RESUMO
The flow stress behaviour of a directionally solidified nickel-base superalloy, MAR-M247, is presented through the combination of experiments and crystal-plasticity simulations. The experimental campaign encompassed quasi-static and dynamic testing in the parallel and perpendicular orientation with respect to the columnar grains. The material showed low strain-rate sensitivity in all cases. Virtual samples were generated with DREAM3d and each grain orientation was established according to the DS nature of the alloy. The elasto-visco-plastic response of each crystal is given by phenomenological-base equations, considering the dislocation-dislocation interactions among different slip systems. The hardening-function constants and the strain-rate sensitivity parameter were fitted with the information from tests parallel to the grain-growth direction and the model was able to predict with accuracy the experimental response in the perpendicular direction, confirming the suitability of the model to be used as a tool for virtual testing. Simulations also revealed that in oligocrystalline structures of this type, the yield-strength value is controlled by the grains with higher Schmid factor, while this influence decreases when plastic strain increases. Moreover, the analysis of the micro-fields confirmed that grains perpendicular to the loading axis are prone to nucleate cavities since the stresses in these regions can be twice the external applied stress.
RESUMO
The development of new interatomic potentials to model metallic systems is a difficult task, due in part to the dependence between the parameters that describe the electron density and the short-range interactions. Parameter search methods are prone to false convergence. To solve this problem, we have developed a methodology for obtaining the electron density parameters independently of the short-range interactions, so that physically sound parameters can be obtained to describe the electron density, after which the short-range parameters can be fitted, thus reducing the complexity of the process and yielding better interatomic potentials. With the new method we can develop self-consistent, accurate force fields, using solely calculations, without the need to fit to experimental data. Density functional theory calculations are used to compute the observables with which the potential is fit. We applied the method to a Ni-based Inconel 625 superalloy (IN625), modelled here as Ni, Cr, Mo and Fe solid solution alloys. The capability of the force fields developed using this new method is validated, by comparing the structural and thermo-elastic properties predicted with the force fields, with the corresponding experimental data, both for single crystals and polycrystalline alloys.
RESUMO
The thermostructural properties of Ni-Cr materials, as bulk and nanoparticle (NP) systems, have been predicted with a newly developed interatomic potential, for Ni/Cr ratios from 100/0 to 60/40. The potential, which has been fitted using experimental data and further validated using Density Functional Theory (DFT), describes correctly the variation with temperature of lattice parameters and the coefficient of thermal expansion, from 100 K to 1000 K. Using this potential, we have performed Molecular Dynamics (MD) simulations on bulk Ni-Cr alloys of various compositions, for which no experimental data are available. Similarly, NPs with diameters of 3, 5, 7, and 10 nm were studied. We found a very rapid convergence of NP properties with the size of the systems, showing already the 5 nm NPs with a thermostructural behaviour similar to the bulk. MD simulations of two 5 nm NPs show very little sintering and thermally induced damage, for temperatures between 300 K and 1000 K, suggesting that materials formed by agglomeration of Ni-Cr NPs meet the thermostructural stability requirements for catalysis applications.
