Thermodynamic and Growth Kinetic Modeling of Solidification From a Metastable Mushy-Zone
the ﬁdelity of modern casting simulation programs requires a detailed
understanding of how solidiﬁcation proceeds during the initial phase where rapid
heat extraction along exterior surfaces strongly inﬂuences local structure and
properties. The overarching goal of this research is to develop a better understanding
of stable-phase rapid solidiﬁcation from a metastable mush... read morey-zone and how it
relates to stable phase microstructures and material properties. With that goal in mind,
this research consists of 3 primary parts: thermophysical property measurements,
thermodynamic modeling, and growth kinetics analyses. Electrostatic levitation (ESL)
experiments were conducted at NASA Marshall Space-Flight Center (MSFC) to deﬁne
stable and metastable phase thermodynamic properties for the Fe-Co alloy system. These
properties were used to optimize the equilibrium phase diagram by investigating the
extension of the metastable liquidus and solidus to allow prediction of the
BCC-FCC-liquid peritectic, deﬁne the driving potential for solidiﬁcation
in undercooling experiments, and identify the partitioning coeﬃcient for each
phase. Growth kinetics were measured using electromagnetic levitation (EML) techniques.
The velocity of the stable phase growing through the mushy zone was found to be greater
than that of the stable phase growing through undercooled liquid, and the velocity was
found to be constant, regardless of the initial undercooling. This is explained, in
part, by an eﬀective change in the heat capacity of the growth environment. This
eﬀective heat capacity considers remelting of the of the metastable phase in
which case the pre-existing BCC δ-phase acts as a heat sink for the stable FCC
γ-phase to release energy during solidiﬁcation. The second matter is the
possibility that heat is conducted back from the tip of the growing dendrite such that
conditions at the tip become non-adiabatic. These two considerations, used in
conjunction with existing dendrite growth models, lead to a new "Non-adiabatic Remelt"
growth model. The results show that for a given heat ﬂux, there will be a minimum
undercooling for which dendritic growth can be supported. The predicted growth velocity,
which corresponds to that minimum undercooling, matches the measured experimental growth
velocity data. This suggests that the growth of the stable phase into the mushy zone
occurs under the minimum conditions required to support dendritic
Thesis (Ph.D.)--Tufts University, 2016.
Submitted to the Dept. of Mechanical Engineering.
Advisor: Douglas Matson.
Committee: Luisa Chiesa, Robert Hyers, and Mary Shultz.
Keyword: Materials Science.read less