Scientific Events

Wide-band-gap oxides have many unique properties that make them ideally suited for applications in energy technologies. They are transparent in the visible, but can be doped to near-metallic conductivities, enabling use as contact layers to optoelectronic devices such as solar cells and light-emitting diodes. They can also be used, passively, in energy-efficient window coatings, or actively in smart windows or transparent electronics for displays. When grown as high-quality heterostructures, oxide-based transistors can be used in power electronics that boost the efficiency of power conversion, currently a large source of loss in applications ranging from hybrid cars to data centers. Complex oxides (containing transition metals or rare-earth elements) offer the prospect of revolutionary new electronic devices. I will discuss how cutting-edge first-principles calculations provide key information about materials properties and enable designing new materials combinations for specific applications. [more]

Workshop for PhD students and early stage PostDocs. The number of participants is limited to 30. Please register until Monday, 20 July 2015 by setting a mail with your contact data to k.huebel@mpie.de. [more]

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NiTi shape memory alloys were first discovered in 1961 by W.J. Buehler and F. Wang - researchers in the United Sate Naval Ordnance Laboratory. Their unique properties immediately drew attention of scientists and engineers, however commercial applications were postponed by more than decade due to tremendous difficulties in melting, processing and machining of the material. There exist other alloys in which shape memory effects occur but NiTi shows the best values of these properties coupled with very good mechanical properties and is the only one shape memory alloy that is highly biocompatible. The shape memory effects i.e. one-way, two-way shape memory effect and superelasticity are caused by thermoelastic reversible martensitic transformation that takes place in the material during temperature change or when an external stress is applied. In NiTi alloys this is the B2 (b.c.c.) phase that transforms to the monoclinic B19' martensite. The transformation characteristics depends very strongly on the alloy chemical composition and its widely understood defect structure. Moreover, in some cases the B19' transformation is preceded by another transition: the R-phase transformation which is also of thermoelastic martensitic type. The transformation sequence, its mechanism and the structure of the R-phase have been for a long time a matter of discussions and discrepancies. Also another problem that was discovered in some NiTi alloys i.e. multistage course of the martensitic transformation was a subject of many studies. The above points will be discussed in the presentation. Additionally, recent trends in NiTi alloys development will be presented. The material can be used in medicine for e.g. stents or osteosythesis clamps. Thus, the surface modification for the medical application is one of the concerns of scientists. [more]

Thermodynamically consistent phase field approach (PFA) for multivariant martensitic phase transformations (PTs) and twinning for large strains is developed [1,2]. A thermodynamic potential is introduced, which allowed us to describe each martensite-martensite (i.e., twin) interface with a single order parameter [3]. These theories are utilized for finite element simulation of various important problems [1-4]. PFA to dislocation evolution was developed during the last decade and it is widely used for the simulation of plasticity at the nanoscale. Despite significant success, there are still a number of points for essential improvement. In our work [5,6], a new PFA to dislocation evolution is developed. It leads to a well-posed formulation and mesh-independent solutions and is based on fully large-strain formulation. Our local potential is designed to eliminate stress-dependence of the Burgers vector and to reproduce desired local stress-strain curve, as well as to obtain the mesh-independent dislocation height H for any dislocation orientation. The gradient energy contains an additional term, which excludes localization of dislocation within height smaller than H but disappears at the boundary of dislocation and the rest of the crystal; thus, it does not produce interface energy and does not lead to a dislocation widening. Problems for nucleation and evolution of multiple dislocations along the multiple slip systems are studied. The interaction between PT and dislocations is the most basic problem in the study of martensite nucleation and growth. Here, a PFA is developed to a coupled evolution of martensitic PTs and dislocations [7,8], including inheritance of dislocation during direct and reverse PTs. It is applied to studying the hysteretic behavior and propagation of an austenite-martensite interface with incoherency dislocations, the growth and arrest of martensitic plate for temperature-induced PTs, the evolution of phase and dislocation structures for stress-induced PTs, and the evolution of dislocations and high pressure phase in a nanograined material under pressure and shear [7-9]. In particular, possibility to reduce PT pressure by an order of magnitude, obtained in our experiments on BN, was confirmed in simulations. Short review of PFAs to other structural changes will be made, including melting of nanoparticles, superheating with ps and fs lasers, interface stresses and nonequilibrium energy, and PT between two solids via intermediate melt within solid-solid interface. 1. V. I. Levitas, V. A. Levin, K. M. Zingerman, & E. Freiman, Phys. Rev. Lett. 103, 025702 (2009). 2. V. I. Levitas, Int. J. Plasticity 49, 85-118 (2013). 3. V. I. Levitas and A. M. Roy, Phys. Rev. B 91, 174109 (2015). 4. V. A. Levin, V. I. Levitas, K. Zingerman & E. Freiman, Int. J. Solids & Struct. 50, 2914-28 (2013). 5. V. I. Levitas and M. Javanbakht, Phys. Rev. B., Rapid Commun. 86, 140101 (2012). 6. V. I. Levitas and M. Javanbakht, J. Mech. Phys. Solids, DOI: 10.1016/j.jmps.2015.05.009 (2015). 7. V. I. Levitas and M. Javanbakht, J. Mech. Phys. Solids, Parts 1 and 2, DOI: DOI:10.1016/j.jmps.2015.05.005 and DOI:10.1016/j.jmps.2015.05.006 (2015). 8. V. I. Levitas and M. Javanbakht, Appl. Phys. Lett. 102, 251904 (2013). 9. V. I. Levitas and M. Javanbakht, Nanoscale 6, 162 - 166 (2014). [more]

We would like to invite you to participate in our three-days workshop, which will be held in June 21st-24th, 2015 at Reisensburg Castle (Günzburg/Donau, Germany). The workshop brings together renowned scientists from the area of theoretical and experimental grain boundary migration. The aim is to provide a distinguished atmosphere and framework for exchanging the newest ideas and concepts in order to resolve the present challenges in the field. more on http://www.mpie.de/3104106/GB2015 [more]

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