Assessment of corrosion-induced damage in the mechanical contact response of cemented carbides at different length scales

Cemented carbides, also referred to as hardmetals, are composite materials consisting of brittle refractory carbide phase (generally WC) embedded in a metallic matrix. Such microstructure leads to an outstanding combination of hardness, wear resistance and toughness. This makes hardmetals first materials in several highly demanding applications, e.g. cutting or forming of metallic alloys, as well as mining operations. Several of the above applications imply exposure to chemically aggressive media, such as lubricants, petrochemical and mine slurries, seawater, etc. Under these conditions, it has been shown that failure induced under applied load is accelerated, and corresponding service life may be significantly shortened. In this regard, the detrimental corrosion-related effects on tribological response and effective wear resistance of cemented carbides have aroused the greatest concern. However, investigations addressing similar information linking corrosion-induced damage and contact mechanical response at different length scales are quite limited. Within the above framework, the first part of this thesis was devoted to carry out a systematic and comprehensive study about corrosion-induced damage and residual strength (damage tolerance) for four microstructurally different hardmetals exposed to various corrosive media. It is found that acidic medium led to higher corrosion rates and more significant strength degradation than those from neutral and basic ones. Regarding degradation mechanisms, it is evidenced that corrosion starts at binder pool centers and evolves towards binder/WC interfaces when exposed to acid solution, while in the basic one it is initially located at binder/WC interfaces and subsequently expands into the ceramic particles. The subsequent sections were focused on assessing the corrosion-induced changes on the mechanical contact response of hardmetals through an increasing length scale (from 100s nanometers to 1000s microns). First, nanoindentation and nanoscrach techniques were employed to assess the influence of corrosion on the mechanical integrity of hardmetals. Corrosion-induced changes on corresponding response and damage scenario are discussed. It is concluded that dissolution of metallic phase becomes critical as it leads to an effectively lessened mechanical integrity. A similar investigation was then extended to a higher length scale range (from 10s to 100s of microns in depth), combining relatively long corrosion times with pyramidal indentation and sliding contact (microscratch) tests, in order to evaluate corrosion-induced changes on both load-bearing capability and damage scenario of hardmetals. Results reveal that mechanical contact strength and resistance to crack extension are significantly reduced after exposure to corrosive media. Such lessening effects are found to depend on the ratio between indentation and/or scratch depth and thickness of the corroded layer. Alike pronounced corrosion influence is evidenced in surface and subsurface damage scenarios regarding the crack propagation behavior. Finally, an even higher length scale (up to 1000 of microns) was introduced by combining Hertzian indentation techniques and variable corrosion times. Corrosion effects on corresponding mechanical response and damage were assessed for hardmetals with metallic binders of different chemical nature. Results indicate significant corrosion-induced changes on indentation stress-strain response and damage scenario. In this regard, critical loads for emergence and evolution of specific damage events - ring and radial cracks, and even specimen failure - are proposed as figures of merit for material selection under the combined action of corrosion and contact loading. It points out the consideration of the synergic interaction between corrosion resistance and hardness/toughness correlation for microstructural design optimization of hardmetals under service-like conditions.Los carburos cementados, también referidos como metal duro, son materiales compuestos constituidos por dos fases: una de carburos refractarios (generalmente WC) y frágil, y otra metálica y dúctil. Dicha configuración microestructural da lugar a combinaciones óptimas de dureza, resistencia al desgaste y tenacidad. A consecuencia de ello, el metal duro es comúnmente elegido para la fabricación de herramientas y componentes cuya aplicación conlleva condiciones de servicio extremas. Muchas de estas aplicaciones incluyen la exposición del material a medios químicamente agresivos. En estos casos, se ha demostrado que la vida útil de las herramientas o componentes puede reducirse significativamente. En este contexto, una revisión bibliográfica muestra el interés de la comunidad por la comprensión de los efectos perjudiciales de la corrosión sobre la resistencia al desgaste del metal duro. Sin embargo, el número de investigaciones que versan sobre el daño inducido por la corrosión en la respuesta mecánica bajo solicitaciones de contacto de estos materiales es muy limitado. En el marco de las ideas descritas, la primera parte de esta tesis se dedicó a realizar un estudio sobre el daño inducido por corrosión y la resistencia mecánica residual de cuatro calidades de metal duro expuestas a diversos medios corrosivos. Se evidencia que el medio ácido conlleva las cinéticas de corrosión más elevadas y una degradación de la resistencia más significativa que los medios neutros y básicos. En cuanto a los mecanismos de degradación, se observa que la corrosión se focaliza en la fase metálica cuando el material se expone a la solución ácida. Por su parte, en el caso de un medio básico, la degradación se centra en la fase cerámica. Posteriormente, la tesis se centró en evaluar los cambios inducidos por la corrosión en la respuesta mecánica bajo solicitaciones de contacto de carburos cementados a escalas dimensionales correspondientes a un rango entre decenas de nanómetros y cientos de micrones. Para ello, se emplearon técnicas de indentación y rayado a diferentes niveles de carga aplicada; técnicas de microscopía confocal y electrónica para evaluar los diferentes escenarios de daño generados, y tiempos de corrosión relativamente largos. Los resultados revelan que la resistencia tanto al contacto mecánico, unidireccional y deslizante, como a la extensión de fisuras se reduce significativamente después de la exposición del material a medios corrosivos. La disminución de la integridad mecánica depende de la relación entre la indentación y/o la profundidad del rayado y el espesor de la capa corroída. Se concluye que la disolución de la fase metálica es crítica para racionalizar los resultados obtenidos. Finalmente, el estudio se llevó escalas dimensionales del orden de milímetros, combinando técnicas de indentación hertziana y diferentes tiempos de corrosión. La influencia de la corrosión sobre la respuesta mecánica y el daño se evaluó para metales duros con ligantes metálicos de distinta naturaleza química. Los resultados indican cambios significativos, asociados a la corrosión previa, en la respuesta tensión-deformación, así como en el escenario de daño. Las cargas críticas para la nucleación y evolución del proceso de fisuración se proponen como parámetros críticos para la selección de carburos cementados bajo la acción combinada de corrosión y solicitaciones de contacto. Ello permite concluir que la optimización del diseño microestructural de estos materiales requiere la consideración simultánea de la resistencia a la corrosión y la correlación dureza / tenacidad para ellos

Doubly Stochastic Generative Arrivals Modeling

We propose a new framework named DS-WGAN that integrates the doubly stochastic (DS) structure and the Wasserstein generative adversarial networks (WGAN) to model, estimate, and simulate a wide class of arrival processes with general non-stationary and random arrival rates. Regarding statistical properties, we prove consistency and convergence rate for the estimator solved by the DS-WGAN framework under a non-parametric smoothness condition. Regarding computational efficiency and tractability, we address a challenge in gradient evaluation and model estimation, arised from the discontinuity in the simulator. We then show that the DS-WGAN framework can conveniently facilitate what-if simulation and predictive simulation for future scenarios that are different from the history. Numerical experiments with synthetic and real data sets are implemented to demonstrate the performance of DS-WGAN. The performance is measured from both a statistical perspective and an operational performance evaluation perspective. Numerical experiments suggest that, in terms of performance, the successful model estimation for DS-WGAN only requires a moderate size of representative data, which can be appealing in many contexts of operational management.Comment: updated version with more explanatory figure

An improved local remeshing algorithm for moving boundary problems

© 2016 The Author(s). Three issues are tackled in this study to improve the robustness of local remeshing techniques. Firstly, the local remeshing region (hereafter referred to as ‘hole’) is initialized by removing low-quality elements and then continuously expanded until a certain element quality is reached after remeshing. The effect of the number of the expansion cycle on the hole size and element quality after remeshing is experimentally analyzed. Secondly, the grid sources for element size control are attached to moving bodies and will move along with their host bodies to ensure reasonable grid resolution inside the hole. Thirdly, the boundary recovery procedure of a Delaunay grid generation approach is enhanced by a new grid topology transformation technique (namely shell transformation) so that the new grid created inside the hole is therefore free of elements of extremely deformed/skewed shape, whilst also respecting the hole boundary. The proposed local remeshing algorithm has been integrated with an in-house unstructured grid-based simulation system for solving moving boundary problems. The robustness and accuracy of the developed local remeshing technique are successfully demonstrated via industry-scale applications for complex flow simulations

Data fusion technology for precision forestry applications

Presently precision forestry is playing an important role in realizing sustainable development and improving societal and economical efficiency for forestry applications. Based on analyzing the features of precision forestry's information requirements, the data needed for precision forestry were classified and the characteristics of the different information were summarized. Data fusion for precision forestry was studied in this paper. The architecture for precision forestry information processing, which integrated information fusion and data mining, was put forward. New and emerging technologies such as Remote Sensing (RS), Geographical Information System (GIS), Global Position System (GPS), Data Base Management System (DBMS), Data Fusion, Decision Support Systems (DSS), and Variable Rate technology (VRT) are applied in forestry production as aids in producers' and managers' decision-making process. Precision irrigation, precision fertilizing, precision pesticide application, precision harvesting, and precision deforestation can promote the realization of minimizing resource inputs, minimizing environmental impacts, and maximizing forest outputs

In vitro corrosion of Mg–1.21Li–1.12Ca–1Y alloy

AbstractThe influence of the microstructure on mechanical properties and corrosion behavior of the Mg–1.21Li–1.12Ca–1Y alloy was investigated using OM, SEM, XRD, EPMA, EDS, tensile tests and corrosion measurements. The results demonstrated that the microstructure of the Mg–1.21Li–1.12Ca–1Y alloy was characterized by α-Mg substrate and intermetallic compounds Mg2Ca and Mg24Y5. Most of the fine Mg2Ca particles for the as-cast alloy were distributed along the grain boundaries, while for the as-extruded along the extrusion direction. The Mg24Y5 particles with a larger size than the Mg2Ca particles were positioned inside the grains. The mechanical properties of Mg–1.21Li–1.12Ca–1Y alloy were improved by the grain refinement and dispersion strengthening. Corrosion pits initiated at the α-Mg matrix neighboring the Mg2Ca particles and subsequently the alloy exhibited general corrosion and filiform corrosion as the corrosion product layer of Mg(OH)2 and MgCO3 became compact and thick