A theoretical investigation of dislocation distributions in two-phase systems

Effects of second phase size and rigidity on stresses associated with blocked screw and edge dislocation array

Enhanced ductility of nanomaterials through cooperative dislocation emission from cracks and grain boundaries

An analytical model is established to explore the cooperative mechanism between the dislocation emission from cracks and grain boundaries driven by grain boundary sliding in deformed nanocrystalline materials. In our model, high local stress concentration nearby the crack actives grain boundary sliding which creates a wedge disclination dipole at the grain boundaries’ triple junctions. The grain size-dependent criterions for the dislocation emission from the crack tip and the grain boundary are respectively derived. Influences of grain boundary sliding and grain size on the cooperative mechanism are discussed. The results show that the dislocation emission from the grain boundary is activated ahead of that from the crack tip for small grain sizes. This can explain that grain boundary sliding can toughen the nanocrystalline materials even though it suppresses dislocation emission from cracks when their grain sizes are relative small, which is because the dislocation emission from grain boundaries is activated. With the increasing grain size, the main dislocation source may transform from grain boundaries to crack tips due to grain boundary sliding. Therefore, the ductility of nanomaterials with different grain sizes can be enhanced through the cooperative dislocation emission from cracks and grain boundaries

Mechanistic studies of stress corrosion cracking in austenitic stainless steels and in 70Cu-30Zn

Hydrogen-induced cracking -- Stress corrosion cracking -- Microscopic fracture process -- Experimental procedures -- Materials -- Geometry of the specimens -- Tests of stress corrosion cracking -- Load pulse tests -- Tensile tests -- Fractographic observations -- Determination of the cracking crystallography of [delta]-ferrite -- Fracture induced by hydrogen evolved from SCC -- SCC behavior of [delta]-ferrite in 316L -- Crystallography of [delta]-ferrite in SCC and PSCC -- Fractographic observations of SCC in 70CU-30ZN -- SCC crack propagation

Twinning induced spatial stress gradients:Local versus global stress states in hexagonal close-packed materials

Length scale dependent microstructural heterogeneities serve as effective pathways in engineering materials for providing simultaneous strength-ductility enhancement. In this regard, hexagonal close-packed (hcp) materials that exhibit a combination of slip and multiple twinning modes potentially act as ideal candidates that generate heterogeneous microstructures. However, such an inhomogeneous distribution of crystallographic defects also results in build-up of spatially heterogeneous local stress gradients that can be distinct from globally applied stress state. In particular, stress fields arising at the vicinity of deformation twins and due to their interaction with grain interfaces often act as precursors to damage nucleation in most hcp metals and alloys. Hence, assessment of such local stresses and their overall impact on plasticity becomes necessary in order to understand the relationship between twinning and fracture in hcp materials. The current work utilizes commercially pure titanium (cp-Ti) as a model material to investigate the impact of twinning induced stress gradients on the local mechanical response. By means of correlative multiscale structural characterization and local stress gradient measurements, we establish a definitive relationship between applied stress vis-à-vis local stress on the local plasticity behavior ahead of a {112¯2} compression twin-grain boundary intersection in cp-Ti. Additionally, the role of twin interfacial structure for tension and compression twinning modes are experimentally determined and their corresponding impact on the local stress fields and associated twin migration mechanisms is assessed.</p

Plastic strain-induced olivine-ringwoodite phase transformation at room temperature: main rules and the mechanism of the deep-focus earthquake

Deep-focus earthquakes that occur at 350-660 km are theorized to be caused by strain-induced olivine-spinel phase transformation (PT). We introduce and apply dynamic rotational diamond anvil cell with rough diamond anvils to deform San Carlos olivine. While olivine was never transformed to spinel at any pressure at room temperature, we obtained olivine-ringwoodite PT under severe plastic shear at 15-28 GPa within seconds. This is conceptual proof of the difference between pressure- and plastic strain-induced PTs and that plastic straining can accelerate this PT from million years to timescales relevant for the earthquake. The PT pressure linearly reduces with increasing plastic strain, corresponding increasing dislocation density and decreasing crystallite size. The main rules of the coupled severe plastic flow, PT, and microstructure evolution are found

Multiscale characterization of ferroelastic deformation in ceramic materials

Ceramic materials offer a variety of useful properties that make them desirable for a wide range of engineering applications, however, ceramics are limited in their utility by low toughness. Ferroelastic deformation provides a mechanism through which ceramics are intrinsically toughened, but the effect of microstructure on the deformation behavior has yet to be fully understood. In this present examination, the behavior of ferroelastic deformation was evaluated on a range of length scales, specifically highlighting the influence of several variables on the domain nucleation behavior. Ferroelastic domain nucleation was first evaluated in micro-scale single crystals. The stress required for domain nucleation was measured while crystal orientation was tracked. Domain nucleation was observed to not follow a critical resolved shear stress criterion, suggesting that orientation alone cannot be used to predict the deformation behavior. Furthermore, multiple types of deformation were observed to act in concert with ferroelastic deformation. This suggests that domain nucleation is a complex process that may involve multiple potential mechanisms of deformation. Domain nucleation in bulk polycrystals was also examined. Statistics collected on grain sizes that more frequently contain mechanically nucleated domains show that larger grains in close proximity to finer grains more frequently deform. The deformation behavior in polycrystals was contrasted against the domain nucleation behavior in single crystal nanopillars. The nanopillars exhibited high deformation stress, while prolific domain nucleation without fracture was observed in polycrystals. These results suggest that local constraints imposed by microstructure play a key role in locally increasing shear stresses responsible for domain nucleation. To design microstructures with specific characteristics, ceramic processing routes must also be developed to control microstructural development during fabrication. To this end, spark plasma sintering (SPS) offers a promising processing route for fabricating dense nanostructured ceramics. The densification mechanisms associated with ceramic processing using SPS have also been investigated in the present work. Results collected on many samples that were processed under identical processing control conditions convey significant variability in the resulting material properties between and within individually produced samples. Furthermore, the results indicate that electric current plays an important role in densifying ionic conducting ceramics during sintering using SPS. Overall, the research presented in this dissertation shows that ferroelastic domain nucleation is a complex process involving several competing and cooperating mechanisms, and that domain nucleation is affected by different microstructural variables. Domain nucleation cannot be predicted based solely on crystal orientation, however, other microstructural variables including grain size do significantly impact the ferroelastic deformation behavior. Microstructures with large ferroelastic grains embedded in a more finely grained matrix promote ferroelastic deformation even without fracture, and the deformation is sensitive to the stress state being applied. Several processing routes presented here result in these favorable bimodal grain size distributions and may be tested more thoroughly in the future to explore the effect that such microstructures have on the intrinsic toughness

Atomistic study of slip transfer in BCC metals

(English) The mechanical properties of structural materials, which are naturally polycrystalline, is defined by a number of physical processes that take place at different time and space scales. On several of those processes, bulk dislocations and grain boundaries (GBs) play a relevant role. The plastic deformation in these materials is mainly due to the mobility of dislocations, therefore the interaction of these defects with other pre-existing defects like GBs is a key factor to explain the evolution of the properties over the time. It has been experimentally observed that degradation in the mechanical properties of the steels in service is connected with the formation of slip-bands. Propagation of slip-bands through grain boundaries increases material heterogeneity, leading to premature failure and detrimental loss of ductility. There are many possible types of GBs and the behavior of one specific GB interacting with dislocations cannot be anticipated and consequently must be analyzed individually. Macroscopically, these reactions are classified as absorption, transmission or reflection of dislocations. The relationship of these reactions with the GB structure as well as the external parameters (stress, temperature, etc.) is the objective of this research. The aim of the work is to predict the result of slip bands interaction with GBs based on a multiscale modeling approach. This work presents of a report on the transferability of dislocations through GBs and the role played by the intrinsic defects at GBs. The main goal to achieve is a set of rules to describe the interaction between dislocations and GBs which can be used in larger scale models (OKMC, DD, FEM). The purpose is to improve the description of the microstructure evolution and subsequently, the predicted long-term evolution of the macroscopic properties of the materials of interest, namely ferritic/martensitic steels, which are widely used in nuclear industry for both fusion and fission applications. In order to investigate the mechanisms of the dislocation – GB interaction it is required to access the atomic level, for that reason it has been chosen the Molecular Dynamics modeling technique to carry out this research work. (Español) Las propiedades mecánicas de los materiales estructurales vienen determinadas por una variedad de procesos físicos que tienen lugar a diferentes escalas espaciales y temporales, en los cuales las dislocaciones y las fronteras de grano (FG) juegan un papel relevante. La deformación plástica está directamente relacionada con la movilidad de las dislocaciones, de modo que su interacción con otros defectos preexistentes, como las FG, es un factor clave para explicar la evolución de las propiedades mecánicas con el tiempo. Existe una gran variedad de FG y el comportamiento particular de cada una con las dislocaciones no se puede anticipar y por tanto debe ser analizada individualmente. A nivel microscópico las reacciones posibles son absorción, transmisión o reflexión de las dislocaciones. Establecer la conexión entre dichas reacciones y la estructura de las FG, así como con parámetros externos (esfuerzo, temperatura, etc.) es el propósito de este estudio. La finalidad es predecir el resultado de la interacción de las bandas de deformación con las FG basándonos en la aproximación de la modelización mulfiescala. Se presenta un informe sobre la transferibilidad de las dislocaciones a través de las FG y el papel jugado por los defectos intrínsecos de la interface. El objetivo principal es investigar el papel que tiene la estructura atómica de las FG en la interacción con dislocaciones a fin de obtener un conjunto de reglas que puedan ser fácilmente transferibles a otros modelos que trabajen a unas escalas espaciales y temporales superiores, por ejemplo, la Dinámica de Dislocaciones. A fin de estudiar los mecanismos de interacción entre la dislocación y la FG se requiere usar una aproximación a nivel atómico, por esta razón el método de modelización escogido ha sido la Dinámica Molecular (DM). Este trabajo presenta los resultados de modelización por DM realizada para estudiar los mecanismos de interacción entre dislocaciones y FG de inclinación simétrica, ya que estas representan a un número significativo de interfaces presentes en materiales reales. Para cada FG considerada se ha realizado un análisis de los resultados que nos ha permitido obtener una descripción de la dependencia del tipo de reacción con la temperatura y el esfuerzo aplicado. Los resultados obtenidos se usan como datos de entrada para la segunda etapa del trabajo, que incluye la ovansión de la caja de simulación para la modelización 3-D, con el fin de estudiar la interacción entre dislocaciones de frontera y defectos de irradiación situados en la interface. Se han obtenido nuevos datos de gran valor que dan una nueva perspectiva respecto de los procesos a nivel atómico asociados a la interacción FG-dislocaciones en acero. Por ejemplo, se ha descubierto el papel clave de las desconexiones elementales en dichas interacciones. Debido a que el esfuerzo crítico necesario para activar su movimiento en la FG {1 12} es muy bajo, el mecanismo de migración de la FG acoplada a la deformación es muy eficiente. Esta FG es la única en la que se ha observado transmisión de la pila de dislocaciones, ya que basta con una sola desconexión para transformar el vector de Burgers de la dislocación absorbida, haciendo que sea capaz de desplazarse hacia el grano adyacente. Para el resto de FG estudiadas no observado transmisión v La-FG {332} es capaz de y absorber varias dislocaciones formando unas interficies asimétricas en la zona de interacción. Respecto a la FG {1 16} es capaz de acomodar la deformación por medio de reacciones con más de un tipo de desconexión elemental. Por el contrario, la ausencia de desconexiones en la FG {1 1 1} impide su migración, no así su transformación en nuevas interficies. La adecuada elección de FG nos ha permitido extraer conclusiones suficientemente generales para ser usadas en el desarrollo de modelos a mayor escala, que son las herramientas indispensables para investigar la evolución a largo plazo ...Física computacional i aplicad