ABOUT CONSTITUTIONAL
SUPERCOOLING
Supercooling, also known as undercooling, is the process of
lowering the temperature of a liquid or a gas below its freezing point without
it becoming a solid. Some of its applications involve refrigeration, organ
preservation etc.
Constitutional
supercooling, is a type of supercooling which occurs during solidification due
to compositional changes, and results in cooling a liquid below the freezing
point ahead of the solid–liquid interface. This effect occurs when the
temperature gradient in the liquid ahead of the interface is small or when
interface velocity is large. Constitutional supercooling leads to the formation
of dendritic structure during solidification of casting.
Fig 1.1 Typical casting grain structureOrder of supercooling:
Chill Zone > Columnar Grains Zone > Central Equiaxed
Chill zone has greater supercooling effect because it is
formed when molten metal comes in contact with the cold mould walls.
Fig 1.2 The origin of constitutional supercooling ahead of
a planar solidification front. (a) Composition profile across the solid/liquid
interface during steady-state solidification. The dashed line shows dXL/dx
at the S/L interface. (b) The temperature of the liquid ahead of the
solidification front follows line h. The equilibrium liquidus temperature for
the liquid adjacent to the interface varies as Te. Constitutional
supercooling arises when TL lies under the critical gradient.
We know that the diffusion of solute into the liquid during
solidification of an alloy is analogous to the conduction of latent heat into
the liquid during the solidification of a pure metal. Now observe the steady
state transformation as shown in fig 1.2(a), it shows the variation of
composition as a function of distance from the solid liquid interface. As a
result of varying composition ahead of the solid liquid interface there is a
variation in equilibrium solidification temperature i.e liquidus temperature as
given by line Te in fig 1.2(b). However, apart from the temperature
of the interface, which is fixed by local equilibrium requirements, the actual
temperature of the liquid can follow any line such as TL. At the
interface TL= Te = T3. If the
temperature gradient is less than the critical value shown in Fig 1.2(b) the
liquid in front of the solidification front exists below its equilibrium
freezing temperature, i.e. it is supercooled. Since the supercooling arises from
compositional, or constitutional effects it is known as constitutional
supercooling.
Microstructure Evolution
Fig 1.3 Constitutional supercooling: a) The phase diagram b) The
composition profile in liquid ahead of the interface, c) and d) The temperature
profiles ahead of the interface, and e) and f) the profiles of the interface.
Referring to the schematic phase diagram in fig 1.3 a) at
temperature T, liquid of composition CL is in equilibrium with solid
of composition CS as given by the tie line. During solidification if
the interface that separates the solid and liquid phase is at temperature T
then the liquid near it will be of composition CL which is greater
than the average composition Co of the melt.
Liquid composition decreases with the increase in distance
from the interface fig 1.3 b). With the help of the phase diagram the
equilibrium liquidus temperature can be plotted as a function of distance, fig
1.3 c) and d). In fig 1.3 c) the actual temperature gradient in the liquid
ahead of the interface is such that the liquid at every point is at the
temperature above the liquidus temperature for that point. Therefore, there is
no supercooling ahead of the interface, which is stable and flat, fig 1.3 e).
Any protrusion that will form is going to feel
superheated and will melt back. In fig 1.3 d) the actual
temperature profile in the liquid is such that the temperature at every point
between x and y is lower than the equilibrium liquidus temperature
corresponding to that point. Any protrusions formed in this region feels
supercooled and does not melt back.
This type of microstructural evolution leads to formation
of two types of microstructures.
1.
Cellular.
2.
Dendritic.
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