Atomistic simulations of dislocations in a model BCC multicomponent concentrated solid solution alloy

Molecular statics and molecular dynamics simulations are presented for the structure and glide motion of a/2(111) dislocations in a randomly-distributed model-BCC Co16.67Fe36.67Ni16.67Ti30 alloy. Core structure variations along an individual dislocation line are found for a/2(111) screw and edge dislocations. One reason for the core structure variations is the local variation in composition along the dislocation line. Calculated unstable stacking fault energies on the (110) plane as a function of composition vary significantly, consistent with this assessment. Molecular dynamics simulations of the critical glide stress as a function of temperature show significant strengthening, and much shallower temperature dependence of the strengthening, as compared to pure BCC Fe as well as a reference mean-field BCC alloy material of the same overall composition, lattice and elastic constants as the target alloy. Interpretation of the strength versus temperature in terms of an effective kink-pair activation model shows the random alloy to have a much larger activation energy than the mean-field alloy or BCC Fe. This is interpreted as due to the core structure variations along the dislocation line that are often unfavorable for glide in the direction of the load. The configuration of the gliding dislocation is wavy, and significant debris is left behind, demonstrating the role of local composition and core structure in creating kink pinning (super jogs) and/or deflection of the glide plane of the dislocation. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Solute Strengthening in Random Alloys

International audienceRandom solid solution alloys are a broad class of materials that are used across the entire spectrum of engineeringmetals, whether as stand-alone materials (e.g. Al-5xxx alloys) or as the matrix in precipitate-strengthening materials (e.g. Ni-based superalloys). As a result, the mechanisms of, and prediction of, strengthening in solid solutions has a long history. Many concepts have been developed and important trends identified but predictive capability has remained elusive. In recent years, a new theory has been developed that builds on one historical model, the Labusch model, in important ways that lead to a well-defined model valid for random solutions with arbitrary numbers of components and compositions. The new theory uses first-principles-computed solute/dislocation interaction energies as input, from which specific predictions emerge for the yield strength and activation volume as a function of alloy composition, temperature, and strain-rate. Being a general model for materials that otherwise have a low Peierls stress, it has broad application and has been successfully applied to Al-X alloys, Mg-Al, twinning in Mg alloys, and recently fcc High-Entropy Alloys. Here, the new theory is presented in a general and systematic manner. Approximations and limiting cases that reduce the complexity and facilitate understanding are introduced, and help relate the new model to various physical features present among the historical array of models, other recent models, and simulation studies. The quantitative predictions of the model in the various materials above is then demonstrated

Méthodes atomiques et élasticité

La connaissance du diagramme de phase d’un alliage est essentielle lorsqu’on élabore un matériau à la recherche de propriétés spécifiques (tenue mécanique, résistance à la chaleur, à l’oxydation, propriétés électriques, magnétiques, etc.). Il est important de connaître la stabilité des différentes phases qui peuvent se développer dans le système et de caractériser les possibles transformations entre elles. Pour cela, une description à l’échelle atomique basée sur des calculs de structure électronique et de physique statistique est nécessaire. Les modèles utilisés dans le cadre de ces calculs dépendent de la nature des matériaux étudiés ainsi que du degré de complexité des mécanismes physiques qui s’y développent. Au-delà de la stabilité des phases, il est aussi intéressant de connaître l’évolution de la microstructure de l’alliage en fonction du temps, de la température et de la concentration. Dans cet exposé, nous regarderons particulièrement les effets de la différence de taille des atomes (effets élastiques) sur la forme de diagrammes de phases d’alliages métalliques massifs et confinés présentant des transformations de phase du type « démixtion » et de type « ordre-désordre ». L’effet de la compétition entre effets chimiques et effets élastiques sera aussi analysé. Deux méthodes numériques à l’échelle atomique sont particulièrement intéressantes pour ce type d’étude : l’approche Monte Carlo avec déplacements et la méthode des fonctions de Green sur réseau (« Lattice statics »). La première est une méthode en principe exacte. Elle permet d’aller au delà de l’élasticité linéaire lorsque les différences de taille sont importantes et permet, en principe, de reproduire d’éventuelles pertes de cohérence. La deuxième méthode se situe dans le cadre de l’élasticité linéaire et intègre les effets élastiques sous la forme d’interactions effectives (à longue portée) sur réseau. Ainsi, seuls des diagrammes de phases et des microstructures cohérents peuvent être étudiés. Son avantage réside dans la possibilité de traiter des volumes de simulation beaucoup plus importants que dans le cas des simulations Monte Carlo avec déplacements. En utilisant ces deux méthodes, nous illustrerons et analyserons l’effet de l’élasticité sur des microstructures d’alliages binaires

Thermal activation parameters of plastic flow reveal deformation mechanisms in the CrMnFeCoNi high-entropy alloy

International audienceTo reveal the operating mechanisms of plastic deformation in an FCC high-entropy alloy, the activation volumes in CrMnFeCoNi have been measured as a function of plastic strain and temperature between 77 K and 423 K using repeated load relaxation experiments. At the yield stress, sigma(y), the activation volume varies from similar to 60 b(3) at 77 K to similar to 360 b(3) at 293 K and scales inversely with yield stress. With increasing plastic strain, the activation volume decreases and the trends follow the Cottrell-Stokes law, according to which the inverse activation volume should increase linearly with sigma - sigma(y) (Haasen plot). This is consistent with the notion that hardening due to an increase in the density of forest dislocations is naturally associated with a decrease in the activation volume because the spacing between dislocations decreases. The values and trends in activation volume agree with theoretical predictions that treat the HEA as a high-concentration solid-solution-strengthened alloy. These results demonstrate that this HEA deforms by the mechanisms typical of solute strengthening in FCC alloys, and thus indicate that the high compositional/structural complexity does not introduce any new intrinsic deformation mechanisms. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Combining experiments and modeling to explore the solid solution strengthening of high and medium entropy alloys

International audienceThe mechanical properties due to solid solution strengthening are explored within the single phase fcc domain of the Co-Cr-Fe-Mn-Ni high entropy alloy (HEA) system. This is achieved by combining an efficient and reproducible metallurgical processing of alloys to X-ray diffraction and nanoindentation characterization techniques, thus enabling to get access to 24 different bulk alloys. Large variations of nanohardness are seen with composition. Experimental results are rationalized in terms of lattice misfit and elastic constant variations with alloy-composition, through the use of an analytical mechanistic theory for the temperature-, composition-and strain-rate-dependence of the initial yield strength of fcc HEAs, with predictions made using only experimental inputs. The good agreement obtained by comparing model predictions to experiments provides the basic framework for mechanical properties optimization within the Co-Cr-Fe-Mn-Ni system; the approach could be systematically applied to all classes of fcc HEAs