Scientific Events

The heat affected zone (HAZ) is most commonly the critical part of welding joint and the comprehension of the thermal cycle it suffers during welding and its effects on the final microstructure is fundamental to predict and reduce the properties degradation on that zone. The traditional approach to study the HAZ involves several welding tests varying the principal parameters (voltage, current and welding speed) with subsequent mechanical testing. These welding trials could be very time, material and cost demanding; could not replicate the plant/field true welding conditions (need for small scale/plant no available for research tests) and still may not provide a profound insight on the mechanisms in play as the thermal history would not be evaluated. In this context, it is very interesting to use simulation techniques that have evolve significantly in the last two decades to optimize the research effort. In one side, we have the material computational simulation development, with the use of finite element methods and double ellipsoid heat source model to describe the process (thermometallurgic – mechanical coupling) and methods like CALPHAD, Phase Field and Cellular Automata to describe the microstructure evolution in details. One the other side, there are equipment (Gleeble) capable of applying very rapid and controlled thermo-mechanical cycles (acquired in the computational simulation) to a sample, so to produce physical simulated specimen that represents the HAZ region of interest, enabling more detailed characterization and some mechanical testing in isolated microstructures. This permits some validation of the computational simulation too. Seizing these techniques potential, LNTSold have been developing a series of studies in welding simulation to characterize the HAZ of different steels for oil and gas industry applications. For the X100M API 5L steel pipe, it was simulated on FEA software (Sysweld) the welding process of the pipe (SAW) and the field pipeline assembly (GMAW). The main concern for this steel is the toughness reduction it may be subject to in the HAZ, with possible formation of local brittle zones due to the evolution of very sensible constituents as the martensita-austenite (MA) constituent. From the bibliography reference, the two HAZ critical regions are the coarse grain region and the intercritically re-heated coarse grain region, so it was studied the thermal cycle of these regions with heat input variation in the FEA software. The thermal cycle was then reproduced in Gleeble samples to produce specimens for microscopy (focus on the MA constituent morphology and quantity analysis) and for Charpy impact test, to assess the toughness losses. The results indicate that the MA morphology depends very much on the peak temperature and that its quantity does not seem to control directly the impact resistance. For an AISI 4130 steel connector, it was performed a study with FEA software (Sysweld) and CALPHAD software (JMatPro) of the coarse grain HAZ region of the last welding passes, focusing in the hardness prediction and considering the post-weld heat treatment. A simulated CCT diagram and an experimental one were developed to include phase and hardness prediction in the FEA modelling. Then some heat treatment conditions (temperature x time) were evaluated using CALPHAD, trying to optimize the production time. All welding and the best heat treatment conditions were physically reproduced in Gleeble. The simulated CCT showed initially a good correlation with the experimental one, but the FEA hardness prediction was more precise using the experimental CCT. It was possible to achieve the hardness requirements and even increase the impact resistance with a faster heat treatment with close relation to simulation results. Finally, the welding of a 9% Ni steel pipe with Ni 625 alloy filler metal was also simulated in the FEA software and the different HAZ regions reproduced in Gleeble with dilatometry analysis to study the reversion and retention of austenite, which plays an important role in this steel tenacity. The goal it is also to isolate the microstructure and study its hydrogen embrittlement susceptibility. [more]

Abstract: In energy storage devices, materials evolve from their initial state due to electrochemical reactions and interfacial instabilities at interfaces. To develop batteries with improved safety, energy density, and lifetime, it is critical to understand transformation mechanisms and degradation processes within these devices. In my research group, multiscale in situ techniques are used to reveal reaction mechanisms and interfacial transformations to guide the development of better batteries and other devices. Our recent work has used in situ transmission electron microscopy (TEM) to reveal phase transformation pathways and mechanical degradation/fracture when sulfide nanocrystals react with different alkali ions (lithium, sodium, and potassium). Surprisingly, mechanical fracture was found to occur only during reaction with lithium, despite larger volume changes during reaction with sodium and potassium. Since fracture is a known capacity decay mechanism in batteries, this result indicates that these materials are useful for the development of novel, high-energy sodium and potassium batteries. In a different study, operando synchrotron X-ray diffraction methods were used to precisely measure crystallographic strain evolution in battery electrode materials; this technique enables measurements beyond what is possible with TEM. In the final portion of the presentation, in situ X-ray photoelectron spectroscopy (XPS) experiments that reveal chemical evolution of solid-state interfaces in energy storage and electronic materials will be presented. Overall, this research demonstrates how fundamental understanding of dynamic processes can be used to guide the design and engineering of new materials and devices with high energy density and long lifetime. [more]

The aim of modern material modeling is the realistic prediction of the behavior of materials and construction parts by numerical simulation. Experimental investigations prove that the microstructure and thus the mechanical properties may vary under loads. It is thus essential to describe the load-dependent microstructure in these cases by material models to close the system of fundamental physical equations. One elegant way for the derivation of such material models is given by the Hamilton principle which belongs to the class of variational, energy-based modeling strategies. The talk starts with fundamental investigations for modeling the simple harmonic oscillator. Afterwards, the presented modeling concept is generalized to the Hamilton principle which is also applicable to deformable solids with evolving microstructure. As first example for such materials, phase transformations in solids are modeled. The numerical results are compared to experimental observations and an industrially relevant application is presented. In the last part of the talk, the universal character of the Hamilton principle is demonstrated by solving the inverse problem of topology optimization. To this end, a growth approach as observed in biological processes is presented which computes component structures with minimal weight at maximum stiffness. [more]

After a short introduction to thin film solar cells, I will review what we know about defects in Cu(InGa)Se2 (CIGS), where we found significant differences between Cu-rich and Cu-poor material. By photoluminescence we recently found fundamental differences between pure CIS and Ga containing CIGS: with Ga the recombination is higher in Cu-rich material. And high Ga content CIGS shows a deep defect which gets more and more shallow when we decrease the Ga content. Finally, I will show that we can use photoluminescence to characterise the tails states in kesterite. [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]

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]