, was studied. Due to the negligible mutual miscibility of copper and chromium, the chosen solid solutions are trapped in metastable states as supersaturated solid solutions with face-centred cubic and body-centred cubic phases. Nano-mechanical probing of the nanostructural evolution as a function of temperature provided interesting insights into the phase separation of these systems.

Laue Microdiffraction at the ESRF

This book highlights emerging diffraction studies of strain and dislocation gradients with mesoscale resolution, which is currently a focus of research at laboratories around the world. While ensemble-average diffraction techniques are mature, grain and subgrain level measurements needed to understand real materials are just emerging. In order to understand the diffraction signature of different defects, it is necessary to understand the distortions created by the defects and the corresponding changes in the reciprocal space of the non-ideal crystals. Starting with a review of defect classifications based on their displacement fields, this book then provides connections between different dislocation arrangements, including geometrically necessary and statistically stored dislocations, and other common defects and the corresponding changes in the reciprocal space and diffraction patterns. Subsequent chapters provide an overview of microdiffraction techniques developed during the last decade to extract information about strain and dislocation gradients. X-ray microdiffraction is a particularly exciting application compared with alternative probes of local crystalline structure, orientation and defect density, because it is inherently non-destructive and penetrating.

In-situ TEM Study of Mechanical Size Effects in TiC Strengthened Steels

It is nowadays well understood that single crystalline metals exhibit a size dependence of the yield stress: with decreasing pillar diameters an increase in the flow stress is measured in micropillar compression tests. This size effect (“smaller is stronger”) is governed by the size of the activated dislocation sources and can be described by a power law equation relating stress and pillar diameter (σ ~ d). For single crystalline metallic pillars containing internal particles three different stress size dependencies are reported in literature: a power law [1, 2], a size-independent (i.e. constant) [3-6], or an intermediate behavior, which can be interpreted as a transition from constant to power law behavior [79]. The differences in size-dependency can be explained by two competing mechanisms, dislocation source activation and dislocation particle interaction [8, 9]. When the stress to activate dislocation sources exceeds the particle strengthening effect, pillars should show the “classical” power law dependence. In the present study we want to elaborate if we can quantitatively deduce the particle strengthening effect of e.g. shearable weak or non-shearable strong particles in metal materials from their transition regimes of size dependency. We have examined this idea in precipitation hardened steel with nanometer-sized titanium carbides (TiC) precipitates.

Microcantilever Fracture Testing of Intermetallic Cu3Sn in Lead-Free Solder Interconnects

Driven by legislation and the abolishment of harmful and hazardous lead-containing solders, lead-free replacement materials are in continuous development. Assessment of the mechanical properties of intermetallic phases such as Cu3Sn that evolve at the interface between solder and copper metalization is crucial to predict performance and meet the high reliability demands in typical application fields of microelectronics. While representative material parameters and fracture properties are required to assess mechanical behavior, indentation-based testing produces different results depending on the sample type. In this work, focused ion beam machined cantilevers were used to unravel the impact of microstructure on the fracture behavior of Sn-Ag-Cu lead-free solder joints. Fracture testing on notched cantilevers showed brittle fracture for Cu3Sn. Unnotched samples allowed measurement of the fracture stress, to estimate the critical defect size in unnotched Cu3Sn microcantilevers.