INTEGRATING SIMULATION, MACHINE LEARNING, AND EXPERIMENTAL APPROACHES FOR HIGH-THOUGHPUT SMALL-SCALE FRACTURE INVESTIGATIONS

From Da Vinci to Galileo to modern experimentalists a variety of characterization methods have been introduced for investigating the fracture of materials. Determining fracture properties of materials at small length scales, with complex shapes, under extreme environmental conditions, is still extremely challenging. We will show how this gap is addressed by introducing two novel methods to investigate fracture. The first one involves light for contactless mechanical testing, while the second method integrates experiments with data-driven approaches to address issues related to complex shapes.
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OPERANDO EXPERIMENTS TO CHARACTERISE BRITTLE FRACTURE-LIKE EVENTS IN CERAMIC ELECTROLYTES VIA PHOTOELASTICITY

Solid electrolytes at current densities that are relevant to real battery operating conditions are prove to the penetration of lithium metal protrusions, also known as “dendritic” events, that are formed during battery charging. In this work, we show via operando photoelasticity experiments that the dendritic events at high current densities can be understood by the classic Griffith – Irwin fracture theory.
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FRACTURE AND FATIGUE BEHAVIOR OF ADDITIVELY MANUFACTURED MAR-M 509 CO-BASED SUPERALLOYS

Mar-M 509 is a Cobalt-based superalloy suitable for elevated temperature applications like nozzle guide vanes and blades in aero engines and gas turbines. Short cycle aging heat treatment of laser powder-bed-fusion processed Mar-M 509 is a novel route explored in this study to enhance the mechanical properties of this alloy, especially tensile ductility and fracture toughness, while retaining room and elevated temperature strengths. A detailed microstructural analysis is carried out using advanced characterisation tools and correlated to miniature, small volume, room temperature tensile tests and fracture toughness and fatigue tests using clamped beam geometry combined with digital image correlation-based in-situ strain mapping across the longitudinal and transverse directions, before and after heat treatment. Mechanisms leading to corresponding changes in fracture and fatigue properties will be discussed.
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DETERMINING THE RATE-CONTROLLING, GRAIN-BOUNDARY-MEDIATED MECHANISMS IN ULTRAFINE GRAINED AU AND AL FILMS

The active grain boundary (GB) mediated mechanisms in ultrafine grained (ufg) Au and Al metallic films, and the extent to which they dictate plastic flow kinetics, are investigated in this work. The approach consists of a synergistic integration of in situ transmission electron microscopy (TEM) deformation experiments, nanomechanical testing, and transition state theory based atomistic modeling, in order to provide a linkage between GB-mediated dislocation processes and their deformation kinetics. The in situ TEM nanomechanical testing experiments are employed to simultaneously identify plastic deformation mechanisms, obtain key details, and measure the sample-level true activation volume in ufg thin films. The activation of relevant GB mediated dislocation mechanisms is modeled using the atomistic free-end nudged elastic band (FENEB) method as a function of representative, experimentally observed GB characters and local stress. Proper integration of experiments (sample-level true activation volume) and atomistic simulations (activation volumes of dislocation processes) to determine strength/rate-controlling mechanisms requires linking the applied stress to the local stress. To that end, a model of grain-size-dependent activation volume previously developed by Conrad is extended to account for the competition between various GB mediated mechanisms.
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INSIGHTS INTO VOID NUCLEATION AND GROWTH IN A DUAL PHASE STEEL BY SMALL SCALE MECHANICAL TESTING [Keynote]

Dual phase (DP) steels are comprised of a soft ferrite matrix and hard martensite islands. They are often used in automotive applications due to their advantageous combination of high strength and good ductility. During forming, DP steels can suffer from ductile damage, i.e. the formation and growth of voids, which typically occur by interface decohesion and martensite fracture [1]. As of now, the void content of a deformed part cannot precisely be predicted and, therefore, safety factors are used to assure the required mechanical properties and component lifetime. These safety factors are opposing sustainability and light-weight design. Consequently, the DFG-funded collaborative research center TRR188 aims at a quantitative characterization, prediction and control of ductile damage during forming.
In the talk, micromechanical experiments on the plasticity and fracture of single ferrite grains and martensite islands of two nominal identical steel grades will be presented. While one steel grade exhibits a low ferrite and a high martensite strength, the other shows a significantly stronger ferrite and lower strength martensite compared to the first steel grade [2]. This results in huge differences in the void nucleation and growth characteristics of the two steel grades.
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IN SITU TRANSMISSION ELECTRON MICROSCOPY STUDY OF NANOMECHANICAL DEFORMATION AND ATOMIC-SCALE FRACTURE IN HIGH ENTROPY ALLOYS

Intergranular fracture plays an important role in polycrystalline materials including high entropy alloys, but the atomic scale fracture mechanisms of individual grain boundaries (GBs) are still not fully understood. In this work, we selectively investigate the fracture behaviors of individual GBs in a single-phase face-centered cubic CoCrFeNi high entropy alloy via in situ transmission electron microscopy (TEM) nanomechanical testing supported by molecular dynamics (MD) simulations. With this set up, the classic mode I crack propagation along GBs can be dynamically visualized and quantitatively analyzed.
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EFFECTS OF IRRADIATION DAMAGE LEVELS ON ACTIVATION VOLUME AND DEFORMATION MECHANISMS IN IRRADIATED GOLD THIN FILMS USING IN SITU TEM STRAINING

The plastic deformation mechanisms of ultrafine-grained gold thin films (average grain size ~150 nm) irradiated with 2.8 MeV Au+ ions at three different levels (0.1, 1 and 5 dpa) have been studied using quantitative in-situ transmission electron microscopy (TEM) nanomechanical testing. This technique allows for the simultaneous observation and comparison of the active deformation mechanisms, measurement of mechanical properties and true activation volume. Some of the observed deformation mechanisms include dislocation nucleation at grain boundaries (GB), dislocation pinning/de-pinning at irradiation induced defects, and stress-induced GB migration. During the early stages of deformation, dislocation nucleation and GB migration occur simultaneously. However, the dense populations of irradiation-induced defects prevent transgranular dislocation motion. As the deformation levels increase, GB migration leads to defect-free zones which then provide avenues for unimpeded dislocation glide. The true activation volume increases from ~10b3 in unirradiated specimens, to ~22b3 in irradiated specimens at 1dpa, for flow stresses ranging from 400 to 550 MPa. The experimentally measured activation volume values are compared with values determined from atomistic simulations (grain size ~10 nm) for different unit dislocation processes to determine the controlling deformation mechanism, using Conrad’s model that provides a Hall-Petch-type relationship of grain size dependent activation volume.
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QUANTIFICATION OF INTERFACE STRENGTH OF A THIN FILM USING A NEW MICROCANTILEVER GEOMETRY.

Interfaces failure occurs not only in structural materials but also in functional material systems including systems for energy conversion and storage. Such failures lead to degradation of mechanical and functional properties, such as battery capacity or electrical conductivity. In bulk scale, there are various experimental methods to investigate the interface strength and its failure mechanisms, for instance, peeling test, superlayer test, or indentation test. One of the disadvantages of these approaches is that it can be applied only to relatively thick coatings [1,2]. Small-scale mechanical testing is a powerful tool for studying interface properties because it can quantify micro- and nanometer-sized thin films, and individual interfaces of interest can be tested by isolating them using focused ion beam (FIB). Single and double cantilever beams have been used to investigate fracture/delamination properties of single interfaces [3,4], however, these methods are prone to experimental imperfections arising from testing geometries.
In this talk, we propose a new in situ scanning electron microscope (SEM) microcantilever design that provides reliable and quantitative interface toughness. In addition, the optimized geometry can promote a pre-notch (or crack) to propagate in a stable manner, which is important to generate a natural crack front without FIB-induced damage/artifacts.
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EVALUATING THE INTERFACIAL TOUGHNESS OF GAN-ON-DIAMOND USING BLISTERING METHOD WITH NANO-INDENTATION

An improved analysis of the interfacial toughness using nanoindentation induced blistering of thin films on stiff substrates is demonstrated on GaN-on-diamond. The Hutchinson-Suo analysis requires accurate measurement of blister dimensions, conventionally measured using 2-D line-scans from 3-D topographical maps. The new meteorology overcomes shortcomings of this technique by fitting the 3-D analytical solution of a clamped Kicrchoff plate to the topological map of the blister. This allowed for quantification of interfacial toughness of smaller blisters in GaN-on-diamond, previously assumed invalid for analysis due to inadequacies of the line-scan analysis. Three samples were investigated and found to have interfacial toughness ranging from 0.6–1 J m−2. Additionally, the relationship between residual stress in the GaN and interfacial toughness was investigated using photoluminescence spectroscopy. In all cases, the GaN was found to be under increased compression at the diamond interface by up to -0.81 GPa, although no correlation with interfacial toughness was observed.
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IMPACT OF GRAIN BOUNDARY MODIFICATIONS ON FRACTURE TOUGHNESS OF TUNGSTEN BASED NANOMATERIALS [Keynote]

Nanostructured materials commonly excel with respect to their strength, but their ductility and toughness remain limiting factors for deployment in safety related applications. In this work, using grain boundary engineering concepts in conjunction with severe plastic deformation for microstructural refinement, we aim to develop nanostructured and nanocomposite materials that overcome these limitations. Since material volumes are limited, we utilize small scale testing approaches to examine the respective material properties such as strength, ductility and fracture toughness. We detail on the one hand challenges and recent advancements in small scale fracture experiments, and on the other hand the effectiveness of the mentioned grain boundary engineering approaches to design outstanding nanomaterials overcoming strength-ductility-toughness limitations.
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