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Graphical abstract:

Abstract: Micromechanical testing techniques can reveal a variety of characteristics in materials that are otherwise impossible to address. However, unlike to macroscopic testing, these miniaturized experiments are more challenging to realize and analyze, as loading and boundary conditions can often not be controlled to the same extent as in standardized macroscopic tests. Hence, exploiting all possible information from such an experiment seems utmost desirable. In the present work, we utilize dynamic in situ microtensile testing of a nanocrystalline equiatomic CoCrFeMnNi high entropy alloy in conjunction with initial feature tracking to obtain a continuous two-dimensional strain field. This enables an evaluation of true stress–strain data as well as of the Poisson’s ratio and allows to study localization of plastic deformation for the specimen. We demonstrate that the presented image correlation method allows for an additional gain of information in these sophisticated experiments over commercial tools and can serve as a starting point to study deformation states exhibiting more complex strain fields. Graphic abstract:

Recently, the miniaturization of devices in the field of microelectronics has become more and more important. This also implies an increased complexity of the devices, where multilayer thin film systems play a major role. The use of various material combinations leads to the development of residual stresses, potentially causing cracks. Therefore, to prevent failures a thorough understanding of material properties such as the fracture toughness at small scales is indispensable, as these may differ significantly from bulk values. In this study we use miniaturized fracture tests to investigate the fracture behaviour of Cu–W–Cu and W–Cu–W trilayer thin film systems, having a thickness of 500 nm per individual W or Cu layer. The films are subjected to differences in elastic properties and residual stress gradients that both influence the fracture behaviour and thus have to be included in all considerations. We demonstrate that for the W layers a valid J-integral can be evaluated. However, we find that the presented advanced treatment does not allow the extraction of valid fracture mechanical quantities for the Cu layers, pointing out the need to develop a more sophisticated approach for ductile materials.

Abstract

The ongoing trend towards miniaturization in various fields of material science requires the capability to investigate the local mechanical properties of the concerned structures by miniaturized mechanical experiments. Besides nanoindentation, miniaturized experiments such as micro-compression, micro-tension, micro-bending, or micro-fracture tests were employed frequently in recent times. A major challenge for these experiments is the fabrication of specimens. Therefore, we present different approaches to prepare miniaturized testing objects in a site specific way, using strategies that employ chemical etching, broad beam ion milling, and focussed ion beam milling. Depending on the required sample size and precision, the typical strategies for sample fabrication will be outlined, and the benefits and drawbacks of the techniques are discussed. Finally, applications of specimens produced by the different procedures are presented.

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.

Using a focused ion beam workstation, micron‐sized bending and compression samples were fabricated from a pure copper single crystal. The bending and compression experiments exhibited a strong size effect on the flow stress of copper, reaching values in the order of 1 GPa for the smallest test structures. Conventional strain gradient plasticity approaches are not capable of explaining this behaviour. The surface damage introduced by Ga+ion implantation during focused ion beam preparation was investigated using Auger electron spectroscopy and its consequence on the mechanical response of the miniaturized test samples is addressed.

The continuous enhancements and developments in the field of power engineering, as well as the uprising of nuclear fusion technology, demand novel high performance materials featuring exceptional strength and damage tolerance as well as durability in harsh environments. Ultra-fine grained bulk materials fabricated by high-pressure torsion, exhibiting a grain size less than 500 nm are promising candidates for these applications. Tungsten, the material of choice for plasma-facing materials in fusion reactors, is expected to exhibit even more enhanced properties by precise doping with impurity atoms, strengthening grain boundary cohesion. In order to allow this meticulous control of chemical composition, in-house mixing of the raw material powders is preferable to use of commercially available alloys. Several challenges arise in powder processing of tungsten via high-pressure torsion, originating in the intrinsic strength and high melting point of the material, and in the affinity of the powder to oxygen. Strategies to overcome these problems will be addressed in this work. Furthermore, we compare ultra-fine grained tungsten produced from a bulk precursor to that from the developed powder approach regarding microstructural features, hardness and rate-sensitive properties. The powder route showed promising and widely comparable results to the material processed from bulk tungsten, rendering it an effective way for fabricating ultra-strong tungsten, while keeping the additional ability to accurately control chemical composition and tailor grain boundary segregation states.