Micromechanics of magnesium and its alloys studied by nanoindentation

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.