Formation of feathery grains with the application of a static
magnetic field during direct chill casting of Al-9.8wt%Zn alloy
Lei Li Æ Yudong Zhang Æ Claude Esling Æ
Zhihao Zhao Æ Yubo Zuo Æ Haitao Zhang Æ
Jianzhong Cui
Received: 1 September 2008 / Accepted: 3 December 2008 / Published online: 14 January 2009
Ó Springer Science+Business Media, LLC 2009
Abstract Effect of a 0.2-T static magnetic field on the
microstructure of a direct chill cast Al-9.8wt%Zn alloy slab
was investigated. The static magnetic field transferred the
microstructure from a mixture of equiaxed and columnar
grains with the primary trunks growing in \100[ direc-
tions to twinned lamellar feathery grains with the primary
and secondary arms growing in \110[ directions. The
application of the static magnetic field results in the
reduction of the heat discharge and solute mixing capacity
through a damping effect on convection and thus a delay of
the melt transformation to solid and a request to reduce the
liquid/solid interface energy through reducing the interface
area due to the loss of undercooling. The delay and the
request account for the growth direction change and the
formation of lamellas. The difference between the Al and
Zn atomic radii and the related incoming flow facilitate the
formation of the twins.
Introduction
Feathery grains, as specific and undesirable microstructure,
are occasionally spotted in aluminum alloys under certain
conditions during direct chill (DC) casting. Several efforts
have been made to reveal the crystallographic character-
istics and the formation conditions of this microstructure. It
was found [1, 2] that they are the result of a \110[ den-
drite growth with each primary trunk split by a twin plane,
showing a transition of the growth direction from the usual
\100[ to \110[. The twin plane is associated with
stacking faults which are favored by a strong shearing rate
related to convection, while the transition of the growth
direction is induced by solute additions and/or solidifica-
tion conditions (thermal gradient, growth rate, and melt
convection). It was further clarified that under forced flow
conditions the feathery growth is an intermediate case
between equiaxed and columnar growth and similar to the
growth of \100[ regular dendrites with the competitive
branching–twinning mechanisms [3]. Despite many years
of investigations [1–6], the formation mechanism of
feathery grains is still not clearly understood and further
investigation is required.
Recently, the application of electromagnetic fields to
material processing, in the field of solid-state phase trans-
formation [7–11] and solidification [12–17], has been
regarded as a new topic. As a unique means, introducing an
external static magnetic field to a DC casting may offer a
new chance to gain new insights into the formation
mechanism of the feathery grains. Since a static magnetic
L. Li Á Y. Zhang Á Z. Zhao Á Y. Zuo Á H. Zhang Á J. Cui
Key Laboratory of Electromagnetic Processing of Materials,
Ministry of Education, Northeastern University,
Shenyang 110004, People’s Republic of China
e-mail: li_lei@live.com
Y. Zhang
e-mail: yudong.zhang@univ-metz.fr
Z. Zhao
e-mail: zzh@epm.neu.edu.cn
Y. Zuo
e-mail: zuoyubo@epm.neu.edu.cn
H. Zhang
e-mail: haitao_zhang@epm.neu.edu.cn
J. Cui
e-mail: jzcui@epm.neu.edu.cn
L. Li Á Y. Zhang Á C. Esling (&)
LETAM, CNRS-UMR 7078, University of Metz, Ile du Saulcy,
57045 Metz, France
e-mail: claude.esling@univ-metz.fr
123
J Mater Sci (2009) 44:1063–1068
DOI 10.1007/s10853-008-3158-0