1) Location of grain boundary by EBSD. 2) Lift-out
of a needle sample by FIB. 3) Reconstruction of
the atom distribution in the needle by APT.
Azimuthal map for a single grain after tensile
loading (L1) and 800 (C4) and 3450 (C5) cycles
with subgrains evolving in the circled area.
Stress corrosion cracking of studs used for ECG
electrodes caused by deformation and a chloridecontaining
environment from the patch gel.
Boron in martensitic creep resistant 9-12% chromium steels
The creep strength of martensitic high temperature
steels with 9-12% chromium limits the
attainable efficiency of energy conversion in
steam power plants and gas turbines. In the
present Phd project, together with Siemens in
Germany, we aim to develop improved 12% Cr
steels alloyed with boron. It has been known
since the 1950s that very small additions of boron
(20-150 ppm) can substantially improve the
creep strength of 9-12% chromium steels, but
the detailed mechanism for boron strengthening
is not well known.
Due to its intermediate atomic size, boron has a
strong tendency to move to (segregate to) lattice
imperfections such as vacancies and grain
boundaries. Boron segregation provides its
unique effect on alloy strength, but it also gives
high risks of unexpected fracture along boron
enriched grain boundaries during manufacturing.
Thus, boron alloying is considered highly
challenging by steel and component manufacturers.
In-situ monitoring of structural evolution during cyclic deformation
Preventing corrosion of ECG electrodes
Our understanding and control of boron segregation
is limited, since it has not been possible
to accurately detect small concentrations of
boron locally in the steel microstructure. With
new microscope techniques such as atom probe
tomography (APT), crystal orientation mapping
(EBSD), and focused ion beam milling (FIB),
we can now sample precisely at the feature of
interest in the microstructure and quantify the
distribution of boron with unprecedented spatial
and chemical resolutions. In the PhD project,
we have developed the sampling technique for
APT at DTU, and, in collaboration with Chalmers
University of Technology in Sweden, we are
producing the first quantitative results on boron
segregation in 9-12% chromium steels with
their APT. Improved understanding of boron
segregation will give us new possibilities to develop
improved boron alloyed steels and metal
alloys for many applications.
Fatigue-related damage due to cyclic deformation
in metals is one of the major failure reasons
of structural materials. While failure occurs
on the macroscale, changing mechanical loads
cause formation of characteristic dislocation
structures at the micro- and nanoscale, which
play a decisive role for the materials life time.
During repeated load reversals, dislocations
organize into ordered structures consisting of
dislocation walls of high dislocation density
separating almost dislocation-free subgrains.
These characteristic low-energy deformation
structures are known to determine the plastic
behavior, but the details of the mechanisms on
the microscale are still insufficiently known, and
their relation to the macroscopic failure unsettled.
Insight into the structural reorganization
within single grains is gained by in-situ monitoring
of the microstructural evolution during
cyclic deformation with high energy synchrotron
radiation at e.g. the Advanced Photon Source at
Argonne National Laboratory. By high resolution
reciprocal space mapping, a large number of
individual subgrains can be resolved in the bulk
of polycrystalline specimens and their fate,
their individual orientation and elastic strains,
tracked during tension and compression. With
this methodology, the evolution of dislocation
structures is followed during progressing cyclic
deformation in tension and compression as well
as during selected load cycles with different
strain amplitudes. Relevant information such as
orientation, size and elastic strain of grains and,
in particular, of subgrains can be extracted from
the reciprocal space maps obtained with high
resolution, leading to a deeper understanding
of the microstructural evolution and its influence
on the macroscopic material properties
during cyclic deformation and fatigue.
An Innobooster project with AH Metal Solutions
A/S has been focusing on preventing corrosion
in disposable electrodes used for measuring
the electrical activity of the heart by electrocardiography
(ECG). The electrode incorporates
a material combination in risk of corroding due
to minor changes in the assembly process or
due to uncontrolled humidity and temperature
storage conditions. Corrosion of the electrode
studs can result in functionality issues when
performing an ECG.
The aim was to gain a deeper understanding of
the existing material interactions leading to corrosion
and to develop solutions that are more
reliable. Rune J. Christiansen was working as a
postdoc on the project, investigating the influence
of annealing, deformation, chloride content
in the conductive gel and various types of
stainless steel and silver chloride electrode. The
corrosion types investigated were pitting (ferric
chloride solution testing), stress corrosion
cracking (boiling magnesium chloride test), and
general corrosion (high humidity exposures).
The project led to a safer choice of materials
minimizing the risk of corrosion. At the same
time “From skin to wire” has kicked off other
projects focusing mainly on the electrode surface
and an industrial postdoc application.
Contact:
John Hald, e-mail: jhald@mek.dtu.dk;
Contact:
Wolfgang Pantleon, e-mail: pawo@dtu.dk
Contact:
Morten S. Jellesen, e-mail: msj@mek.dtu.dk
materials and surface engineering 37