Projects
Study
of influence of precipitates on corrosion behavior of aluminium alloys by
in-situ local electrochemical techniques
PhD
student Ali Davoodi, Supervisors Doc. Jinshan Pan and Prof. Christofer
Leygraf
Div. of Corrosion Science, Dept. of Materials Science & Engineering,
KTH
Localized
corrosion behavior of chosen materials in aggressive solutions and the
influence of precipitates will be studied in-situ
by advanced local techniques, such as electrochemical AFM/STM, as well
as scanning electrochemical microscopy (SECM), which provide information
of corrosion/passivation processes occurring at sub-micron scale. An
additional local technique, scanning Kelvin probe force microscopy
(SKPFM), can also be used to reveal lateral variations in Volta
potential associated with intermetallic precipitates (particles), and
their tendency to galvanic corrosion relative to the alloy matrix. The
materials will primarily be aluminium alloys
of the AA3000 and AA6000 series, or other alloys of interest. The
experimental work will be performed to investigate the influence of
various types of precipitates (phase, size and distribution) on the
localized corrosion behaviour. Certain emphasis will be given to
precipitates formed during fast cooling processes. In
multi-component aluminium alloys of the series AA3000 and AA6000,
silicon, iron, manganese, chromium, copper and magnesium are introduced
for improved mechanical strength. This leads to new intermetallic
precipitates whose detailed corrosion action to the matrix is not well
known. The information obtained from the planned project will provide
more detailed knowledge on localized corrosion behavior of the aluminium
alloys, and will be used for the optimisation of new aluminium alloys.
This
project has additional funding from Sapa Technology.
Electronic
structure and properties of interfaces in materials
PhD-student:
Vitaly Baykov, David Andersson, Supervisors: Dr. Pavel Korzhavi, Prof. Börje
Johansson
Div. of Applied Physics,
Dept. of Materials Science & Engineering, KTH
It
is proposed to perform theoretical studies of alloy phases and
interfaces using ab initio calculations of their electronic structure
and total energy, as well as atomistic modeling. The research will be
focused on ordered phase precipitates and precipitate-matrix interfaces
in steels alloyed with Al, Si, V, Cr, Co, Ni, and Mo, as well as in
high-temperature g/g’ superalloys. As a result, it is planned to
achieve a deeper understanding the effects of the alloying elements on
the structure and properties of the studied systems.
Outline
of the studies for 5 years
Year
1: Practicum II and III (Ab initio calculations for selected ordered
compounds and random alloys, surface energies).
Year
2: Modeling the structural disorder and the site preference for ternary
alloying elements in binary intermetallic compounds and sigma phases.
Years
3 and 4: Calculations of the surface energies and atomic structure for
g/g’ and other kinds of interfaces in binary systems. Systematic
studies of the orientation dependence and the effects of ternary
alloying.
Year
5: Analysis of the calculated thermodynamic data and systematic trends;
interpretation of the results in terms of electronic structure.
Preparation of the PhD thesis.
Enhanced dehydrogenation in ladle treatment - to meet the demand on the
optimization of mechanical properties of steels
PhD
Student Jenny Brandberg, Supervisor Prof. Du Sichen
Div. of Metallurgy,
Dept. of Materials Science & Engineering, KTH
Dehydrogenation is usually carried out in ladle treatment, very often by
vacuum degassing. In order to optimize the process taking into account
of the quality of the steel, the energy saving and environmental
aspects, a description of the dehydrogenation rate is essential. The
optimization of the steels would require the knowledge of the
relationship between the hydrogen concentration and mechanical
properties, the thermodynamic constrain regarding the lowest hydrogen
level that could be achieved under given industrial conditions and the
rate of dehydrogenation.
Dehydrogenation in a ladle involves both slag-metal as well as gas-metal
reactions. It is a common knowledge that dehydrogenation would be more
difficult in humid weather, as slag formers pick up moisture. Very
often, slag supplies hydrogen to the steel rather than picks it up from
the metal. The development of sub-models for the description of the
reactions in the slag-metal mixing zone requires a good understanding of
the slag-metal mixing, viz. the dispersion of slag drops in at metal or
vice versa. It also requires a knowledge of the capacity of the slag
holding hydrogen ( HO-). On the other hand, the micro modelling of
gas-metal reaction is directly related to adsorption and disorption.
The present proposal envisages a further examination of the effect of
hydrogen content on the mechanical properties. Sub-models describing the
reactions of dehydrogenation are also to be developed. Each basic
sub-model development will be done in the following steps:
1. Mechanism study – To gain a better
understanding and chemical as well as physical pictures of the
reaction phenomena.
2. Mathematical formulation – To develop
sub-models on the basis of the mechanism study.
A generalized model of
dehydrogenation for ladle treatment is to be developed on the basis of
the sub-models.
The industrial experimental costs are covered by Uddeholm Tooling.
Property tools towards process
optimisation
PhD student Madeleine Lundberg: Supervisor Prof. Seshadri Seetharaman
Div. of Metallurgy,
Dept. of Materials Science & Engineering, KTH
Aim
The planned project targets generation of thermochemical and
thermophysical properties and suitable extrapolation models with a view
to incorporate them in process optimizations. The research efforts have
a slightly stronger emphasis on slag phase, where there is a dearth of
data.
The thermochemical properties targeted are:
1.
thermodynamic activities
2.
sulphide capacities
3.
enthalpies
4.
molar volumes
The thermophysical properties being looked into are:
1.
viscosities
2.
thermal diffusivities
3.
surface- and interfacial tensions
These properties should enable process optimization on the basis of
thermodynamics as well as mass, heat and momentum transfer.
Project plan
The work carried out so far is as follows:
1.
Development of a theoretical model for the heat conduction of porous
solids, which can be applied in the characterization of coke and even
for other porous solids.
2.
Development of a theoretical model for the viscosities and thermal
diffusivities of mould flux slags with a view to optimize the
properties towards faster casting speeds.
3.
Measurement of fluoride evaporation from mould flux slags and
characterization the same from the view points of chemistry as well as
environment.
4.
Interlinking of thermochemical and thermophysical properties of slags
in order to generate self-consistent data for process simulation.
Further work planned:
1.
Completion of the work on the characterization of mould flux slags
2.
Model the melting rate of the mould flux by X-ray imaging
3.
Measurement and modelling of the tensile strength of solidified slag
films in order to minimize the frictional effects on the strand and
consequent surface defects. The study is also supposed to provide
structural insights into slag structure.
Thermodynamic
modelling of molar volume and thermal expansion
PhD student
Xiaogang
Lu, Supervisor Dr. Malin Selleby
Div. of Computational Thermodynamics,
Dept. of Materials Science & Engineering, KTH
Research
activities
Combination of the Calphad method and
theoretical calculations provides new possibilities for the study of
materials science. This work is aimed to combine these methods to
facilitate modeling and to extend the thermodynamic databases with
critically assessed volume data. In this work, the theoretical
calculations refer to first-principles and Debye-Grüneisen calculations.
The first-principles (i.e. ab initio)
electronic structure calculations, based on the Density- Functional
Theory (DFT), are capable of predicting various physical properties at 0
K, such as formation energy, volume and bulk modulus. The ab initio
simulation software, VASP, is used to calculate the binding curves (i.e.
equation of state at 0 K) of metallic elements, cubic carbides and
nitrides. From the binding curves, the equilibrium volumes at 0 K are
calculated for several metastable structures as well as stable
structures.
The vibrational contribution to the free
energy is calculated using the Debye-Grüneisen model combined with
first-principles calculations. Two different approximations for the
Grüneisen parameter, γ, are used in the Debye-Grüneisen model, i.e.
Slater’s and Dugdale-MacDonald’s expressions. The thermal electronic
contribution is evaluated from the calculated electronic density of
states.
By fitting experimental heat capacity and
thermal expansivity around Debye temperatures, it is possible to optimal
Poisson’s ratio values and to use them to evaluate Young’s and Shear
moduli. In order to reach a reasonable agreement with the experiments,
it is necessary to use the logarithmic averaged mass of the
constitutional atoms.
A new model describing thermodynamic
properties at high pressures was implemented in Thermo-Calc. The model
is based on an empirical relation between volume and isothermal bulk
modulus. Pure Fe and solid MgO were assessed using this model. Solution
phases will be considered in a future work to check the model for
compositional dependence.
Phase field simulation of grain growth
PhD student
Henrik Strandlund,
Supervisor Prof John
Ågren
Div. of Physical Metallurgy,
Dept. of Materials Science & Engineering, KTH
The phase field method is based on the
theory of diffuse interfaces (Landau-Ginzburg, Cahn-Hilliard) and has
been applied frequently to study different processes and transformations
in materials the last decade. Especially dendritic solidifications,
precipitations in solid state and grain growth in multicomponent systems
have been simulated and studied.
The phase field method has been studied at
the division of physical metallurgy for almost 10 years. Earlier
projects have been devoted to study dendritic solidification,
recrystalization and widmanstätten precipitation. Some software
development have also been performed to increase the numerical stability
and to computational speed. Experiments concerning grain growth have
also been performed to enable quantitative comparisons between phase
field simulations and experiments.
In this project we will use the software
FEM-LEGO, developed at the department of mechanics at KTH, to study
grain growth. A phase field model will be formulated and implemented.
Some of the earlier developed tools and models enable stable and fast
calculations will be applied. Simulations under different conditions
will be performed and quantitative comparisons with experimental data
concerning grain growth in low alloyed steels will be made.
Development of
tools for integrated optimisation of materials – MatOp - stainless
steels
PhD student
Henrik Sieurin,
Supervisor Prof. Rolf
Sandström
Div. of Materials Technology,
Dept. of Materials Science & Engineering, KTH
Stainless steels are used in highly demanding applications in for
instance the processing and power industry. In order to meet these
demands, the composition and micro structure of the materials must be
carefully controlled during manufacturing. The aim of this project is to
be able to predict micro structure development in complex processes and
to link microstructure components and material properties.
Prediction of micro structures and properties will be performed for
highly alloyed stainless steels intended for advanced requirements on
corrosion and mechanical properties such as vessels for aggressive
media. General corrosion, pitting corrosion, crevice corrosion,
strength, ductility, hot cracking (weldability), hot workability,
ferrite content, and presence of TCP-phases such as sigma will be
analysed. Most of these quantities need the phase information from
Thermo-Calc to evaluate the property values with the help of CMPRs
(Composition-Microstructure-Property Relations).
As
another example of process optimisation, discontinuous annealing after
cold working of structural steels for forming applications will be
studied. Computational models will be applied to the manufacturing
process. Grain growth, recrystallization, texture formation,
carbonitride precipitation and austenite to ferrite phase transformation
will be taken into account. The features included in the optimisation
system such as microstructural evolution on the basis of chemical
thermodynamics and classical nucleation and growth theory will be
utilised. The role of the yield to tensile strength ratio will be
analysed.
Development of
tools for integrated optimisation of materials – MatOp – aluminium
alloys
PhD student
Johan Zander,
Supervisor Prof. Rolf
Sandström
Div. of
Materials Technology,
Dept. of Materials Science & Engineering, KTH
The
use of aluminium alloys in the transportation sector is increasing
rapidly due to the low weight of the materials and their good properties
in manufacturing and usage. To ensure the competitiveness of the
materials, these properties must be carefully optimised for the
applications.
The
properties of materials depend in a complex way on composition and micro
structure. During the last few years new modeling tools have become
available for the prediction of micro structure and properties, enabling
a mathematical optimization of materials for specific applications. The
purpose of this project is to use these tools to develop the
understanding of the factors controlling properties in aluminium alloys.
The
work includes
-
Development of models for micro structure development in aluminium
alloys.
- Find
and formulate connections between composition, micro structure and
properties.
-
Optimization of materials for vehicle applications using FEM.
Creep modelling
of particle strengthened materials
PhD student
Hans Magnusson,
Supervisor Prof. Rolf
Sandström
Div. of
Materials Technology,
Dept. of Materials Science & Engineering, KTH
The
use of particle hardening is typically the most efficient way of
improving the strength of creep resistant materials. This is utilised
for example in nickel-base superalloys and modern 9 and 12%Cr-Mo-V
steels in fossil fired power plants. To get an optimised long time creep
strength the behaviour of the particles must be well controlled
metallurgically. If the particles coarsen too rapidly the high
temperature strength is lost.
This
work will primarily focus on the behaviour of 9 and 12%Cr-Mo-V steels.
Extensive international work has been performed on these steels in the
framework of COST 522. In spite of the extensive international work two
pieces of information are still lacking, influence of particles on creep
rupture and a more fundamental model for the development of
substructure during creep.
It is
the purpose of this project to generate models for these two items and
in international collaboration integrate them with existing models for
particle formation and coarsening.
Creep
data and microstructure information is collected from the open
literature for creep resistant low alloy and high chromium steels. In
particular, the influence of particle structure on creep strength is
analysed.
With
the help of thermodynamical modelling (Dictra), the nucleation and
growth of strength controlling particles is predicted.
A
previously developed model for dislocation climb across particles in
austenitic steels will be reviewed, expanded and compared with other
available models. In particular, the temperature dependence of the creep
properties must be represented well. In this way the precipitation
kinetics can be integrated in the model and the role of different phases
can be taken into account fully. One important aspect is the type of
interaction between dislocations and particles that is present.
In the
past the role of the dislocation substructure has been taken into
account in a qualitative way. In this project the critical interaction
between the substructure and the particles will be studied and modelled.
Existing models and observations for the development of substructure
during creep will be analysed.
Process-microstructure relations
PhD-student: Johan Jeppsson,
Supervisor Prof John
Ågren
Div. of Physical Metallurgy,
Dept. of Materials Science & Engineering, KTH
Models available in the literature will be assessed and modified to fit
the aims of the Materials Optimisation project. Phenomena like
nucleation, growth and coarsening of precipitates as well as grain
growth and recrystallisation will be considered. The models will include
classical nucleation theory and its well-known modifications to account
for heterogeneous nucleation on grain boundaries and dislocations, as
well as Zener-Hillert type of equations to calculate growth rates of
individual precipitates and pearlitic or bainitic colonies and
Kolmogorov-Johnson-Mehl-Avrami type of equations to account for
impingement.
For
simultaneous nucleation and coarsening a numerical treatment of
classical LSW theory (Lifshitz, Slyozov, Wagner) will be applied. A
correction factor, taking into account a finite fraction of secondary
phase, will be used. This factor is based on previous results from
random walk and stochastic theory.
The
goal is to have models which simulate the final and intermediate
structures for a given alloy composition and applied process route. The
models will be developed to such a level that they are good enough to
lead the industry in the best direction for alloy and process
development. Simulations by advanced models and databases speed up this
development and reduce the number of laboratory and industrial scale
experiments.
Thermodynamic models of metallurgical processes
PhD-student: Lina Kjellqvist, Supervisor: Prof. Bo Sundman
Div. of Computational Thermodynamics,
Dept. of Materials Science & Engineering, KTH
The
aim of the project is to connect thermodynamic models with software for
simulation of metallurgical processes, e.g. fluid flow simulations for
decarburization of alloyed steels. A full simulation where you calculate
local equilibrium in several thousand grid points in a 3D simulation of
flows is computationally impossible, so different methods to simplify
the simulation should be examined. As a first approach, the
decarburization process is considered as one equilibria, via different
reactor models that divides the process into different segments with
flow between these, to a full fluid flow simulation with simplified
thermodynamics.
We
will start by looking at the system Fe-Ca-Si-O-C. The liquid metal,slag,
various solid oxides and metal phases shall be taken into consideration.
Simulations with Mn, Cr, Ni and S should be studied later. The result
will be compared with measurements on real systems.
Thermodynamic models assessed independent of the studied process shall
beused. The connection have to be done so that it is easy to exchange
partsof the software, i.e. the integrity of the software should be
maintained.A simulation package with the opportunity to choose the
degree ofrefinement of the process model will be developed, with a user
interfacethat make it possible to use the simulator in industrial
applications.
Dennis Andersson
2005-05-13 |
