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The study of grain boundaries (GBs) in polycrystalline materials is a field of major interest, as many fundamental properties are influenced by their actual structure. One of the main challenges in investigating GBs is that electron microscopy techniques capable to resolve their atomistic arrangement require very small sample volumes of a few tens of nanometers and might therefore change the natural state of the GB via removal of occurring material constraint. To counteract this influence one could apply indirect measurements such as internal friction to probe for changes of the GB structure. However, with the drive towards smaller volumes most of these techniques are at their limit. The current work proposes a new approach based on mechanical spectroscopy of micron sized specimens applied in-situ in a scanning electron microscope. Applying this novel concept to ultra-fine grained Ta we investigate changes in the defect structure between the as-deformed condition and after a heat treatment. In fact, we detect a decrease in internal friction of about 50% upon annealing. This pronounced change occurs in conjunction with an increase in flow stress and hardness and is related to thermally induced GB relaxation.

To investigate the mechanism of { 10 1 ¯ 2 } twinning in magnesium (Mg) single crystal and its influence on mechanical size effects and strain rate dependent deformation behavior, in-situ microcompression of Mg [ 2 1 ¯ 1 ¯ 0 ] pillars of various sizes from 0.5 μm to 4 μm was carried out in a scanning electron microscope (SEM) and also in a transmission electron microscope (TEM), covering strain rates from 10−4 to 10−2 s−1. The in-situ observations directly showed that the pile-up of prismatic dislocations acts as local stress concentration for the twin nucleation. Preceding the twin nucleation, the lead dislocation from the dislocation pile-up cross-slips to the basal plane and dissociates into partial dislocations, one of which trails a stacking fault (SF) behind. The twin nucleus of a finite size formed at the junction between prismatic dislocations and basal SFs and subsequently propagated rapidly across the pillar. The present in-situ observations reveal that not only the dislocation pile-up but also the dissociation reaction of dislocations play critical roles in the nucleation of { 10 1 ¯ 2 } twins. Furthermore, the { 10 1 ¯ 2 } twinning exhibits a relatively strong size effect in terms of the twin nucleation stress (size exponent n = 0.7). This pronounced size effect may arise from the fact that the precursor to twin nucleation, namely dislocation pile-up and junction formation, depends more strongly on the crystal size than the ordinary dislocation source operation. Moreover, a noticeable effect of the strain rate is that a higher rate (10−2 s−1) promotes the activation of basal slip within the { 10 1 ¯ 2 } twin. While the twin nucleation occurs more easily at a high strain rate, here the twin growth rate cannot cope with the applied strain rate, so that strain energy accumulation increases with applied strain. When the twin grows to reach the required twin thickness for basal slip, the basal slip promptly activates within the twinned region to release the accumulated strain energy and plastic deformation swiftly catches up with the applied strain rate.

Measuring the local behaviour of a propagating crack in a quantitative manner has always been a challenge in the field of fracture mechanics. In-situ microcantilever testing inside a scanning electron microscope (SEM) is one of the most promising techniques for the investigation thereof. However, quantifying such experiments is fairly challenging. Here, for the first time we utilize a continuous measurement of the dynamic compliance in-situ to permit evaluation of the crack length. Microcantilever experiments have been performed on brittle single crystalline Si and nanocrystalline Fe to assess the stability of the setup, the applicability of the technique inside an SEM and to establish a correlation between stiffness and crack length. Subsequently, micromechanical fracture tests were performed on single crystalline, 〈001〉{001} oriented tungsten as a model material and continuous J-Δa curves were measured. The gathered data was evaluated with close relation to standardized fracture mechanics testing and showed an overall agreement with literature data. This novel possibility to measure J-Δa curve behaviour continuously and locally while also following the crack extension through in-situ imaging inside an SEM is generally applicable and will allow new insights in the crack propagation of modern materials. Keywords: In-situ microcantilever testing, Elastic-plastic fracture mechanics, Dynamic compliance measurement, Tungsten

Service materials are often designed for strength, ductility, or toughness, but neglect the effects of cyclic time-variable loads ultimately leading to macroscopic mechanical failure. Fatigue originates as local plasticity that can first only be observed on the micro scale at defects serving as stress concentrators such as inclusions or grain boundaries. Thus, a recently developed technique to perform in-situ observation of micro scale bending fatigue experiments was applied. Micro-beams fabricated from copper, single grained and ultrafine grained (ufg), and an oxide dispersion strengthened (ODS) steel were subject to cyclic deformation and subsequent damage. The elastic stiffness, yield strength, dissipated energy, and maximum stress were measured as a function of cycle number and plastic strain amplitude. From these properties, cyclic stress-strain curves were developed. Initial pronounced monotonic hardening and an increasing Bauschinger effect were observed in all samples with increasing strain amplitude. Cyclic stability was maintained until plastic strain amplitudes reached a critical value. At this point, dramatic cyclic softening and microcracking occurred. The critical strain amplitude was found to be approximately 5.4×10 −3 for the copper with a refined grain structure and 1.2×10 −2 for the steel specimen. Grain rotation and noticeable changes in sub-grain structure were evident in the ufg copper after a critical strain amplitude of e a =8.3×10 −3 . In-situ micro fatigue bending couples the cyclic evolution of bulk mechanical properties measurements with real-time electron microscopy analysis techniques of damage and failure mechanisms, which renders it a powerful method for developing novel fatigue resistant materials.

The advent of miniaturised testing techniques led to excessive studies on size effects in ma-terials. Concomitantly, these techniques also offer the capability to thoroughly examine deformation mechanisms operative in small volumes, in particular when performed in-situ in electron micro-scopes. This opens the feasibility of a comprehensive assessment of plasticity by spatially arranging samples specifically with respect to the crystal unit cell of interest. In the present manuscript, we will showcase this less commonly utilised aspect of small-scale testing on the case of the hexagonal metal Mg, where, besides dislocation slip on different slip planes, twinning also exists as a possible deformation mechanism. While it is close to impossible to examine individual deformation mechanisms in macroscale tests, where local multiaxial stress states in polycrystalline structures will always favour multiple mechanisms of plasticity, we demonstrate that miniaturised uniaxial experiments conducted in-situ in the scanning electron microscope are ideally suited for a detailed assessment of specific processes. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.

Hard protective coating materials based on transition metal nitrides and carbides typically suffer from limited fracture tolerance. To further tune these properties non-metal alloying – substituting C with N – has been proven favorable for magnetron sputtered Hf-C-N based thin films. A theoretically predicted increase in valence electron concentration (from 8.0 to 9.0 e/f.u. from Hf C to Hf N) through nitrogen alloying lead to an increase in fracture toughness (KIC obtained during in-situ SEM cantilever bending) from 1.89 ± 0.15 to 2.33 ± 0.18 MPa·m1/2 for Hf0.43C0.57 to Hf0.35C0.30N0.35, respectively. The hardness remains close to the super-hard regime with values of 37.8 ± 2.1 to 39.9 ± 2.7 GPa for these specific compositions. Already the addition of small amounts of nitrogen, while sputtering a ceramic Hf C target, leads to a drastic increase of nitrogen on the non-metallic sublattice for fcc single phased structured HfC1-xNx films, where x = N/(C + N). The here obtained results also provide experimental proof for the correlation between fracture characteristics and valence electron concentration.

The present work provides an analytic solution for the stiffness to crack length relation in microscopic cantilever shaped fracture specimens based on classical beam theory and substitution of the crack by a virtual rotational spring element. The resulting compact relationship allows for accounting of the actual beam geometry and agrees very well with accompanying finite element simulations. Compared with the only other model present in literature the proposed relationship reduces the deviation between model and data to a maximum of 1.6% from the previous minimum of 15%. Thus, the novel solution will help to reduce the necessity for individual simulations and aim to increase the comparability of elastic-plastic microcantilever fracture experiments in the future.

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.

The importance of lightweight materials such as titanium and magnesium in various technical applications, for example aerospace, medical implants and lightweight construction is well appreciated. The present study is an attempt to combine and improve the mechanical properties of these two materials by forming an ultra-fine grained composite. The material, with a composition of 75 vol% (88.4 wt%) Ti and 25 vol% (11.4 wt%) Mg , was synthesized by powder compression and subsequently deformed by high-pressure torsion. Using focused ion beam machining, miniaturised compression samples were prepared and tested in-situ in a scanning electron microscope to gain insights into local deformation behaviour and mechanical properties of the nanocomposite. Results show outstanding yield strength of around 1250 MPa, which is roughly 200 to 500 MPa higher than literature reports of similar materials. The failure mode of the samples is accounted for by cracking along the phase boundaries.

The importance of lightweight materials such as titanium and magnesium in various technical applications, for example aerospace, medical implants and lightweight construction is well appreciated. The present study is an attempt to combine and improve the mechanical properties of these two materials by forming an ultra-fine grained composite. The material, with a composition of 75 vol% (88.4 wt%) Ti and 25 vol% (11.4 wt%) Mg , was synthesized by powder compression and subsequently deformed by high-pressure torsion. Using focused ion beam machining, miniaturised compression samples were prepared and tested in-situ in a scanning electron microscope to gain insights into local deformation behaviour and mechanical properties of the nanocomposite. Results show outstanding yield strength of around 1250 MPa, which is roughly 200 to 500 MPa higher than literature reports of similar materials. The failure mode of the samples is accounted for by cracking along the phase boundaries.

With modern materials applications continually decreasing in size, e.g., microelectronics, sensors, actuators, and medical implants, quantifying materials parameters becomes increasingly challenging. Specifically, addressing individual constituents of a system, such as interfaces or buried layers in a multilayer structure, emerges as a topic of great importance. We demonstrate herein a technique to assess fracture parameters of different interfaces of a Cu-WTi-SiOx-Si model system based on in situ microcantilever testing in a scanning electron microscope. Positioning the initial notch position with respect to the interface of interest enabled selection of different crack paths, while an additional overlaid sinusoidal signal permitted continuous measurement of stiffness changes and thereby experimental measurement of the actual crack extension. We thus achieved continuous J–Δa curve measurements for the interface between Cu and WTi, the bulk WTi, and the interface between WTi and SiOx. The localized nature of this novel approach makes it generally applicable to testing specific interfaces.

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