B01 - Interfaces in Mg based Alloys in Dependence on Chemical Potential and Temperature

Project Leader(s)

  • Christina Scheu

Summary

The research activities of B01 are dedicated to get an atomistic understanding of the occurring interfaces and to discover their changes during thermal treatment and corrosion. The atomic structure, chemical composition and bonding behaviour of the heterophase boundaries, i.e. interfaces and surface phases (e.g. oxides/hydroxides) formed during corrosive attack will be studied by advanced aberration corrected scanning transmission electron microscopy (S/TEM) techniques including various spectroscopy methods. Possible interfacial phase transitions occurring during annealing, e.g. complexion transitions, will be investigated in B01 using post-mortem and in-situ S/TEM methods. A strong focus is laid on the oxides/hydroxides, which form at the surface due to the contact with air or aqueous solution. For the latter case, specific pH values will be chosen in conjunction with the project B05. Different oxides/hydroxides are expected to grow on top of the intermetallic (Mg, Al)2Ca Laves phase (C14, C15 or C36) in comparison to the Mg matrix which will be analyzed in detail. The distribution of the different elements obtained by electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDX) will be correlated to the ones obtained by atom probe tomography (B03). Precise interfacial atomic positions will be extracted from the experimental STEM and high resolution TEM (HRTEM) images by mathematical algorithms provided in (A04). The results of the oxide/ hydroxide structure determination will also serve as input for (B04, B05) to develop a model of the processes happening during corrosion.

Motivation

Internal and external interfaces in Mg-alloys play a major role during mechanical loading, thermal treatment and during corrosion. For example, the internal interfaces are weak points for electro-chemical attacks occurring when the alloy is exposed to a corrosive environment. The interfaces are considered to be easy diffusion pathways for the ionic species of the electrolyte leading finally to crack formation. Stabilizing the interfaces by segregation of alloy elements and/or complexions might be a promising tool for improved corrosion resistance. Likewise, these concepts can be applied for improving the mechanical properties of Mg alloys. However, to develop such concepts, a detailed knowledge of the interfaces and the oxide/hydroxide phases which form at the surface when in contact with an electrolyte is required. In particular their changes occurring as a function of the chemical potential and temperature are often not understood in detail. This is related to the experimental difficulties to address these points as imaging and spectroscopic techniques are required, which possess the necessary spatial resolution down to the sub-nanometer scale. Such atomic scale characterisation will be performed in B01.

Aims

  • Develop protocols to image and analyze the internal and external interfaces in Mg alloys using advanced S/TEM techniques with minimal alterations in structure and composition
  • Determine the internal interface structure between the metallic matrix and the different intermetallic phases in dependence on the chemical potential and temperature
  • Analyze the oxides/hydroxide phases which are present at the surface after exposure to an aqueous solution with specific pH values and characterize their bonding to the underlying phases

Methods

  • advanced aberration corrected S/TEM imaging such as high-angle annular dark-field (HAADF) and high resolution TEM imaging for atomic structure determination
  • EELS and EDS measurements in STEM mode for chemical composition analyses of the interfaces
  • Analysis of the energy-loss near-edge structure (ELNES) associated with each element specific edge in the EELS data to study the electronic structure and bonding behavior

B02 - Combinatorial Synthesis of Mg-Al-Ca Thin Film Model Systems

Project Leader(s)

  • Jochen Michael Schneider

Summary

The goal of this project is to investigate the correlation between chemical composition, phase formation and morphology of intermetallic as well as solid solution Mg-Al-Ca thin films. To this end, binary and ternary Mg-based solid solution and intermetallic (Mg,Al)2Ca Laves phase thin films will be synthesized combinatorically by high vacuum physical vapour deposition. Using this approach, thin films with composition gradients are obtained which allows for an efficient investigation of the composition induced changes in phase formation within the Mg-Al-Ca material system (see Fig B02.1). The as-grown thin films will be characterized regarding phase formation by X-ray diffraction (XRD) and (global) chemical composition by energy dispersive X-ray spectroscopy (EDX) along the composition gradient. Furthermore, the correlation between synthesis conditions and thin film morphology (STEM) will be identified in order to synthesize tailored model samples. Hereby, the technologically relevant Mg-based composites, consisting of a skeleton phase and a metallic matrix, are mimicked by bi- and multi-layered composite thin films of (Mg,Al)2Ca Laves phase with alternating Mg-based solid solution layers. By varying the individual layer composition, thickness, morphology and overall multilayer configuration, the synthesized films serve as model systems. In collaboration the effect of composition and layer architecture on the phase formation, defect structure, local chemical composition as well as the corrosive and mechanical behaviour is studied systematically. These data are required to compile predictive defect phase diagrams.

Motivation

The motivation is to contribute towards understanding of the relationships between chemical composition, phase formation and morphology of Mg-Al-Ca intermetallic thin films, Mg-Al-Ca solid solution thin films as well as multilayers thereof.

Aims

  • Identification of synthesis conditions for the phase formation and morphology evolution of Mg-based solid solution and (Mg,Al)2Ca Laves phase thin films as well as intermetallic/solid solution composite architectures.
  • Investigation of the effect of the constituent element concentration on the solid solution (solubility limits) and intermetallic phase formation (Mg/Al ratio).

Methods

  • UHV combinatorial magnetron sputtering
  • Energy-dispersive X-ray spectroscopy (EDX)
  • X-ray diffraction (XRD)
  • Scanning transmission electron microscopy (STEM)

B03 - Local Chemical composition of Defects

Project Leader(s)

  • Marcus Hans

Summary

Within this project the local chemical composition of Mg(Al,Ca) solid solutions, (Mg,Al)2Ca Laves phases and solid solution/intermetallic composite thin films as well as bulk samples will be investigated by atom probe tomography. Identification of the atomic distribution of the constitutional elements within the different types of defects (e. g. phase boundaries, precipitates, grain boundaries, dislocations) as a function of the matrix composition will enable us to understand their influence on the phase formation as well as deformation and corrosion behavior. Simultaneous information on the nanostructure and local chemical environment will be obtained by correlative transmission Kikuchi diffraction on atom probe tips for selected samples.

The knowledge of the defect composition in dependence of the constitutional elements present in the matrix is of paramount importance to establish predictive defect phase diagrams and enable control over the deformation and corrosion resistance of materials.

Motivation

The knowledge of the local defect composition as a function of the matrix composition for different types of defects contributes to establish predictive defect phase diagrams and enables control over the deformation and corrosion resistance of materials.

Aims

  • Exploration of the local chemistry for various types of defects such as phase boundaries, precipitates, grain boundaries and dislocations as a function of selected matrix compositions.
  • Identification of correlations between the local defect composition and underlying deformation and corrosion mechanisms by analysis of mechanically and/or electrochemically stressed samples
  • Comprehensive characterization of nanostructure and chemical environment at the nanometer scale using correlative methods involving atom probe tomography and transmission Kikuchi diffraction for selected samples.

Methods

  • Atom probe tomography (APT)
  • Transmission Kikuchi diffraction (TKD)

B04 - Corrosion of Mg Alloys and Intermetallics

Project Leader(s)

  • Mira Todorova

Motivation

In this project, the role of chemical composition and presence of structural defects, both of which strongly affect the corrosion behavior of Mg and its alloys, will be addressed using ab initio methods. The stability of surface and interface structures of Mg (with addition of alloying elements Al and Ca) and its oxide/hydroxide phases will be investigated as a function of environmental conditions (T, pH, U), as well as the role of defects and alloying elements for the formation of corrosion products. Chemical potentials will be essential for the construction of defect diagrams and determination of local equilibria. Approaches, in particular related to the presence and influence of a double layer and electric fields at the solid-liquid interface on the formation of observed defect phases will have to be developed.

Aims

  • Structural reconstructions at surfaces and in micro-cracks in dependence of alloying elements and corrosive environment (dry and wet corrosion)
  • Formation of oxidized phases (including internal oxidation)
  • Thermodynamic stability in an electrochemical environment as a function of chemical composition
  • Construction of surface phase diagrams (ΔG, p, T), surface and bulk Pourbaix diagrams (ΔG, pH, U), defect phase diagrams (μ, EF)
  • Interpretation of experimental structures and phenomena in collaboration with experimental projects (B01, B02, B03, B05, C03) and other computational projects (A02, A06, C04, C05). Construction of Pourbaix diagrams for Laves phases in collaboration with C04.

Methods

  • Density functional theory
  • Ab-initio molecular dynamics
  • Concepts of thermodynamics and statistics

B05 - Selective Corrosion and Passivation of Intermetallic-Metallic Interfaces

Project Leader(s)

  • Daniela Zander

Summary

Corrosion mechanisms, such as selective corrosion and passivation are dominated by the local chemistry and defects of the microstructure. It is well known that alloying elements added to enhance the mechanical properties affect the corrosion behavior by changing the chemistry of the solid solution crystals and the kind and distribution of the precipitated secondary phases, besides other factors. Alloying chemistry and processing routes very often result into localized corrosion by introducing different redox potentials at interfaces e.g. between secondary phases in respect to the matrix, local enrichment and/or depletion in solid solution and secondary phases and severe strain induced energetic crystalline defects at high angle grain boundaries. This, on the other hand, also leads to the ability to form more protective passivation layers. However, hardly any systematic and fundamental studies have been carried out concerning the specific electrochemical properties determined by internal and surface interfaces and their influence on corrosion mechanisms. The project focusses on developing a mechanistic understanding of the role of internal intermetallic/metallic and surface interfaces of Mg-Al-Ca alloys on selective corrosion mechanisms and passivation in simple aqueous environments.

Motivation

Mg-Al-Ca, used as a model material within the SFB, has already been studied extensively in the context of the development of magnesium alloys with improved corrosion properties. However, the detailed investigations on the influence of alloying elements on the formation of interfaces has never been assessed in this context and is expected to result in a fundamental understanding of localized corrosion mechanisms, such as micro-galvanic corrosion, and passivation.

Aims

The aim of this project is to explore the role of intermetallic/metallic interfaces of Mg-Al-Ca alloys on selective corrosion mechanism and passivation by electrochemical, chemical and microstructural investigations. This project is expected to give valuable insights into the electrochemical reactions in simple aqueous electrolytes associated to selective micro-galvanic corrosion and passivation and directed by the type and amount of alloying elements and the processing route. The key scientific questions which will be addressed are:

  • What are the electrochemical properties/reactions of the dissolution, passivation initiation and growth of polycrystalline solid solution crystals, complex phases and interfaces of solid solution/complex phases of Mg-Al-Ca alloys?
  • Which are the dominant interface effects on aqueous corrosion in dependence of the electrochemical character of anodic (Ca) and cathodic acting (Al) alloying elements?
  • How can these be transferred to an interface related mechanistic corrosion model?

Methods

  • Direct current (DC) / Alternating current (AC) corrosion measurements
  • Scanning vibrating electrode techniques / Scanning Ion-selective Electrode Technique (SVET/SIET)
  • Inductive coupled plasma mass spectrometry (ICP-MS)
  • X-ray photoelectron spectroscopy (XPS)
  • In-situ TEM/EELS (with B01)
  • Scanning electron microscopy (SEM + EDS)

B06 - Fracture of Intermetallic Phases and Intermetallic - Mg Interface Strength: Effects of Structure, Composition and Temperature

Project Leader(s)

  • Gerhard Dehm

Summary

For ternary Mg-Al-Ca alloys the complex Laves phases (Mg,Al)2Ca as well as the intermetallic phase A12 (β-Mg17Al12) are the most prominent intermetallic compounds, which are detrimental for the resulting mechanical performance. The project B06 explores the fracture properties of the intermetallic phases as a function of their structure and chemistry by site-specific miniaturized mechanical tests in cooperation with projects A03, A05, B01, B03 & C02. Additionally, first experiments on bi- and multi-layered structures with alternating layers of matrix and intermetallic phase are performed to probe the crack arresting capacity of the matrix. The experimental studies include (i) downscaling the fracture experiments to the relevant micro- and submicrometer dimensions, (ii) establishing protocols for quantitative fracture experiments as a function of crystal orientation and (iii) chemical composition on (sub-)micron-sized intermetallic phases, (iv) expanding fracture experiments to elevated temperatures (25°C to ≤600°C), (v) studying the role of defects such as grain boundaries within the intermetallic phases on the fracture toughness and finally (vi) starting to establish test procedures to analyze the potential for arresting cracks from the intermetallic phases by the hexagonal matrix. In case of limited plasticity within the intermetallic phases (e.g. at elevated temperature), the fracture concepts have to advance from linear elastic fracture mechanics to elasto-plastic fracture mechanics. This objective will be accompanied by finite element modelling to optimize the fracture sample geometry and dimensions and to clarify data interpretation. The elasto-plastic fracture results will be linked with the study of dislocation based plasticity in project A05. Additionally, bi and multi-layered composite thin films of intermetallic phases with alternating Mg-based solid solution layers of project B03 will be fracture tested to sense the crack arresting capacity of the Mg-based matrix and to advance towards the skeleton-like structure of real alloys.

Motivation

The limited ductility, fracture toughness and corrosion resistance of Mg alloys are some of the main challenges which hamper their technological breakthrough. In addition, these properties also limit the required reliability and safety requirements for applications. In order to increase the strength and creep resistance of Mg alloys solute additions, such as Ca, Zn and Al, are incorporated, which lead to the formation of intermetallic phases. For ternary Mg-Al-Ca alloys the complex Laves phases C14, C15 and C36 ((Mg,Al)2Ca) as well as the intermetallic phase A12 (β-Mg17Al12) are the most prominent intermetallic compounds. These phases enhance the strength but can become detrimental and induce failure due to their brittle nature. As a consequence a fundamental understanding of their fracture properties and the crack arresting capacity at the matrix-intermetallic phase as a function of temperature and chemical composition is required to provide design information for optimized alloys combining strength and ductility. Mapping the fracture behavior as a function of temperature and chemical composition (including point defects like antisite or constitutional vacancies) opens the possibility to link these information to phase diagrams for point defects, dislocations and interfaces in cooperation with projects A03, A05, B01, B03 & C02.

Aims

  • Establish test protocols for miniaturized fracture experiments on the complex Laves phases C14, C15 and C36 ((Mg,Al)2Ca) as well as the intermetallic phase A12 (β-Mg17Al12) including sample dimensions and internal defects in the phases (e.g. grain boundaries).
  • Extend the fracture experiments to elevated temperatures to relate brittle to ductile transitions with crystal structure, chemical composition, and point defects (constitutional antisite atoms or vacancies).
  • Establish micromechanical tests to probe the interface between the matrix and the intermetallic phase and to provide quantitative data.

Methods

  • Miniaturized fracture experiments (e.g. notched cantilevers, pillar splitting) up to 600°C
  • In situ SEM combined with micromechanical testing and/or nanoindentation
  • SEM based electron spectroscopy (EDX, WDX) and electron diffraction (EBSD) to measure local chemistry, phase identification and crystal orientation