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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Themes: Small Scale Specimen Testing
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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ANALYSIS OF FRACTURE BEHAVIOUR OF MULTILAYERS BY CANTILEVER AND CLAMPED BEAM BENDING GEOMETRY
Multilayering of metal/ceramic combinations can help to achieve better strength and toughness than the individual material constituents. The effect of elastic-plastic mismatch in multilayers on the crack driving force and eventually on fracture resistance has been analyzed in this work. The enhancement in fracture toughness by decreasing layer spacing has been predicted from finite element calculations and verified by micro-cantilever fracture tests. Further, calculations have been carried out for a more stable clamped beam bend geometry to determine R-curve behavior in such multilayers.
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OPTIMIZATION AND USE OF HIGH-THROUGHPUT MICROMECHANICAL TESTING DESIGN FOR 3D-PRINTED POLYMERS
Modern materials behave differently on a micro-scale level than in bulk applications. Therefore, with ever present miniaturization, the materials’ testing on a micron-level is gaining importance. 3D printing with a sub micron precision, such as direct laser writing by two-photon lithography, allows for relatively fast manufacturing of miniaturized specimens for micromechanical testing. In combination with precise loading by a nanoindenter tip, high throughput micromechanical testing is enabled. Presented research shows design process of miniaturized cantilever and push to pull device specimens for fracture mechanics testing, aided finite element modelling, together with high throughput testing of polymeric materials with varied printing parameters and loading conditions. Such in situ and ex situ experimental setup allows for systematic fracture mechanics testing on the small scale for common materials used in small-scale 3D printing.
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MICRO-BENDING FOR MULTI-SCALE FRACTURE CHARACTERIZATION OF CEMENT-BASED MATERIALS AND CERAMICS
The last few years have seen the development of several testing techniques at the micro-scale to characterize the mechanical properties of multi-scale materials. One such novel methods are micro-cantilever bending tests to assess mechanical properties of materials. Micro-cantilever tests allow for a variety of test configurations, including scaled chevron-notch geometry allowing controlled crack-growth prior to critical failure load. Chevron notches are convenient at such length scales since they allow for ex-situ measurements of crack length – avoiding the need to directly measure crack length, which could be a challenge at such scale. The test approach provides access to R-curve behavior for quasi-brittle materials on the micro-scale and opens the door to investigate more sophisticated topics, i.e., creep crack growth. Moreover, the technique complemented with visualization and scanning tools, such as Scanning Electron Microscopy and Microtomography, allows for a proper and extensive analysis of the crack growth phenomena.
The motivation of this work revolves around expanding currently available knowledge on multiscale characterization of cement-based materials with the focus on fracture testing at the micro- and mesoscale through the novel micro-bending technique. This test opens the door to understanding fracture, toughening mechanisms, and evolution of these properties regarding processing variables.
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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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