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NanoMat 2018 Nano Research & Applications
ISSN: 2471-9838 Page 82
April 26-27, 2018
Rome, Italy
17th Edition of International Conference on
Emerging Trends in
Materials Science and
Nanotechnology
An undercooled melt possesses an enhanced free enthalpy
that gives access to crystallize metastable solids. Crystal
nucleation selects the crystallographic phase whereas the
subsequent crystal growth controls the microstructure evolution.
Electromagnetic and electrostatic levitation techniques are
very efficient to produce a highly undercooled melt since
heterogeneousnucleationoncontainer-wallsisavoided.Moreover,
a freely suspended drop is accessible for in situ observation of
crystallization far away from equilibrium. We combine levitation
technique with the diagnostic means of neutron scattering to
investigate short range order in undercooled melts and energy
dispersive x-ray diffraction of synchrotron radiation to observe
phase selection processes upon undercooling. Measurements of
the statistics of nucleation undercooling are performed in order
to study the physical nature of crystal nucleation. Nucleation
is followed by crystal growth. In undercooled melts the crystal
grows with dendritic morphology since a planar interface is
destabilized by the negative temperature gradient ahead the
solid liquid interface. In highly undercooled melts dendrites
propagate very rapidly. A high speed camera is used to record
the advancement of the solidification front. Dendrite growth
velocities are measured as a function of undercooling of pure
metals, solid solutions and intermetallics. Non-equilibrium
crystallization effects are evidenced. Crystal growth is governed
by heat andmass transport. To explore the influence of convection
on dendrite growth comparative experiments in microgravity
are performed using an electromagnetic levitator on board the
International Space Station. Metals show dendritic growth in
a mesoscopic scale with a rough interface at the microscopic
scale. In case of semiconductor, the solidification front is facetted
in a mesoscopic scale with a smooth interface in a microscopic
scale. The entropy of fusion of the compound Ni2B is located in
between that of metals and semiconductors. A transition from
dendritic to facetted growth is observed induced by convection in
the undercooled drops.
Containerless undercooling and solidification of metals and alloys
Dieter Herlach
Ruhr-Universität Bochum, Germany
Nano Res Appl, Volume:4
DOI: 10.21767/2471-9838-C1-009