Scientific Events

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]

Recent advances in topological optimization methodologies for design of internal material architecture, coupled with the emergence of micro- and nanoscale fabrication processes, 3D imaging, and micron scale testing methodologies, now make it possible to design, fabricate, and characterize lattice materials with unprecedented control. This talk will describe a collaborative effort that employs scalable 3D textile manufacturing, location specific joining, and vapor phase alloying to produce metallic lattices with a wide range of internal architectures, alloy compositions, and mechanical and functional properties. The project involves three length scales. The highest level (component scale) spans centimeters to meters and encompasses gradients in unit cell architecture, porosity, and the creation of sandwich structures. The second level (architectural unit cells) spans tens of microns to millimeters and employs architectural optimization to design the geometry of the braided/woven structure. The smallest level (microstructure) spans nanometers to tens of microns focuses on vapor phase alloying of the wires after textile manufacturing. Topology optimization allows properties to be decoupled and tailored for specific applications. Dramatic enhancements in permeability have been balanced with modest reductions in stiffness and are being used to develop heat exchanger materials with high thermal transport, low impedance, low thermal gradients and high temperature strength. In a parallel effort, architectural designs to maximize both structural resonance and inter-wire friction are also being employed to develop metallic lattices capable of mechanical damping at elevated temperatures. These examples will be used to highlight the benefits to be gained by development of metallic lattice materials with a wide range of tailorable properties. [more]

Thermoelectric materials can convert waste heat into electricity, which is of significant technological and environmental interest. In my talk I will give a short introduction into the field of thermoelectrics including the measurement of the thermoelectric properties of bulk materials at low and elevated temperatures. I will introduce a selection of general concepts, which allow to improve and optimize thermoelectric materials and I will briefly talk about a selection of new directions in the field, where some of them (will) heavily rely on and benefit from the fields of metallurgy and atom probe tomography (e.g. phase boundary mapping and antiphase boundaries as a new route towards low thermal conductivities). [more]

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]

Heterogeneous deformation in metallic polycrystals arises from several factors, including anisotropy in elastic properties and plastic slip. The ability to accurately simulate heterogeneous deformation requires physically based models of slip that includes grain boundary properties, as grain boundaries are usually barriers to slip. As slip transfer across boundaries occurs in some boundaries, grain boundary properties have been installed in a dislocation density based crystal plasticity model to enable slip transfer, and used to examine idealized bicrystal tensile samples. This code will be used to simulate deformation of annealed pure aluminum foil multicrystal experiments, in order to examine thresholds for slip transfer. An analysis of slip transfer events indicates that for near-cube oriented grains, the threshold is higher than observed in hexagonal materials, and potential reasons for this will be discussed. Secondly, as computational simulations of polycrystals normally assume a zero-stress initial condition, this assumption is questionable in non-cubic metals where the coefficient of thermal expansion (CTE) is anisotropic. To assess the effect of the anisotropic CTE on initial stress states, two pure titanium samples with different textures were examined using in-situ high energy x-ray diffraction microscopy to measure the evolution of the internal stresses in each grain during heating and cooling. These data show a significant change in expansion rates in the <a> and <c> directions at about 700 C. A simulation of this experiment shows good agreement with experimentally measured data, indicating that it is possible to start a simulation with a good estimate of the internal stress state arising from the anisotropic CTE. This work was supported by grants from US DOE/BES and the Community of Madrid [more]

3D printing of metals has advanced rapidly in the past decade and is used across a wide range of industry. Many aspects of the technology are considered to be well understood in the sense that validated computer simulations are available. At the microscopic scale, however, more work is required to quantify and understand defect structures, which affect fatigue resistance, for example. Synchrotron-based 3D X-ray computed microtomography (µXCT) was performed at the Advanced Photon Source on a variety of AM samples using both laser (SLM) and electron beam (EBM) powder bed; this showed systematic trends in porosity. Optical and SEM characterization of powders used in additively manufacturing (AM) reveals a variety of morphologies and size distributions. Computer vision (CV), as a subset of machine learning, has been successfully applied to classify different microstructures, including powders. The power of CV is further demonstrated by application to detecting and classifying defects in the spreading in powder bed machines, where the defects often correspond to deficiencies in the printed part. In addition to the printed material, a wide range of powders were measured and invariably exhibited porosity to varying degrees. Outside of incomplete melting and keyholing, porosity in printed parts is inherited from pores or bubbles in the powder. This explanation is reinforced by evidence from simulation and from dynamic x-ray radiography (DXR), also conducted at the APS. DXR has revealed a wide range of phenomena, including void entrapment (from powder particles), keyholes (i.e., vapor holes) and hot cracking. Keyhole depth is linearly related to the excess power over a vaporization threshold. Concurrent diffraction provides information on solidification and phase transformation in, e.g., Ti-6Al-4V and stainless steel. High Energy (x-ray) Diffraction Microscopy (HEDM) experiments are also described that provide data on 3D microstructure and local elastic strain in 3D printed materials, including Ti-6Al-4V and Ti-7Al. The reconstruction of 3D microstructure in Ti-6Al-4V is challenging because of the fine, two-phase lamellar microstructure and the residual stress in the as-built condition. Both the majority hexagonal phase and the minority bcc phase were reconstructed. [more]

Deformation in metallic glasses occurs by initiation and propagation of multiple thin shear bands. This mode is rather difficult to analyse since generally, a single band soon propagates to a large extent in the specimen leading to a catastrophic failure. Exceptions are for example in creep tests under very low stress and moderate temperature or in confined deformation tests. We used instrumented nano-indentations to perform series of independent experiments at room temperature on a Mg65Cu12.5Ni12.5(Ce75La25)10 metallic glass. Loading part of the curves shows serrations which size and duration were measured using an automatic procedure. To make analyses consistent, data were considered only in the domain with similar strain rates, in the range of 1 to 0.3 s-1. Times between successive serrations follow a normal distribution characterizing a random occurrence of deformation burst in the glass. It was then conjectured, first that serration occurs through activation of appropriate zone in the glass that should naturally scale with a multiple of an elementary domain size characterizing the deformation mechanism. Second, as activated zones leading to serration are very few in the glass, the model should be described by the Poisson statistics. Data analyses reveals that serration size are well fitted by a Poisson distribution. The model predict an elementary size which scale with that of the activation volume of 3 atoms, measured from creep test at constant load in the same series of experiments. Eventually, energy dissipated during serration is analyzed as to define shear bands dynamics characteristics.Depending on time, I shall present the use of nano-indention for investigating dynamics of nanoporous metallic materials deformation. N. Thurieau, L. Perriere, M. Laurent-Brocq, Y. Champion, J. Appl. Phys., 118 (2015) 204302. [more]