Plenarvorträge - DPG-Tagungen

Metallphysik Montag
[1] S.G. Mayr and R.S. Averback, Phys. Rev. Lett. 87 (2001), 196106
[2] S.G. Mayr, Y. Ashkenazy, K. Albe and R.S. Averback, Phys. Rev.
Lett. 90 (2003), 55505
M 8 Phasenumwandlungen I
Zeit: Montag 14:45–16:00 Raum: H4
M 8.1 Mo 14:45 H4
Precipitates in Al-Li: The existence of a precursor phase —
•Stefan Müller1 , Walter Wolf2 , and Raimund Podloucky3 — 1Universität Erlangen-Nürnberg, Staudtstr. 7, D-91058 Erlangen —
2Materials Design s.a.r.l., 44.av.F.-A. Bartholdi, 7200 Le Mans, France —
3Universität Wien, Institut fuer Physikalische Chemie, Liechtensteinstr.
22A, A-1090 Wien
Although it is generally believed that the δ ′ phase of Al3Li is directly
formed from the fcc-based α solid solution by quenching and aging the
sample, X-ray small angle scattering results report the existence of a precursory
structure in the early stage of the decomposition. By combining
density functional theory calculations with a mixed-space cluster expansion
and Monte-Carlo simulations we will show that the existence of such
a precursor phase sensitively depends on the chosen Li-concentration.
Our calculations allow to predict the ground-state diagram, short-range
ordering, the size-shape-temperature relation of the L12 precipitates, as
well as the coherent phase boundaries for both the precursor phase and
the δ ′ phase.
M 8.2 Mo 15:00 H4
Magnetic effects on the nucleation in undercooled Co-Pd melts
— •Dirk Holland-Moritz 1,2 and Frans Spaepen 2 — 1 DLR, Institut
für Raumsimulation, D-51170 Köln — 2 Harvard University, Division
of Engineering and Applied Sciences, Cambridge, MA 02138, USA
For all known ferromagnetic metallic materials the Curie temperature
is lower than the liquidus temperature. Therefore, ferromagnetism has
not been observed in stable metallic melts. By use of advanced undercooling
techniques it is however possible to undercool melts of Co-based
alloys near their Curie temperature. Preceding undercooling experiments
on melts of Co-Pd alloys as a function of the alloy composition indicated
a change of the nucleation mechanism if the temperature of the liquid
is approaching the Curie temperature. Therefore the measured composition
dependence of the nucleation temperatures cannot be described in
the framework of classical nucleation theory.
In this work we present a thermodynamic model which extends classical
nucleation theory by adding magnetic contributions to the driving
free energy for crystallization. The inclusion of these contributions allows
a quantitative description of experimental results on the undercoolability
of Co-Pd melts in the entire composition range from 50-100 at.% Co.
This work was supported by the Deutsche Forschungsgemeinschaft
(DFG) under contract Nos. Ho1942/1 and Ho1942/2, by the Alexander
von Humboldt Stiftung and by the National Space and Aeronautics
Administration (NASA).
M 8.3 Mo 15:15 H4
In situ observation of nucleation processes in Ti-V during
phase separation — •Ingo Ramsteiner, Andreas Schöps, Harald
Reichert, and Helmut Dosch — Max-Planck-Institut für Metallforschung,
Heisenbergstr. 3, 70569 Stuttgart
Ti and V form a disordered bcc solid solution at high temperatures,
which phase separates into various structures at low temperatures. We
M 9 Flüssige und amorphe Metalle III
performed time resolved in situ x-ray measurements on the phase separation
process in Ti2V3, TiV and Ti3V2 single crystals. High energy
x-rays (70-80 keV) allow to penetrate bulk samples of more than 1 mm
thickness in transmission geometry. Using a fast 2D detector system we
resolve the evolution of the diffuse scattering on an almost flat plane in
reciprocal space within seconds. Upon cooling the sample from the high
temperature equilibrium phase into the miscibility gap we observe the
formation and evolution of Bragg reflections indicating the nucleation
of precipitates. The corresponding positions in reciprocal space allow to
determine the alignment of the nuclei with respect to the parent lattice.
The experiments demonstrate, that a controlled variation of the thermal
history leads to nucleation of different structures.
M 8.4 Mo 15:30 H4
Nucleation processes in Au-Ni alloys — •H. Reichert, A.
Schöps, I. Ramsteiner und H. Dosch — MPI für Metallforschung,
Heisenbergstr. 3, D-70569 Stuttgart
Au-Ni alloys exhibit a pronounced competition between ordering and
phase separation. In the ground state the system is phase separated. We
have investigated the early stages of phase separation in Au-Ni alloys after
quenching alloy single crystals from the disordered high temperature
phase into the miscibility gap. Time-resolved measurements of the diffuse
scattering of Au3Ni2 alloy single crystals demonstrate the existence of
two distinct nucleation channels in the precipitation of Au-rich particles
from the host matrix. We find a fast nucleation channel which is producing
extremely well-aligned Au precipitates in the host matrix. From
the scattering data we can deduce a direct influence of the electronic
structure of the host matrix onto the phase separation process
M 8.5 Mo 15:45 H4
Characterization of hard magnetic phase in mould-cast and directionally
solidified Nd-Fe alloys — •G. Kumar 1 , R. Sato 2 , J.
Eckert 3 , W. Löser 1 , L. Schultz 1 , and R. Grössinger 2 — 1 IFW
Dresden, Institute for Metallic Materials, D-01171 Dresden, Germany
— 2 Institut für Experimentalphysik, TU Wien, A-1040 Vienna, Austria.
— 3 TU Darmstadt, FB 11 Materials- und Geowissenschaften, FG
Physikalische Metallkunde, D-64287 Darmstadt, Germany.
In this work, Nd80Fe20 alloys were prepared by copper mould casting
and directional solidification (DS) to obtain a coarse microstructure. The
mould-cast samples exhibit a fine globular eutectic structure and show
high room temperature coercivity ( 5 kOe). The DS samples exhibit
large grains (composition close to NdFe2) in peritectic morphology but
show no coercivity. The NdFe2 grains (formed in the DS samples) show
clear magnetic domains with uniaxial anisotropy indicating that NdFe2
is not a cubic phase. HRTEM equipped with selected area diffraction
(SAD) was used to identify the crystal structure of NdFe2-type phase
observed in the DS samples. AC susceptibility and Mössbauer studies
were used to identify different magnetic phases. Acknowledgement The
work was supported by the German Science Foundation (DFG) via the
DFG priority program ”Phasenumwandlungen in mehrkomponentigen
Schmelzen”under grant Ec 111/11-1,2.
Zeit: Montag 16:30–18:00 Raum: H16
M 9.1 Mo 16:30 H16
Phase separation in bulk metallic glasses — •Shantanu Madge 1 ,
Gerhard Wilde 1 , and Lindsay Greer 2 — 1 Institute of Nanotechnology,
Forschungszentrum Karlsruhe, P.O.B. 3640, D-76021, Germany —
2 Department of Materials Science and Metallurgy, University of Cambridge,
Cambridge CB2 3QZ, U.K
Phase separation has recently seen much attention in explaining
nanocrystallisation in bulk metallic glasses. In the present paper, we explore
two bulk glass-forming systems, namely, Mg-Ni-Nd and Zr-Ta-Cu-
Ni-Al. Energy-filtered TEM shows that the alloy Mg65Ni20Nd15 phase-
separates into Ni-rich and Ni-poor liquids during quenching. However,
very surprisingly, annealing a phase-separated glass causes homogenisation,
instead of crystallisation. The nature of the miscibility gap is
considered. Secondly, a range of Zr-Ta-Cu-Ni-Al bulk glasses has been
cast. Based on preliminary results and thermodynamic considerations, it
appears that these alloys could be phase-separating prior to crystallisation.
The prospects of using phase separation for optimising mechanical
properties would be discussed.