Scientific Events

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]

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The austenite-ferrite transformations are a key metallurgical tool to tailor properties of advanced low-carbon steels. Even though significant progress has been made to develop knowledge-based process models for the steel industry it remains critical to improve the predictive capabilities of these models by developing next generation modelling approaches with a minimum of empirical parameters. Computational materials science now offers tremendous opportunities to formulate microstructure evolution models containing fundamental information on the underlying atomistic mechanisms that can be implemented across different length and time scales. The phase transformation kinetics depends critically on interface migration rates which are significantly affected by the presence of alloying elements, e.g. Mn, Mo and Nb in steels. Here, an approach is illustrated that links atomistic scale models for the solute-interface interaction with phase field modelling and conventional diffusion models. The overall status of this multi-scale phase transformation model approach will be analyzed for intercritical annealing of dual-phase steels and the rapid heat treatment cycles in the heat affected zone of linepipe steels. [more]

III-V semiconductor nanowires (NWs) grown onto silicon substrate may become new building blocks of modern optoelectronic and electronic devices. For success in technical application it is necessary to explore their physical properties on the nanoscale. In MBE grown semiconductor NWs axial stacking faults separating zinc-blende and wurtzite entities are the major structural defects influencing the physical properties. Structural composition, phase arrangement and residual strain of individual GaAs NWs grown on Si(111) can be investigated X-ray nano-diffraction employing a focused synchrotron. It is found that even neighbouring NWs grown on the same sample under the same growth conditions differ significantly in their phase structure Moreover, the misfit strain at the substrate to NW interface releases within few monolayers due to relaxation towards the NW side planes [1]. The evolution of stacking faults is no constant but depends on growth time and the growth mode. In case of InAs NWs grown catalyst-free along the [111] we explored the dynamic relation between the growth conditions and the structural composition of the NWs using time-resolved X-ray scattering and diffraction measurements during the MBE growth. The spontaneous buildup of liquid indium droplet in the beginning of the growth process is accompanied by the simultaneous nucleation of InAs NWs predominantly grown in the wurtzite phase with low number of stacking faults. After nucleation the In droplets become consumed resulting in structural degradation of NWs due to the formation of densely spaced stacking faults [2]. For the first time the particular phase structure of single GaAs NWs could be correlated with their electrical properties. Here the V-I characteristics was measured in a dual Focused Ion Beam chamber the resistance and their effective charge carrier mobility was modeled in terms of thermo-ionic emission theory and space charge limited current model, respectively. Both resistance and inverse mobility show a qualitatively similar electric behavior comparing the inspected NWs. The same single NWs electrically measured have been inspected by X-ray nano-diffraction. The NWs were found to be composed by zinc-blende and twinned zinc-blende units separated by axial interfaces and a small plastic displacement. It turns out that the measured value of the extracted resistance and the inverse of effective mobility increases with the number of intrinsic axial interfaces, whereas the small plastic displacement has less influence on electrical properties [3]. We acknowledge support by BMBF and DFG. [more]