For those interested in the subject :
A crystal dendrite is a crystal that develops with a typical multi-branching
tree-like form. Dendritic crystal growth is very common and illustrated by
snowflake formation and frost patterns on a window. In metals, dendrites are
growing during solidification. The Computational Materials Laboratory,
directed by Prof. M. Rappaz, has demonstrated that the range of possible
dendrite growth directions in Al-Zn alloys is much richer than previously
anticipated. A paper has been published in Nature materials.
During normal solidification of metallic alloys such as steels, nickel based
super-alloys or aluminium alloys, dendrites are growing like in snowflakes.
It is favourable for the solid to grow according to this dendritic
morphology to allow the diffusion of the chemical elements initially present
in the liquid metal and rejected by the newly formed solid (water diffusion
in the air during snowflakes formation). These growth morphologies are
taking into account the surface tension between the solid and the liquid.
From those two antagonist forces, namely the diffusion and the capillary
forces, the nature chooses an optimum in the micrometer range, hence the
name "microstructure". Primary dendrite growth directions can vary between
different crytallogaphic directions, as a function of the crystalline
anisotropy parameters, themselves function of composition. In cubic
crystals, growth directions will be the main axis of the crystal, actually
<100> directions.
Aluminium is characterized by a low anisotropy of interfacial energy.
Recently, it has been discovered that alloys elements could change this
anisotropy and then, change dendrites growth directions. For example,
aluminium-zinc system, which constitutes the base of 7000 aluminium alloys,
frequently used in aeronautics or as coatings on steel sheets, present
interesting features : on one hand, zinc has an hexagonal structure with a
strong anisotropy, on the other hand, he can be incorporated in aluminium up
to 95 wt% while keeping a cubic structure. A recent collaboration between
the Computational Materials Laboratory (EPFL) and the Northeastern
University (Boston), demonstrates that the growth directions of aluminium
can vary progressively from <100> at low Zinc content to <110> near the
eutectic composition. This transition, called Dendrite Orientation
Transition (DOT), generates unusual structures, called "seaweeds".
27.10.2006
More information:
Links Computational Materials Laboratory (LSMX)
Contacts Prof. Michel Rappaz
Documents
Orientation selection in dendritic evolution (pdf)
http://newsletter.epfl.ch/sti/index.php?module=Newspaper&func=viewarticle&np
_id=182&np_eid=24&catid=22
Meilleures Salutations
Very Best Regards
Roland
www.PCBspecialist.com
Roland Jaquet
14 Champ Budin
1258 Perly, GENEVA
SWTZERLAND
TEL+41-22-880-0405 GSM+41-79-203-3723
There are three constant in life... Change, Choice & Principles.
Stephen R. Covey
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