Scientific Events

Room: Large Conference Room No. 203

Mechanistic View on Electric Current Induced Kinetic Enhancement and its Various Examples in Materials

Pushing the boundaries of micro and nanomechanics

Pushing the boundaries of micro and nanomechanics
Current level of miniaturization in everyday devices indicates that micro and nano architectures have become functional elements in electronics and diminutive mechanical-based systems. Yet, the potential of such multiscale functional elements is not fully realized due to incomplete understanding of their deformation mechanisms in application relevant loading conditions such as high strain rates (mimicking drops and impacts) and high/cryo temperatures. Even the state-of-the-art micro/nano mechanical testers are currently incapable of conducting experiments in such harsh loading environments. Thus, the mechanical properties of micro and nano scale materials are largely unknown at strain rates beyond 0.1/s and temperatures beyond 250°C or below room temperature. This premise forms the motivation of my research vision: “To investigate the small scale plasticity and failure mechanisms under extreme conditions, using novel micro/nano mechanical experimental platforms”. In this presentation, I will highlight three aspects from my previous research: i) Instrumentation and protocols for conducting extreme micro and nanomechanical testing, ii) Case studies of micro/nano scale metals and amorphous materials tested at high strain rates and high temperature combinations and iii) Sample manufacturing techniques for high through-put micro/nanomechanical testing. Specifically, I will present the work on in situ nanomechanical testing at high strain rates enabled by a custom-built hybrid piezo and microelectromechanical systems (MEMS) based testing system and the case-study on silver nanowires tested at strain rates upto ~200/s. Further, the instrumentation and protocols for micromechanical testing at combinations of high strain rates and extreme temperatures will be explained, with a case study on fused silica and silicon micropillar compression at strain rates upto 1000/s and temperatures upto 400°C. The final part of the talk will focus on my recent work with unique manufacturing methods: two-photon lithography/electrodeposition combination and localized electrodeposition, which are capable of manufacturing ideal damage-free test-beds of metallic micro/nano architectures including arrays of micropillars, microsprings and complex microlattices. [more]

Manipulation of individual defects in 2D and layered Materials

Manipulation of individual defects in 2D and layered Materials
Defects decisively influence the properties of virtually any material. It is therefore desirable to control the occurrence and properties of defects down to the atomic scale. While many methods have been successfully developed to influence defects in an indirect way (e.g. heat treatments, ion implantation, etc...), the direct interaction and control over individual defects is still in its infancy. This method of direct control promises to greatly deepen our understanding of the properties of single defects and may even lead to the discovery of novel physical phenomena. In this work, we demonstrate ways of directly controlling and testing individual defects in the form of dislocations and grain boundaries. Bilayer graphene, being the thinnest material to host extended dislocations, serves as the perfect model material for dislocation manipulation. Using a precisely controlled micromanipulator it is possible to directly interact with individual line defects in situ in scanning electron microscopy1. Besides showcasing fundamental properties of dislocations such as line tension and interaction with free edges, a novel switching reaction at threading dislocations was observed. For the manipulation further developments were made in the form of a mechanical cleaning approach as well as a setup for diffraction in SEM2. Furthermore, using a layered crystal (VSe2) the sliding behavior of twist grain boundaries is analyzed. By cutting and compressing inclined micropillars made from a single-crystalline specimen, twist grain boundaries can be created. After compression, grain boundary sliding can be tested using micromanipulation combined with spring-table based force measurement. Ultra-low sliding friction and self-retraction is observed for twist grain boundaries. Finally, an experimental pathway towards the analysis of the atomic-scale grain-boundary sliding mechanisms in layered systems will be demonstrated. 1. Schweizer, P., Dolle, C. & Spiecker, E. In situ manipulation and switching of dislocations in bilayer graphene. Sci. Adv. 4, (2018). 2. Schweizer, P., Denninger, P., Dolle, C., Rechberger, S. & Spiecker, E. Low Energy Nano Diffraction (LEND) – Bringing true Diffraction to SEM. Microsc. Microanal. 25, 450–451 (2019). [more]

Multi-scale design and analyses of advanced materials: Experimental approaches

Multi-scale design and analyses of advanced materials: Experimental approaches
When a 100-tonne steel forging die fails during industrial processing; the root causes are often localised to small length scales. Advanced materials therefore need to be designed at the characteristic material length scales; incorporating environmental considerations such as local defects or temperature, amongst many others. This talk highlights two recent examples of project work led by Dr. Best with industrial partners. The first focuses on the bottom-up design of ceramic thin-film coatings using nano- and micro-mechanical approaches, where high-temperature fracture toughness measurements were primarily utilised to design multi-layered protective coatings with improved lifetimes for steel forging dies. The second addresses the top-down analysis of a 3D-printed bulk metallic glass, where the connection between bulk toughness and local short range order was linked through in-situ micro-pillar compression. In both examples, the interplay between structure-property relations at multiple length scales is emphasised. [more]

Close Packed Phases in Nickel-Based Superalloys - Investigation by Diffusion Multiples

Close Packed Phases in Nickel-Based Superalloys - Investigation by Diffusion Multiples
Precipitation of close-packed phases is a common problem of modern nickel-based superalloys, containing refractory or higher melting point elements such as Re, Ru, Cr, Mo and W. Thus, a fundamental understanding of phase stabilities of close-packed phases governed by these elements is of high relevance regarding the improvement of databases for nickel-based superalloys and the development of next generation superalloys. Diffusion multiples have been used to investigate the ternary systems Ni-Mo-Cr, Ni-Mo-Re and Ni-Mo-Ru at 1100°C and 1250°C. A novel manufacturing technique for diffusion multiples based on a two-step casting process will be presented. EDS and EBSD measurements lead to isothermal sections of phase diagrams. Additionally investigations of certain quaternary systems will be shown. Solubility limits of sigma-, P-, delta- and hcp-phase were determined. Adaptation of the MatCalc database to the experimental results by project partners in Vienna lead to significant improvements in predictions for multicomponent alloys. [more]

Lessons learned from nano scale specimens tested by MEMS based apparatus

Lessons learned from nano scale specimens tested by MEMS based apparatus
Materials at small scale behave differently from their bulk counterparts. This deviation originates from the abundance of interfaces at small scale. Quantifying the properties and revealing the underlying mechanisms requires experiments with small samples in situ in analytical chambers. However, small size poses the challenge of sample handling, but offers the opportunity of in situ inspection of mechanism during testing in analytical chambers. In order to overcome the challenge and take advantage of the opportunity, we developed a MEMS based micro scale testing stage where the sample and the stage are co-fabricated. The stage suppresses any misalignment error in loading by five orders of magnitude. The stage allows in situ inspection of samples during testing in SEM and TEM. We employed the stage in two scenarios. (1) Exploring the effect of microstructural heterogeneity, such as grain size and orientation, on the deformation mechanisms in nano grained polycrystalline metals. Here the test specimens are free standing thin films subjected to uniaxial tension. We found that heterogeneity introduces two apparently dissimilar, but fundamentally linked, anomalous behaviors. The samples undergo plastic deformation during unloading, i.e., exhibit Bauschinger type phenomenon. Upon unloading, they recover a significant part of plastic deformation with time. The underlying mechanism, verified by in situ TEM inspection, is as follows: during loading, the relatively larger grains undergo plastic deformation and relax by employing dislocations, while the smaller grains remain elastically deformed. During unloading, the smaller grains apply reverse stress on the larger grains causing reverse plasticity resulting in a deviation from linear stress-strain response. Upon complete unloading, the residual stress of the elastically strained small grains continue to apply reverse stress on the larger grains resulting in biased jumps of dislocation in the larger grains and strain recovery. (2) Exploring the effect of size on brittle to ductile transition (BDT) temperature (540C) in single crystal silicon. Here the sample is a micro scale single crystal silicon beam subjected to bending which limits the high stress region to a small volume in the sample, and minimizes the probability of premature failure from random flaws. We found that silicon indeed deforms plastically at small scale at temperatures much lower than 540C. Ductility is achieved through a competition between fracture stress and the stress needed to nucleate dislocations from the surface. Our combined SEM, TEM and AFM analysis reveals that as a threshold stress is approached, multiple dislocation nucleation sites appear simultaneously from the high stressed surface of the beam with a uniform spacing of about 200 nm between them. Dislocations then emanate from these sites with time lowering the stress while bending the beam plastically. This process continues until the effective shear stress drops and dislocation activities stop. A simple mechanistic model is presented to relate dislocation nucleation with plasticity in silicon. [more]

Molecular dynamics simulations and beyond for plasticity and wear of metals

Molecular dynamics simulations and beyond for plasticity and wear of metals
While the general principles underlying the plastic response of metals are mostly understood—especially for the crystalline state—advanced tailoring of their properties and the development of novel, high-performance materials requires detailed insights into the mechanisms at the atomic scale. This talk will discuss how computer models and simulations can be the tools of choice to discover such mechanisms in the context of our work on amorphous metals and wear. I will first introduce some concepts of molecular dynamics computer simulations and how we used them to investigate the plastic deformation of metallic crystal/glass composites, where localized and collective shear transformations govern the macroscopic behavior. In particular, I will address the questions of when and how precipitates can enhance the mechanical properties of metallic glasses and what the difference between a nanocomposite and a nanocrystal is. In the second part, I will discuss our current work on wear of materials and what to do when the time and length scale limitations of molecular dynamics become a problem. When surfaces in contact slide relative to each other, they are in fact only in contact in some small areas due to their roughness. At this length scale, we employ molecular dynamics with different model materials in order to elucidate how detachment of matter occurs in the form of individual particles, which in the end comes down to the details of nanoscale plasticity and fracture processes. In order to gain insights on relevant figures of merit for applications, though, we have to collect statistics of wear particle formation at the meso to macroscale, using continuum methods. Since some of this work is in its early stages, I will finish the talk with a preview of promising future research directions and methods, such as taking the microstructure evolution of the material into account. [more]
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