Scientific Events

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]

In the search of a truly high-temperature superconductor we tried to metalize dirty (i.e. yttrium doped) hydrogen under high pressure. Shining light during hydrogenation of an yttrium film in a diamond anvil cell was the key ingredient of our discovery of switchable mirrors. Since then the transition from shiny metal to transparent semiconductor or from metal to highly absorbing black has been observed in many hydrides. Even in metals that remain metallic during hydrogenation, the optical changes induced by absorption of hydrogen are easily observable. This opened the way to Hydrogenography, a new high-throughput optical technique to measure optically and simultaneously on thousands of (nano)structured samples, pressure-composition isotherms, enthalpies and entropies of hydride formation. We demonstrated for example that the thermodynamics of hydrogen absorption in Mg/TM (TM: transition metal) (multi)layers can be tuned by engineering suitable elastic constraints or by reducing particle sizes down to 1 nm. Hydrogenography provides also unique possibilities to determine the intrinsic hydrogen permeability of alloys and to explore whether or not observed enthalpy-entropy correlations are phantom effects. Reaction kinetics and catalytic activities, long-range diffusion, switchable metal-hydrides for smart windows, fiber optic H sensors and nanoantennas for active plasmonics are also efficiently investigated with Hydrogenography. [more]

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]

Properties of materials are sensitively influenced by the microstructure inherited from their synthesis and processing. In response to high stress, temperature and composition gradients microstructures evolve in a complex way that involves nucleation of new phases, interface migration and mass redistribution that lead to complex morphological evolution on the mesoscale. Understanding this evolution and the ways it influences properties can be key to optimizing materials for targeted applications. Due to their importance related to grain structure evolution and properties of polycrystals, significant effort has been devoted to calculation of free energies and mobilities of isolated grain boundaries using atomistic simulations. It is commonly assumed that interface properties are continuous functions of temperature, pressure and chemical composition. On the other hand there is accumulating evidence suggesting that interfaces are capable of first order structural transitions, in which case the properties like segregation, excess volume, mobility, cohesive strength and sliding resistance may change discontinuously. This talk will review the results of recent atomistic computer simulations, investigating the nature of structural phase transitions in metallic grain boundaries, induced by changes in temperature and composition. We start by reviewing changes in the structure of elemental boundaries that are observed when these interfaces are exposed to very high homologous temperatures, and the nature of the qualitatively different types of structural disordering that can arise. The transitions involve changes in atomic density in the grain-boundary plane, which was discovered only when new simulation methodologies were developed that permit such variations. We show that interfaces can absorb large number of point defects through a first order phase transition, which may play important role in recovery of materials from radiation damage. Our simulations demonstrated strong effect of the transitions on self and impurity diffusion as well as grain boundary migration and shear strength. [more]

The processing regime relevant to superplasticity in the Ti-6Al-4V alloy is identifed. The effect is found to be potent in the range 850 to 900 deg ?C at strain rates between 0.001/s and 0.0001/s. Within this regime, mechanical behaviour is characterised by steady-state grain size and negligible cavity formation; electron backscatter diffraction studies confirm a random texture, leaving grain boundary sliding as the overarching deformation mechanism. Outside of the superplastic regime, grain size refinement involving recrystallisation and the formation of voids and cavities cause macroscopic softening; low ductility results. Stress hardening is correlated to grain growth and accumulation of dislocations. The findings are used to construct a processing map, on which the dominant deformation mechanisms are identified. Physically-based constitutive equations are presented which are faithful to the observed deformation mechanisms. Internal state variables are used to represent the evolution of grain size, dislocation density and void fraction. Material constants are determined using genetic-algorithm optimisation techniques. Finally, the deformation behaviour of this material in an industrially relevant problem is simulated: the deformation of diffusion-bonded material for the manufacture of hollow, lightweight structures, e.g. those used for fan blades of aeroengines. [more]

In this talk, I will give an overview of our attempts to design various solid catalysts for industrially important chemical conversions. These reactions include conversion of biomass derived platform chemicals to other value-added chemicals, lower alkane activation and CO2 activation. We try to understand how the structural aspects of catalysts affect the reactivity, which is crucial in developing better catalysts. For this purpose, we try to make use of various characterization techniques. Recent results from our work in this direction will be described, taking a few examples. [more]