C01 - Multiphysics Description of Metal-Intermetallic Composites at the Grain Scale

Project Leader(s)

  • Stefanie Sandlöbes
  • Franz Roters

Summary

Project C01 will focus on the co-deformation of a solid solution matrix reinforced with an intermetallic phase skeleton using crystal plasticity simulation and deformation experiments. In a first approach the intermetallic reinforcement phases will be considered as elastic inclusions while the matrix phase is computed as plastic phase to determine optimum network structures in terms of volume fractions, morphology and spatial arrangement. Experimental validation of the mechanical behaviour and microstructure evolution of selected composite network structures will be performed. Specifically, composite samples with varying volume fraction and morphology of constituent phases will be investigated using quasi in-situ and in-situ deformation experiments. In a second step the mechanical properties and deformation mechanisms of the intermetallic phases will be implemented as physical constitutive models using the data provided by experiments of the intermetallic phases (A05) and the interfaces (C02) and atomistic simulations (A06, C05) in other projects.

Motivation

Deformation of solid solution / intermetallic composite alloys is controlled on the one hand by the intrinsic plasticity mechanisms of the individual phases, their interfaces, and their co-deformation mechanisms. On the other hand, the three-dimensional network structure and morphology of solid solution matrix and intermetallic phase(s) determine the lower and upper bounds of the strengthening / reinforcement potential.

Aims

Key scientific questions to be addressed in sub-project C01 are:

  • What are the geometrical effects of the three-dimensional composite network on the strength and ductility range of composite alloys?
  • To what extent does the plasticity of intermetallic strengthening phases effect the mechanical properties of composite alloys?

The overall aim of project C01 is to identify three-dimensional network structures and morphologies of composite alloys with a high geometrical strengthening potential on the one hand and on the other hand to provide a multiphysics description of the (co-)deformation mechanisms identified in projects studying the solid solution or the intermetallic phases separately.

Methods

  • Crystal plasticity simulations and coupled crystal plasticity – phase field simulations
  • In-situ DIC measurements
  • Electron microscopy (SEM, EBSD, conventional TEM)

C02 - Co-Deformation of Intermetallic-Metallic Composites

Project Leader(s)

  • Sandra Korte-Kerzel

Summary

This project is concerned with the co-deformation of an intermetallic skeleton (Laves phases and Mg17Al12) and a metallic matrix (Mg solid solution). While the deformation mechanisms of each phase are investigated in dedicated projects (e.g. experimentally in A01, A05 and B06) the focus of this project is to elucidate their interaction and competition in the metallic-intermetallic composite.

Motivation

In the design of multi-component alloys as composites of intermetallic and metallic phases it is essential to consider the properties of the individual phases but also how these interact and compete when deformed as one unit. This understanding forms the basis for the required microstructures design in which strengthening and embrittlement need to be balanced and the three-dimensional structures and their metallic-intermetallic interfaces may govern deformation.

Aims

  • Identification of the dominant deformation mechanisms governing deformation at different strains, temperatures and in different microstructure morphologies and alloy compositions
  • Understanding of slip transfer mechanisms at the intermetallic-metallic interface
  • Quantification of strain partitioning and localization
  • Microstructure design in collaboration with closely linked projects considering phase properties, fracture properties of phases and interfaces, microstructure morphology and processing conditions to achieve these

Methods

  • In-situ micromechanical testing
  • Digitial image correlation to quantify local strain
  • Image analysis supported by artificial intelligence to quantify deformation mechanisms (intermetallic fracture, interface failure, slip band formation,…) on panoramic high resolution micrographs
  • Scanning and transmission electron microscopy including diffraction techniques (SADP, EBSD) to analyse slip transfer mechanisms
  • Dedicated nanomechanical experiments at intermetallic-metallic interfaces

C03 - Corrosion Mechanism Influenced by Complex Metallic Phases

Project Leader(s)

  • Daniela Zander

Summary

Useful strategies controlling corrosion and designing more corrosion resistant alloys and/or microstructures depend on a deep understanding of the underlying corrosion mechanisms: in alloys, these depend on the formed solid solution crystals, the kind and distribution of the precipitated secondary phases, and severe strain induced energetic crystalline defects, besides other factors. It is broadly accepted that depending on content, distribution and morphology, secondary intermetallic phases can either act as corrosion barrier or accelerate the corrosion rate due to galvanic coupling. The important fact is that not only the composition, but especially the size and the distribution of precipitates are control variables for the corrosion process. Moreover, complex metallic phases are known to precipitate in several advanced alloys. Nevertheless, hardly any systematic and fundamental studies have been carried out concerning their electrochemical properties and influence on corrosion mechanism within a complex microstructure consisting of intermetallic skeleton, matrix, nano-precipitates, and previously introduced dislocations.

The project focusses on developing a mechanistic understanding of the corrosion mechanism influenced by volume fraction, phase distribution and morphology of complex intermetallic phases and interlinked with dislocation based plasticity.

Motivation

Mg-Al-Ca, used as a model material within the SFB, was already studied extensively in the context of the development of magnesium alloys with improved corrosion properties. However, the approach of engineering atomic complexity has never been assessed in this context and is expected to result in a fundamental understanding of the influence of different complex crystalline phases, provided within the Mg-Al-Ca system, on the corrosion mechanism interlinked to previously introduced dislocation plasticity.

Aims

The aim of this project is to explore corrosion mechanisms by 2D and 3D investigations that serve as a basis for developing a mechanistic understanding of the influence of complex intermetallic phases, such as Al12Mg17, Mg2Ca, (Mg, Al)2Ca, within a complex microstructure consisting of intermetallic skeleton, matrix, nano-precipitates and dislocations on aqueous corrosion. Electrochemical and microstructural (SEM, TEM/EELS (B01-TS), XANES, ICP-MS) 2D investigations after corrosion are expected to give valuable insights into the electrochemical properties and reactions, e.g. dissolution, passivity, of the intermetallic phases acting between different phases. 3D investigations, e.g. by “quasi” in-situ synchrotron tomography, are expected to give a sufficient basis for developing a mechanistic understanding of the corrosion process, influenced by e.g. volume fraction, phase distribution and morphology. The key scientific questions which will be addressed are:

  • Influence of complex intermetallic phases on localized corrosion mechanism within the microstructure of Mg-Al-Ca alloys consisting of intermetallic skeleton, matrix and nano-precipitates and interlinked with dislocation based plasticity.
  • Transfer to a generalized 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)
  • X-ray Diffraction (XRD)
  • In-situ TEM/EELS (with B01)
  • Scanning electron microscopy (SEM + EDS)
  • (quasi) in-situ Synchrotron Tomography and XANES/XRF (applications will go to DESY, ESRF, BESSY)

C04 - Combinatorial Microstructure Design

Project Leader(s)

  • Hauke Springer

Motivation

The motivation of this project is to compliment the novel materials design concept of the CRC by providing the basis for leveraging the derived atomistic understanding towards bulk microstructure / property relationships. This is targeted by a twofold approach: (i) High throughput combinatorial screenings (top down; main part of the project) and (ii) knowledge-based microstructure design (bottom up) in close interaction with other projects within the CRC. The investigated materials and their specific deformation phenomena span a wide range and are complex regarding the relevant parameters controlling them. This project therefore aims at providing a solid mechanical, microstructural and thermodynamic basis from which the materials can be matured towards industrial application at a later stage.

Aims

  • Provide property maps of the ternary Mg-Al-Ca system by bulk high throughput investigations. Identify the chemical composition and processing parameters (especially the solidification rate) for optimum material performance, and verify Calphad predictions. Develop the methodology for future material systems.
  • Obtain bulk materials with designated microstructures to validate and exploit theory-based predictions regarding optimal deformation processes and corrosion behavior.

Methods

  • High throughput screenings: Combinatorial casting into stepped moulds for simultaneous variation of the solidification rate. Small scale rolling coupled with heat treatments to evaluate suitability for thermo-mechanical treatments. Basic microstructure characterization by optical and scanning electron microscopy, x-ray diffraction. Mechanical properties by bulk tensile and bending tests.
  • Knowledge-based design: Various liquid metallurgy synthesis techniques from directional solidification to splat-quenching coupled with complex thermal processing. Microstructure characterization and mechanical / chemical properties as above.

C05 - Ab initio Thermodynamics of Defect Phases

Project Leader(s)

  • Tilmann Hickel

Motivation

In this project, the multi-dimensional role of defects in Mg alloys will be addressed with ab initio methods. The simulation of the stability, structure and composition of these defects as a function of processing parameters, like composition, temperature and stress, requires concepts that go beyond established routes of bulk thermodynamics. Chemical potentials will play an essential role in the development and connection of defect phase diagrams. To evaluate them, bulk phases and their point defects will be studied. In addition, the soft bonds in Mg may require the inclusion of temperature effects beyond configurational entropy as well as non-adiabatic coupling effects. The methods will be applied to planar defects such as twin boundaries and stacking faults in the Mg matrix material and the intermetallic phases. In the same way, also the interfaces between these phases are considered. The segregation of alloying elements to these defects and their impact on the local atomic structure are calculated. The research is performed in close collaboration with projects that characterize defects experimentally and that model the thermodynamics and mechanical properties of the Mg-Al-Ca system.

Aims

  • Thermodynamic stability of defects as a function of chemical composition in Mg and in the precipitate phases (Laves phases and Mg17Al12)
  • Ab initio phase diagrams of low-dimensional structures including the impact of structural constraints
  • Segregation profiles (concentration and extension) of solutes around defects including the interface between matrix and precipitates
  • Mechano-chemical coupling mechanisms during defect nucleation and growth
  • Interpretation of experimental phenomena and determination of parameters for other computational projects (particularly A2, A6, B4, C1, C5)

Methods

  • Density functional theory for bulk and defect structures
  • Concepts of thermodynamics and statistics, incl. concepts for chemical disorder
  • Nudged elastic band method and kinetic Monte-Carlo simulations (3rd period)

C06 - Calphad Description of the Mg-Al-Ca System and its Defect Phases

Project Leader(s)

  • Bengt Hallstedt

Summary

In this project, a completely new 3rd generation Calphad description for the Mg-Al-Ca system will be developed, which will describe all important phases over the complete concentration range and from 0 K to well above melting temperatures. This will be possible by completely basing the modelling on ab initio calculations (from project C05) and by using new models for the elements currently under development under the auspices of SGTE (Scientific Group Thermodata Europe). While knowledge about the phase diagram and the thermodynamic properties of its (bulk) phases is necessary for the development and processing of alloys, many of their properties are determined by defects, in particular grain boundaries, phase interfaces and dislocations. However, the thermodynamic properties of defects and their transitions between different states (or phases) are much less understood. A central theme within the CRC is the development of defect phase diagrams, i.e. the investigations of transitions between different defect states. In this project, thermodynamic models for different defect states will be developed, which can be used to calculate equilibria between different states in order to build defect phase diagrams. In addition a framework for the inclusion of composition and temperature dependent crystal dimensional (lattice parameters and angles) and elastic constants into Calphad thermodynamic databases will be developed.

Motivation

Phase diagrams are indispensable tools for development and processing of alloys. Thermodynamic models for stable and metastable states form the basis for kinetic simulations of the microstructural evolution of alloys. Defects are responsible for many properties of alloys, but their thermodynamic properties are not well known, and, in particular, transitions between different states (phases) are poorly understood.

Aims

  • Complete thermodynamic description of stable and metastable states in the Mg-Al-Ca system from 0 K to well above melting temperatures.
  • Exploration of the thermodynamic modelling of defect phase diagrams (grain boundaries, phase interfaces, dislocations).
  • Inclusion of molar volumes based on crystal dimensional data and elastic constants

Methods

  • Thermodynamic modelling using the Calphad approach will be extended to model properties and phenomena that have so far only rarely been modelled or not at all.