Scientific Events

Reinforcement with ordered intermetallic precipitates is a potent strategy for the development of strength alongside damage tolerance and is central to the success of fcc nickel-based superalloys. Such a strategy is equally of interest within bcc-based systems for their increased melting point and acceptable cost. However, only limited studies have been made on refractory metal (RM) or titanium based alloys strengthened by ordered-bcc precipitates (e.g. B2 or L2₁). Are such “bcc superalloys” possible? Do they offer useful properties? In this talk, opportunities for refractory-metal-based superalloys systems will be discussed, including a review of Cr-Ni₂AlTi, Mo-NiAl, Ta-(Ti,Zr)₂Al(Mo,Nb) and Nb-Pd₂HfAl systems together with newly developed alloys. These alloys exploit an extensive two-phase field that exists between A2 (RM,Ti) and B2 TiFe to produce nanoscale precipitate reinforced microstructures that increase strength by over 500 MPa. This work was supported through EUROfusion Researcher Grant & EPSRC Doctoral Prize Fellowships, EPSRC ‘DARE’ (darealloys.org) EP/L025213/1 and Rolls-Royce/EPSRC Strategic Partnership EP/H022309/1 and EP/H500375/1. [more]

Nearly all classes of materials show non-equilibrium phase transitions and the first technological use of quenching metals for designing properties is documented as ~800 BC. However, the decomposition towards equilibrium is still difficult to understand due to the strong non-equilibrium kinetics. Two examples are discussed: First the decomposition of a quenched super saturated solid solution and second the decomposition of a quenched metallic melt. In the first example the technological important AlMgSi alloys are addressed. Low temperature solute clustering, its implications on aging and the effect of trace elements are discussed. Moreover, it is shown which physical pre-requisites need to be fulfilled to modify diffusion by orders of magnitude and to examine a “diffusion on demand” concept. In the second example the first solid–solid transition via melting in a metal, detected upon the decomposition of a metallic glass, is demonstrated. The transformation path is discussed under its thermodynamic and kinetic prerequisites. Moreover, the capabilities of the applied novel technique of fast scanning calorimetry is addressed. Finally, it is outlined how this technique links the two examples via its potential for in-situ measuring the non-equilibrium vacancy evolution. [more]

Dislocations in oxides are typically heavily charged and are surrounded by compensating electric charges. As such they are kinetically more stable than chemical dopants. Adepalli et al. termed dislocations a means for “one-dimensional doping” [1]. As they are often introduced by mechanical methods, they may also be termed “mechanical doping” or “self-doping”, as the charges derive from local concentration of the matrix elements. In the literature dislocations have been demonstrated to enhance oxygen conductivity [1] and improve the figure of merit of thermoelectrics by reducing thermal conductivity through phonon scattering by dislocations [2]. Dislocations have been suggested to improve interfacial reaction kinetics and have been theoretically predicted to pin domain walls in ferroelectrics. In Darmstadt we have so far focused on establishing a set of techniques to introduce dislocations into single crystals at room temperature or enhanced temperature and to study (dislocation) creep. Structural investigations have been performed by dark-field X-ray diffraction, rocking curve analysis [3], TEM, NMR and EPR techniques. The first property evaluations have been done with respect to electrical and thermal conductivity and domain wall pinning. All this has to be seen with the perspective of a just developing field, with many opportunities, many obstacles and a lot of exciting uncertainty. Select examples will be provided on dislocation structures, electrical and thermal conductivity in SrTiO₃ and our first attempts on dislocation creep in BaTiO₃. Time provided, I will show 4 slides on the small brother field: “Elastic-deformation tuned conductivity in piezoelectric ZnO." [1] Adepalli, K. K., Kelsch, M., Merkle, R., and Maier, J., "Enhanced ionic conductivity in polycrystalline TiO2 by "one-dimensional doping''," Phys. Chem.Chem. Phys., 16[10] 4942-51 (2014). [2] S. Il Kim, K. H. Lee, H. A. Mun, S. H. Kim, S. W. Hwang, J. W. Roh, D. J. Yang, W. H. Shin, X. S. Li, Y. H. Lee, G. J. Snyder, S. W. Kim, “Dense dislocation arrays embedded in grain boundaries for high-performance bulk thermoelectrics“, Science, 348, 109-114 (2015). [3] E.A. Patterson, M. Major, W. Donner, K. Durst, K.G. Webber and J. Rödel, „Temperature dependent deformation and dislocation density in SrTiO3 single crystals”, J. Amer. Ceram. Soc., 99, 3411-120 (2016). [more]

This presentation provides an overview of recent developmental efforts at Oak Ridge National Laboratory (ORNL) on heat-resistant ferrous materials with Laves-phase strengthening for fossil-fired energy conversion systems. Laves phases are attractive as second-phase strengtheners in Fe-base alloys, including ferritic and austenitic stainless steels, since most of the Fe-rich Laves phases (Fe₂M intermetallic compounds, M: Nb, Mo, W, Zr, Ti, etc.) are thermodynamically equilibrated with BCC- or FCC-Fe solid solution. Because of the characteristics, relatively easy control of second-phase dispersion is expected through a traditional “solution-and-annealing” process combined with proper alloying additions. The thermal stability of the Laves phase precipitates at elevated temperature was found to be controlled and improved through combinations of multiple Laves-phase forming elements, which guides the alloy design and provides effective strengthening of high-temperature structural materials for the extended periods of time. Laves-phase precipitation in Fe-base matrix can be expected in relatively large composition/temperature ranges, which also allows designing the alloys with proper surface protections, such as chromia- or alumina-scale formation on the surface. This leads to proposing and designing new high-temperature structural materials to be used in extreme environments such as Advanced USC or supercritical CO₂ cycle applications. The presentation will also introduce various developmental efforts in Fe-base, Cr-base, and Cu-base alloys with Laves-phase strengthening at ORNL in the last decades. Research supported by the U.S. Department of Energy, Office of Fossil Energy, the Crosscutting Research Program. [more]

Laves phases constitute the largest class of intermetallic phases. Within the inter-institutional research initiative “The Nature of Laves Phases” of the Max Planck Society (2006-2011) fundamental aspects of Laves phases have been investigated. Since then, advances in high resolution analytical methods and modelling gave new insight. Simultaneously interest in development and application of alloys strengthened by Laves phases has considerably increased. The workshop is devoted to summarise our current understanding of Laves phases and to identify topics for future research.The workshop is jointly organised by Forschungszentrum Jülich, Max-Planck-Institut für Chemische Physik fester Stoffe (Dresden), Tokyo Institute of Technology and Max-Planck Institut für Eisenforschung GmbH. [more]

Advances in 3D additive manufacturing techniques have enabled the fabrication of nanostructures with remarkable mechanical properties. Using the latest 3D printing techniques, novel material structures with specific architectures, often referred to as metamaterials, can be produced. They can exhibit superior mechanical and physical properties at extremely low mass densities and, thus, expand the current limits of the yet stiff and strong architectures, architectures with high mechanical resilience or with negative Poisson’s ratio. Mechanical size effects were shown to result in extraordinary strength values of different specific architectures. Understanding the underlying characteristics of these complex new materials, such as the deformation and failure mechanisms, and how they impact behavior of the structure, is critical and increasingly challenging. In this presentation, the principles underlying ultra-strong yet light 3D nano-architected metamaterials as well as strategies to tailor their properties will be discussed. [more]

We formulate a theory of non-equilibrium statistical thermodynamics for ensembles of atoms or molecules. The theory is an application of Jayne's maximum entropy principle, which allows the statistical treatment of systems away from equilibrium. In particular, neither temperature nor atomic fractions are required to be uniform but instead are allowed to take different values from particle to particle. In addition, following the Coleman-Noll method of continuum thermodynamics we derive a dissipation inequality expressed in terms of discrete thermodynamic fluxes and forces. This discrete dissipation inequality effectively sets the structure for discrete kinetic potentials that couple the microscopic field rates to the corresponding driving forces, thus resulting in a closed set of equations governing the evolution of the system. We complement the general theory with a variational meanfield theory that provides a basis for the formulation of computationally tractable approximations. We present several validation cases, concerned with equilibrium properties of alloys, heat conduction in silicon nanowires, hydrogen desorption from palladium thin films and segregation/precipitation in alloys, that demonstrate the range and scope of the method and assess its fidelity and predictiveness. These validation cases are characterized by the need or desirability to account for atomic-level properties while simultaneously entailing time scales much longer than those accessible to direct molecular dynamics. The ability of simple meanfield models and discrete kinetic laws to reproduce equilibrium properties and long-term behavior of complex systems is remarkable. [more]