Plenarvorträge - DPG-Tagungen

Metallphysik Dienstag
M 11 Hauptvortrag Stefan Müller
Zeit: Dienstag 09:30–10:00 Raum: H16
Hauptvortrag M 11.1 Di 09:30 H16
Ab-initio thermodynamics of metal alloys: From the atomic to
the mesoscopic scale — •Stefan Müller — Universität Erlangen-
Nürnberg, Lehrstuhl für Festkörperphysik, Staudtstr. 7, D-91058 Erlangen
The combination of density functional theory calculations (DFT) with
Cluster Expansion methods (CE) gives access to both huge parameterspaces
(e.g. for ground-state searches) and systems containing more than
a million of atoms (e.g. for microstructure studies). It will be shown how,
together with Monte-Carlo simulations, this ansatz can be applied to
M 12 Phasenumwandlungen II
study alloy properties which possess a delicate temperature-dependence
like mixing enthalpies, short-range order or microstructure evolution
without any empirical parameters, but with an accuracy that a quantitative
comparison with experimental data becomes possible. Regarding
microstructures I will focus on so-called coherent precipitates whose sizeshape
relations can be of technical relevance in alloys like Al-Li, Al-Zn,
or Al-Cu. The presented methods will be extended to alloy surfaces in
order to investigate ordering at surfaces and surface segregation, i.e. the
deviation of the alloy’s composition in the near-surface region compared
to the bulk.
Zeit: Dienstag 10:15–11:15 Raum: H16
M 12.1 Di 10:15 H16
Screening effects in the binary alloy systems Cu-Au, Cu-Pd, and
Au-Ni — •H. Reichert, A. Schöps, I. Ramsteiner und H. Dosch
— MPI für Metallforschung, Heisenbergstr. 3, D-70569 Stuttgart
Electronic screening effects are one of the origins of long-range interactions
in many binary metallic alloys. Experimentally this can be
studied via the singularities in the response functions at specific points
in reciprocal space (Kohn anomalies, 2kF Fermi surface nesting). These
singularities are the origin of a marked fine structure of the diffuse scattering
in binary systems. We have investigated the fine structure of the
diffuse scattering in-situ as a function of temperature in various binary
alloy systems (Cu-Au, Cu-Pd, Au-Ni) using high energy x-ray scattering.
In the diffuse scattering maps we found unique signatures of electronic
screening in all systems. The corresponding nesting constructions are immediately
apparent from the scattering data. The data sets we present
are a test bench for electronic structure calculations.
M 12.2 Di 10:30 H16
In-situ Determination of Phase Selection Sequences in Undercooled
Ti-Fe-Si-O Melts — •Oliver Heinen, Dirk Holland-
Moritz, Thomas Volkmann, Jörn Strohmenger, Thomas Lierfeld,
Sven Reutzel, and Dieter M. Herlach — DLR, Institut für
Raumsimulation, D-51170 Köln
The alloy system Ti-Fe-Si-O shows a great variety of complex stable
and metastable phases. Depending on the composition and the undercooling
different solidification pathways are found. The large number of alloy
phases in Ti-Fe-Si-O leads to a strong competition of phase selection
during rapid solidification of undercooled melts. In order to determine
the phase formation sequence in these alloys as function of composition
and to examine possible dependencies on the short-range order of the
undercooled liquid, in situ investigations of the solidification pathways
and the short-range order of the liquid were performed. The containerless
processing technique of electromagnetic levitation for deeply undercooled
metallic melts was combined with energy dispersive diffraction of
synchrotron radiation at the European Synchrotron Radiation Facility
(Grenoble). This enables us to directly determine the crystal structure of
the solid phases formed during rapid solidification with a time resolution
of 1-2 seconds and study the short-range order of the liquid in the same
experiment. Depending on the undercooling, several different solidification
sequences were found. This work was financially supported by DFG
under contract No. Ho1942/4 and by ESRF.
M 12.3 Di 10:45 H16
Metastable Phase formation in electromagnetically levitated
Nd-Fe-B-melts investigated by in-situ diffraction experiments
with synchrotron radiation. — •Jörn Strohmenger 1 , Thomas
Volkmann 1 , Jianrong Gao 2 , Dirk Holland-Moritz 1 , Oliver
Heinen 1 und Dieter Herlach 1 — 1 Institute of Space Simulation, German
Aerospace Center (DLR), D-51170 Cologne — 2 Key Lab of Electromagnetic
Processing of Materials (EMP), Northeastern University, Shenyang
110004, China
Nd-Fe-B alloys are of special interest due to the excellent hardmagnetic
properties of the intermetallic compound Nd2Fe14B1 (Φ-phase).
Competitive crystallisation of stable and metastable phases in undercooled
Nd-Fe-B melts with Nd-contents between 11.8 and 18.0 at.% at
the ratio of Nd
B
= 2
1
was investigated by the electromagnetic levitati-
on technique in combination with in-situ diffraction experiments at the
European Synchrotron Radiation Facility (ESRF) in Grenoble. Under
equilibrium conditions the solidification sequence is primary γ-Fe followed
by the peritectic formation of the Nd2Fe14B1 phase. It is shown that
in the undercooled melt the crystallisation of the γ-Fe-phase is suppressed
in favour of the Φ phase. Moreover, the crystallisation of a metastable
phase could be directly observed. A phase selection diagram showing the
different solidification pathways as a function of undercooling and alloy
composition will be presented.
Financial support by the Deutsche Forschungsgemeinschaft under contract
No. HE1601/14 and the European Synchrotron Radiation Facility is
gratefully acknowledged. J.G. appreciate support from AvH-foundation.
M 12.4 Di 11:00 H16
Nucleation and Orientation Relationships during partial
alpha-gamma-Phase Transformation in microalloyed Steels
— I. Lischewski, C.-M. Park, G. Gottstein, •I. Lischewski,
C.-M. Park, and G. Gottstein — Institut fuer Metallkunde und
Metallphysik, RWTH Aachen, Kopernikusstr.14, D-52056 Aachen,
Germany
The orientation relationships and the selection of crystallographic variants
during partial ferrite to austenite transformation in a microalloyed
steel were studied, to obtain new insights into the nucleation of this phase
transformation. The samples were encased with a CrNi-18/9 steel foil and
annealed at a low transformation temperature. Nickel, which diffused
from the steel wrap into the transformed sample, stabilized the austenite
nuclei.Therefore it was possible to establish a direct correspondence between
prior ferrite and subsequent austenite crystallography. The local
crystallography was measured by EBSD in a high resolution FEG-SEM.
The austenite was observed to nucleate at random triple junctions of
pre-existing ferrite grain boundaries and satisfied the K-S (Kurdjumov-
Sachs) orientation relationship surprisingly, no apparent variant selection
was observed during incipient stages of austenite nucleation. The preferential
nucleation of austenite at random ferrite triple junctions was
considered in a triple junction weighted orientation distribution which
compared well with experimental results.