Publication: Micromechanics of magnesium and its alloys studied by nanoindentation
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2016-11
Defense date
2017-02-20
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Abstract
La fabricación de vehículos ligeros constituye una estrategia prometedora en cuanto a
la reducción del consumo de combustibles fósiles y las emisiones de gases de efecto invernadero.
El magnesio es un material muy conveniente para tal fin debido a su excelente
resistencia específica. Sin embargo, ciertos aspectos, tales como su baja ductilidad y alta
anisotropía mecánica a temperatura ambiente, obstaculizan su uso general a nivel industrial.
La aleación de magnesio con otros elementos químicos es una estrategia prometedora
dado que este material reacciona fácilmente con otros compuestos formando precipitados y/o
fases intermetálicas que afectan de una manera severa a la competición entre los diferentes
mecanismos de deformación y microestructura, y por lo tanto, a sus propiedades mecánicas.
Las estrategias tradicionales enfocadas al desarrollo de nuevas aleaciones de magnesio
con propiedades mecánicas avanzadas se han basado en extensas, largas y costosas campañas
experimentales para evaluar las propiedades de las nuevas aleaciones. Esta limitación podría
ser resuelta aplicando Métodos Combinatorios en este proceso. Dicha metodología, aplicada
inicialmente en la industria química y farmacéutica, permite producir y caracterizar un
gran número de nuevos materiales en muy poco tiempo. Sin embargo, su implementación
efectiva en procesos metalúrgicos requiere el desarrollo de diversas nuevas tecnologías. En
concreto, se pueden destacar dos aspectos tecnológicos sin resolver: en primer lugar, un
nuevo método que permita la caracterización de los diferentes mecanismos de deformación
del magnesio y sus aleaciones de una forma rápida requiriendo poco material; y segundo,
nuevos modelos matemáticos que sean capaces de reproducir el comportamiento mecánico
real de estos materiales a nivel tanto micro- como macroscópico. En cuanto al primero,
nanoindentación es una técnica muy prometedora ya que, además de ser fácil de usar y
rápida, requiere cantidades de material muy reducidas. Respecto al segundo aspecto, los
modelos de plasticidad cristalina cumplen con los requisitos dado que son capaces de capturar
la deformación plástica por deslizamiento cristalográfico y maclado.
La presente tesis doctoral constituye un elemento fundamental en cuanto a la superación
de estas dos limitaciones. El principal objetivo de esta tesis doctoral ha sido el estudio de la competición de los diferentes mecanismos de deformación del magnesio y sus aleaciones
bajo diferentes condiciones, combinando técnicas avanzadas de caracterización, como
nanoindentación de monocristales, microscopía de fuerza atómica o microscopía de electrones
retrodispersados, junto con avanzadas técnicas de simulación basadas en novedosos modelos
de plasticidad cristalina. Se ha constatado que la dureza y la deformación residual y la
microtextura alrededor de la indentación dependen de una manera notoria de un efecto combinado
de la orientación del cristal indentado así como de la temperatura de ensayo. Dichas
dependencias han sido explicadas satisfactoriamente desde un punto de vista tanto analítico
como numérico debido a la activación de diferentes modos de deslizamiento y maclado en
las áreas cercanas a la zona de ensayo. Se ha demostrado que, mientras que los principales
mecanismos de deformación a temperatura ambiente son el deslizamiento basal y el maclado
de extensión, la deformación plástica a alta temperatura es dominada por deslizamiento basal
y prismático. Se mostrado que el incremento de actividad prismática con la temperatura es
compensada con una reducción muy importante de la actividad de maclado de extensión.
Además, se ha desarrollado un novedoso modelo de plasticidad cristalina que considera la
micromecánica real del maclado de extensión. Además, se ha constatado que es fundamental
tener en cuenta que la nucleación de una macla es un proceso que requiere estados tensionales
mucho más severos que su propagación a la hora de reproducir la evolución de la actividad
de dicho mecanismo de deformación con la temperatura.
Además, se ha demostrado que el maclado de extensión es un mecanismo de deformación
afectado severamente por efectos tamaño. Nuestros resultados experimentales evidencian que
la activación de una macla requiere la concentración de altas tensiones en un determinado
volumen. Finalmente, se ha desarrollado una nueva y práctica metodología para estimar la
tensión crítica resuelta de aleaciones de magnesio de interés industrial. Esta metodología, que
se basa en la variación de la dureza con la orientación cristalográfica del grano indentado, ha
sido diseñada teniendo en cuenta estándares industriales de manera que pueda ser utilizada
fácilmente por la comunidad industrial. La metodología propuesta, validada inicialmente
para magnesio puro, ha sido utilizada para estudiar los mecanismos de deformación de una
aleación de magnesio de tierras raras aleada con un 1% de manganeso y 1% de neodimio.
Se ha demostrado que la adicción de tierras raras conlleva una importante reducción de la
resistencia relativa de los sistemas de deslizamiento basales y no basales, lo que justifica
la mayor isotropía mecánica mostrada por este material en comparación con aleaciones de
magnesio convencionales.
Weight reduction is a cost-effective approach to decrease the fossil fuel consumption and greenhouse gas emissions of the transport industry. Magnesium, the lightest structural metal, constitutes a significant alternative as weight-saving material. However, some issues hinder its widespread use in the industry, like its low ductility and high mechanical anisotropy at room temperature. Due to its high chemical activity, alloying is a promising strategy to overcome these limitations, as magnesium easily reacts with other compounds to form precipitates and/or intermetallic phases which heavily affect the competition between the different deformation modes and microstructure, and therefore, its physical properties. Traditional approaches to develop novel magnesium alloys with enhanced mechanical properties rely on vast and time-consuming experimental campaigns in order to assess the mechanical properties of the new alloys. This limitation could be solved with the application of Combinatorial Methods in this process. This new methodology, initially applied in the chemist and pharmaceutical industries, allows to produce and characterise a great number of new materials in a very sort time. However, the effective implementation of such approach requires the development of several new technologies. Among others, two main unresolved technological issues can be mentioned: first, a new approach to characterise the different deformation modes of magnesium and its alloys in an easy and fast way; and second, new material models that are able to reproduce the real mechanical behaviour of magnesium and its alloys at the macro- and micro-mechanical scale. Regarding the first one, nanoindentation seems to be a perfect candidate as, in addition to being easy and fast, requires small amount of material. Regarding the second one, crystal plasticity models meet perfectly the requirements as they are able to capture plastic deformation by crystallographic glide and mechanical twinning. The present Ph.D. thesis constitutes a milestone in order to overcome these two last limitations. The main objective of this research has been to study the competition between the different deformation modes in magnesium and its alloys under different testing conditions, combining advanced characterisation techniques, like single crystal nanoindentation, atomic force microscopy and electron backscatter diffraction, with novel crystal plasticity modelling approaches. It has been shown that the hardness and the residual deformation and microtexture around an indent highly depend on the combined effect of the orientation of the indented plane and testing temperature. Such dependencies have been successfully explained from an analytical and numerical point of view due to the activation of different slip and twin modes in different areas in the surroundings of the indent. It is demonstrated that, while the main deformation modes at room temperature are basal slip and tensile twinning, the plastic deformation at high temperature is dominated by basal and prismatic slip. It is shown that the increase of prismatic activity with temperature is compensated by a dramatic decrease of tensile twin activity as temperature increases. In addition, a novel crystal plasticity model which takes into account the micromechanics of tensile twinning has been developed. In is shown that, in order to properly reproduce the evolution of tensile twin activity with temperature, it is fundamental to take into account the fact that twin activation is a process which requires a much bigger stress than twin propagation. It has been also demonstrated that mechanical twinning is a process highly affected by size effects. Our results provides experimental evidences that twin activation requires the concentration of high stresses in a certain activation volume. Finally, a novel and practical methodology has been developed in order to estimate the critical resolved shear stresses of industrial magnesium alloys. This methodology, which is based on the variation of the hardness with the crystallographic orientation of the indented grain, is designed taking into account industrial standards so it can be easily applied by the alloy development community. First validated in pure magnesium, the proposed methodology is applied to study the deformation modes of a rare-earth magnesium alloy containing 1% of Manganese and 1% of Neodymium. It is shown that the addition of rare-earth elements lead to an important reduction between the relative strength of the basal and non-basal slip systems, which justifies the much more isotropic mechanical behaviour of this material in comparison with conventional magnesium alloys.
Weight reduction is a cost-effective approach to decrease the fossil fuel consumption and greenhouse gas emissions of the transport industry. Magnesium, the lightest structural metal, constitutes a significant alternative as weight-saving material. However, some issues hinder its widespread use in the industry, like its low ductility and high mechanical anisotropy at room temperature. Due to its high chemical activity, alloying is a promising strategy to overcome these limitations, as magnesium easily reacts with other compounds to form precipitates and/or intermetallic phases which heavily affect the competition between the different deformation modes and microstructure, and therefore, its physical properties. Traditional approaches to develop novel magnesium alloys with enhanced mechanical properties rely on vast and time-consuming experimental campaigns in order to assess the mechanical properties of the new alloys. This limitation could be solved with the application of Combinatorial Methods in this process. This new methodology, initially applied in the chemist and pharmaceutical industries, allows to produce and characterise a great number of new materials in a very sort time. However, the effective implementation of such approach requires the development of several new technologies. Among others, two main unresolved technological issues can be mentioned: first, a new approach to characterise the different deformation modes of magnesium and its alloys in an easy and fast way; and second, new material models that are able to reproduce the real mechanical behaviour of magnesium and its alloys at the macro- and micro-mechanical scale. Regarding the first one, nanoindentation seems to be a perfect candidate as, in addition to being easy and fast, requires small amount of material. Regarding the second one, crystal plasticity models meet perfectly the requirements as they are able to capture plastic deformation by crystallographic glide and mechanical twinning. The present Ph.D. thesis constitutes a milestone in order to overcome these two last limitations. The main objective of this research has been to study the competition between the different deformation modes in magnesium and its alloys under different testing conditions, combining advanced characterisation techniques, like single crystal nanoindentation, atomic force microscopy and electron backscatter diffraction, with novel crystal plasticity modelling approaches. It has been shown that the hardness and the residual deformation and microtexture around an indent highly depend on the combined effect of the orientation of the indented plane and testing temperature. Such dependencies have been successfully explained from an analytical and numerical point of view due to the activation of different slip and twin modes in different areas in the surroundings of the indent. It is demonstrated that, while the main deformation modes at room temperature are basal slip and tensile twinning, the plastic deformation at high temperature is dominated by basal and prismatic slip. It is shown that the increase of prismatic activity with temperature is compensated by a dramatic decrease of tensile twin activity as temperature increases. In addition, a novel crystal plasticity model which takes into account the micromechanics of tensile twinning has been developed. In is shown that, in order to properly reproduce the evolution of tensile twin activity with temperature, it is fundamental to take into account the fact that twin activation is a process which requires a much bigger stress than twin propagation. It has been also demonstrated that mechanical twinning is a process highly affected by size effects. Our results provides experimental evidences that twin activation requires the concentration of high stresses in a certain activation volume. Finally, a novel and practical methodology has been developed in order to estimate the critical resolved shear stresses of industrial magnesium alloys. This methodology, which is based on the variation of the hardness with the crystallographic orientation of the indented grain, is designed taking into account industrial standards so it can be easily applied by the alloy development community. First validated in pure magnesium, the proposed methodology is applied to study the deformation modes of a rare-earth magnesium alloy containing 1% of Manganese and 1% of Neodymium. It is shown that the addition of rare-earth elements lead to an important reduction between the relative strength of the basal and non-basal slip systems, which justifies the much more isotropic mechanical behaviour of this material in comparison with conventional magnesium alloys.
Description
Mención Internacional en el título de doctor
Keywords
Metallic alloys, Nanoindentation, Combinatorial methods, Metales, Ensayo de materiales, Nanotecnología