Scientific Events

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The analysis of deformation and failure mechanisms in small-scale devices and thin films is a critical issue, not yet solved. In this presentation, we describe recent advances and developments for the measurement of fracture toughness at small scales by the use of nanoindentation-based methods including techniques based on micro-cantilever, beam bending and micro-pillar splitting. A critical comparison of the techniques is made by testing a selected group of bulk and thin film materials. For pillar splitting, cohesive finite element simulations are used for analysis and development of a simple relationship between the critical load at failure, pillar radius, and fracture toughness for a given material. The minimum pillar diameter required for nucleation and growth of a crack during indentation is also estimated. An analysis of pillar splitting for a film on a dissimilar substrate material shows that the critical load for splitting is relatively insensitive to the substrate compliance for a large range of material properties. Micro-pillars are then produced by Focused Ion Beam (FIB) ring milling, being the pillar diameter approximately equal to its length; this ensures full relaxation of pre-existing residual stress in the upper portion of the specimen. Nanoindentation splitting tests are performed in-situ and the deformation mechanisms corresponding to each class of materials have been investigated. Experimental results from a selected group of materials show good agreement between single cantilever and pillar splitting methods, while a discrepancy of ~25% is found between the pillar splitting technique and double-cantilever testing. The limitations of the method are finally discussed. In particular, a minimum pillar’s diameter for the nucleation and growth of a crack during indentation is identified and quantified for a wide range of materials properties. It is concluded that both the micro-cantilever and pillar splitting techniques are valuable methods for micro-scale assessment of fracture toughness of brittle ceramics, provided the underlying assumptions can be validated. Although the pillar splitting method has some advantages because of the simplicity of sample preparation and testing, it is not applicable to most metals because their higher toughness prevents splitting, and in this case, micro-cantilever bend testing is preferred. [more]

Interfaces play a decisive role in the deformation of any polycrystalline metal or precipitate-strengthened alloy. Perhaps best known is the role of grain boundaries (GBs) as obstacle to dislocation motion as evidenced by the Hall-Petch strengthening. However, GBs can also serve as initiation sites for fracture and provide easy pathways for crack propagation. When the grain size is reduced below 100 nm, GBs become furthermore the dominant sources and sinks for dislocations, and pinning of dislocations at GBs becomes an important hardening mechanism. At very small grain sizes below about 10 nm, the contribution of grain boundary glide and grain rotation becomes significant. All these processes take place at the atomic scale. Consequently, atomistic simulations have played a key role in studying grain- and interphase boundaries (IPBs), and their interactions with dislocations. However, most of the detailed studies on dislocation – interface interactions were performed on quasi-two dimensional simulation setups with straight dislocation lines interacting with perfectly planar interfaces. Similarly, the deformation of nanocrystalline metals is commonly studied using artificial structures generated by means of the Voronoi tessellation. This procedure creates planar GBs and non-equilibrium triple junction topologies, as well as unrealistic numbers of neighboring grains and distributions of triple line lengths. Here we give an overview on our recent atomistic studies on dislocation – interface interactions, with the focus on non-planar boundaries and more realistic GB topologies. Simulations on twinned nanoparticles and nanowires are used to demonstrate that the presence of twin boundaries can change the deformation mechanism, thereby explaining experimentally observed dislocation structures. Controlled studies on dislocations interacting with various high-angle GBs in a bicrystal setup allow to quantify changes in the stress field and energy of absorbed dislocations and show the importance of GB curvature on slip transmission through GBs. We then compare the processes taking place in various nanocrystalline samples with different degrees of GB curvature as well as different GB network topologies. Here, a statistical analysis shows clear differences in terms of stress states and contributions of dislocation glide versus GB-mediated processes, however the distribution of critical stresses for dislocation nucleation and dislocation depinning from GB as well as on the distribution of plastic strain caused by individual slip events remains unaffected by the GB topology. Finally, we report on simulations on atom probe tomography – informed superalloy samples, which reveal the importance of interface curvature and chemical composition on the misfit dislocation network and subsequent interactions with matrix dislocations. [more]

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The lifetime of aerospace engineering components is often limited by fatigue. Traditional management strategies presuppose an existing defect length and estimate time to failure through short crack growth and propagation methods, using empirical approaches such as fitting of a Paris’ law. New advances in material processing and production render this argument insufficient to exploit tackle the next generation clean and well-engineered materials, as these growth based empirical studies are too conservative for effective engine management. In this talk, I will outline our recent work focussing on exploiting the next generation of characterisation tools, such as high (angular) resolution electron backscatter diffraction, high (spatial) resolution digital image correlation, combined with geometrically faithful and relatively simple (i.e. limited free parameters) lengthscale based crystal plasticity approaches. These have been brought to bear on a experimental and modelling campaign that focusses on tracking deformation and damage accumulation in single, directionally solidified, polycrystalline and polycrystalline Ni-superalloys with inclusions. In this talk I will outline some highlights from this body work which include: a comparison of ability of HR-DIC and HR-EBSD to recover components of the deformation tensor; understanding accumulated damage and the onset of cracking near non-metallic inclusions; and predicting and understanding accumulated slip in fatigue. [more]

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Metal additive manufacturing (AM) techniques are powder-based, layer by layer methods which can directly build 3D structures onto substrates with complex geometries. They offer a unique ability to dynamically mix materials during the deposition process and produce functionally graded structures, new composite microstructures and perhaps even new material classes. Some of the challenging issues related to the energy beam based process are the very high heating and cooling rates, leading to non-equilibrium microstructures, which are usually harder, less ductile, and often exhibit high residual stresses; the strongly textured, anisotropic microstructures inherited from the solidification conditions; or the pronounced residual stresses resulting from the large thermal gradients in the AM fabricated parts. However, the very rapid consolidation of the material in a small material volume and the achieved high solidification rates allow for the manufacture of components containing meta-stable materials. In this talk some relevant results of the AM related research at Empa will be presented. The first part of the presentation deals with the development and characterization of a novel oxide dispersion strengthened (ODS) titanium aluminide alloy (Ti-45Al-3Nb ODS) for beam-based AM processes. The alloy design and selection process was supported by computational thermodynamics based on the CALPHAD approach, taking into account requirements for processing as well as long term alloy behavior under service conditions. Besides, an in situ method to study the behavior of alloys during rapid heating and cooling combining laser heating with synchrotron micro X-ray diffraction (microXRD) and high-speed imaging was developed. In the second part, the feasibility of producing metal-diamond composites by SLM was studied. A Cu-Sn-Ti alloy powder was mixed with 10-20 vol.% artificial, mono-crystalline diamonds. The influence of the processing parameters on the density and microstructure of the composites as well as on the stability of the diamonds was studied. It was shown that stable specimens containing intact diamonds could be produced. [more]