HIGH-CYCLE FATIGUE IN THE TEM: NANOCRYSTALLINE METALS [Keynote]

In-situ TEM high-cycle fatigue experiments on electron transparent thin films of nanocrystalline Pt and Cu have revealed not only microstructural-sensitive crack propagation, but also unexpected microstructural-scale crack healing. Based on the experimental observations, atomistic modeling, and continuum-scale microstructural modeling, the mechanism appears to be crack flank cold welding facilitated by local compressive microstructural stresses and/or grain boundary migration. While these observations are specific to pure nanocrystalline metal thin films under a high-vacuum environment, there are potentially much broader ramifications. The existing observations can be used to help rationalize suppressed fatigue crack propagation rates in vacuum, subsurface, or under contact-inducing mixed-mode stresses; and even the precipitous decline in propagation rates near the fatigue threshold.
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UNDERSTANDING OF TOUGHENING IN CEMENTED CARBIDES BY MEANS OF SMALL-SCALE MECHANICAL TESTING AND CHARACTERIZATION

Small-scale mechanical testing (massive nanoindentation, compression of micropillars and fracture of notched microcantilevers) and characterization (cross-section FIB-tomography and FESEM inspection) are proposed and validated as effective tools for studying fracture mechanics and toughening mechanisms governing stable crack growth in cemented carbides. Crack growth resistance behavior of cemented carbides and corresponding microstructural effects are sucessfully described and understood on the basis of ductile ligament reinforcement behind the crack tip as the key toughening mechanism for these materials.
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RELATING NANOSCALE STRUCTURE AND PROPERTIES TO MACROSCALE FRACTURE TOUGHNESS FOR BULK METALLIC GLASSES [Keynote]

Bulk metallic glasses (BMGs) can range from exceptionally tough to brittle depending on their structural state; however, quantifying their structure-property relationships has been an unresolved challenge. Our findings revealed that local hardness variations within the BMG microstructure strongly affect the fracture behavior. Moreover, the hardness heterogeneities are controlled by the size and volume fraction of FCC-like medium-range order (MRO) clusters. We have proposed a model of ductile phase softening whereby relatively soft FCC-like MRO clusters sit in a matrix of harder icosahedral dominated ordering, while micropillar compression testing has revealed how the activation of these clusters into shear transformation zones can be negatively affected by oxygen impurities which in turn lower the fracture toughness.
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CHARACTERIZATION METHODOLOGY OF PIPELINE STEELS USING MINIATURE SPECIMENS

A possible solution to check the fitness-to-service of existing pipeline steels for hydrogen transport is to extract small coupons without interrupting supply operations. From these coupons, it is possible to machine sub-size specimens to characterize ductility and fracture toughness of the base and weld (weld metal and heat affected zone) materials in both air and pressurized hydrogen environments. Using sub-size requires specific facilities. This paper describes a new setup and the associated methodology developed to test sub-size specimens. The method is applied to tests under pressurized hydrogen gas.
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IN SITU SEM HIGH-THROUGHPUT CYCLIC TESTING OF FREESTANDING THIN FILMS

This work presents a small-scale high-throughput technique to characterize the cyclic behavior of freestanding thin films. The technique consists of the microfabrication of a Si carrier composed of an array of grips and freestanding dogbone thin films and of the automated in situ Scanning Electron Microscope (SEM) fatigue testing of the microfabricated carrier. The Si carrier functions as a nanomechanical testing device in which multiple dogbones can be simultaneously and independently tested under the same applied mechanical conditions. As a proof-of-concept, the fatigue behavior of nanocrystalline Al thin films was investigated. The technique allows for the simultaneous evaluation of crack nucleation and propagation across the fatigue life of several dogbones, facilitating the understanding of deformation mechanisms in nanocrystalline metals and providing statistically significant data. This technique reduces total testing time by orders of magnitude and allows for the investigation of the stochastic variability in fatigue failure. The current technique can be further expanded to account for different materials, new geometries and different loadings modes.
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EFFECT OF ELECTRIC CURRENT ON PRE-CRACKED THIN METALLIC SHEETS: FROM CRACK PROPAGATION TO CRACK HEALING [Keynote]

Both scientifically and technologically, it is important to study the effects of electric current pulses on the structural integrity of metallic components. As an electric current reverses its direction across a crack, massive current crowding occurs at the crack tip, thereby generating a non-uniform temperature field sourcing away from the crack tip, and considerable electromagnetic forces are generated on the crack faces that open the crack in Mode I. Recent studies have shown that due to the synergistic effects of the above two stimuli, a pre-existing flaw may grow as well as heal upon application of an electric current pulse of high density. While one is a bane for structural integrity, the other one is a boon to in-service components. Here, we will discuss the reasons behind crack propagation upon application of an electric current and then explore the attributes responsible for a transition from flaw propagation to flaw healing upon passage of an electric current pulse. Furthermore, the synergetic role of mechanical load and magnetic field in the propagation of a pre-existing flaw will be discussed. We will establish the complementary roles of electric current, magnetic field and mechanical load in the failure of pre-cracked metallic sheets.
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UNDERSTANDING FATIGUE DAMAGE PROGRESSION IN A STRUCTURAL STAINLESS STEEL THROUGH CYCLIC BALL INDENTATION TESTING

Understanding fatigue damage progression and estimating remaining life of dynamically loaded components has been a major challenge for several safety critical in-service components. Towards this, a few small specimen fatigue test methods are available, such as, cyclic ball indentation, cyclic small punch test and cyclic bulge test etc. Cyclic ball indentation has the potential to be deployed in-situ during plant maintenance to record fatigue response of localized spots. The method uses a spherical indenter of 1/16” (~1.58 mm) diameter which applies cyclic compression-compression loading on the material at selected location and monitors the load-displacement response continuously to identify failure event due to fatigue.
To capture a complete picture of this, controlled experiments using carefully prepared dog bone fatigue specimens of SS 304 have been conducted. The dog bone specimen is fatigue cycled under tension-tension uni-axial loading till failure, with Acoustic Emission (AE) signature capture during fatigue cycling. the fatigue cycling is interrupted periodically and cyclic ball indentation tests are carried out again at some locations of gage length to identify failure life cycle data of fatigue cycled specimen through displacement sensing and hysteresis area. Data obtained from cyclic ball indentation is then correlated with loss of stiffness.
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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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