Scientific Events

Due to their ubiquity, magnetic materials play an important role in improving the efficiency and performance of devices in electric power generation, conversion and transportation 1. Permanent magnets are essential components in motors and generators of hybrid and electric cars, wind turbines, etc. Magnetocaloric materials could be the basis for a new solid state energy efficient cooling technique alternative to compressor based refrigeration 2. The talk focuses on rare earth and rare earth free permanent magnets 3 and magnetocaloric materials, with an emphasis on their optimization for energy and resource efficiency in terms critical element utilization. The concept of criticality of strategic metals is explained by looking at demand, sustainability 4 and the reality of alternatives of rare earths. Modelling, synthesis, characterization, and property evaluation of the materials will be discussed considering their micromagnetic length scales, hysteresis and phase transition characteristics. Specifically I will give examples for how to (a) reduce Dy needs by grain boundary diffusion processes, (b) utilize excess Ce in (Nd,Ce)Fe14B balance magnets, (c) tailor anisotropy by 5d elements in (Fe1−xCox)2B alloys and (d) assess magnetic moments in iron nitrides 5-7. 1 Magnetic Materials and Devices for the 21st Century: Stronger, Lighter, and More Energy Efficient, Adv. Mat. 23 (2011) 821. 2 Giant magnetocaloric effect driven by structural transition, Nature Mat. 11 (2012) 620. 3 Towards high-performance permanent magnets without rare earths, J. Phys.: Condens. Matter 26 (2014) 064205. 4Recycling Used Nd-Fe-B Sintered Magnets via a Hydrogen-Based Route to Produce Anisotropic, Resin Bonded Magnets, Advanced Energy Materials 3 (2013) 151. 5Temperature dependent Dy diffusion processes in Nd-Fe-B permanent magnets, Acta Mat. 83 (2015) 248 6Effect of doping by 5d elements on magnetic properties of (Fe1−xCox)2B alloys, Phys. Rev. B 92 (2015) 174413. 7Increased magnetic moment induced by lattice expansion from α-Fe to α′-Fe8N, J. Appl. Phys. 117 (2015) 173911. [more]

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In this talk an overview of the room temperature grain coarsening effect in polymer-supported thin gold and copper films under cyclic mechanical loading will be presented. Detailed EBSD analysis, as the major characterization method, allows to capture extensive statistical data about the evolution of thousands of grains with the cycle number but also to observe the motion and elimination of single grain boundaries. It will be shown that very strong and homogeneous grain coarsening occurs in 500 nm gold films where the average grain size grows from 200 nm to approximately 2 µm during cyclic loading. In contrast, 500 nm thick copper films with bi-modal grain size distribution exhibit rather moderate grain coarsening which leads to the reduction of the fraction of ultra-fine grained areas. The correlation between the grain coarsening and the development of fatigue damage will be discussed along with background mechanisms of motion and elimination of grain boundaries. [more]

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]