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In this video we look at our phase diagram
completely miscible metals and so it's
a relatively
simple phase diagram compared to the ones we can have when we have partial miscibility
but it'll help to describe the behavior
and the properties of the system. So what we're looking at
just a hand-drawn view. So these should be
of course smooth curves where temperatures plotted versus the mole fraction of
bismuth vs. high-temperature. This region is all liquid and low temperature
this is just a solid solution
and then between this region here is liquid plus
solid and so we're going to talk about number of aspects of this
diagram first let's look at this point corresponds to the melting temperature
a solidification temperature for bismuth and then likewise
this point is the melting temperature for antimony
What you notice then for the in between region is
that the melting temperature depends
on the composition of the alloy composition solid or to solidification
temperature
a composition of the liquid and further that the
melting or solidification depending on which direction we're going.
for mixtures now takes place over temperature range whereas for pure
material of course it melts at just one temperatures. So let's look at the case where
we start with liquid that's at 600 degrees centigrade
and it mole fraction of bismuth is 0.6 and
as we lower the temperature when we get to this point we start forming solid so
a couple of things that are important to note
The solid that forms has to be at the same temperatures
and the composition of the solid is quite different from the compositional liquids
so we have liquid and solid for equilibrium
the liquid composition does not equal
the solid composition. In a solid in equilibrium they have
different compositions so for this particular example
we had x of the liquid was 0.6
and the mole fraction of bismuth in the solid
is about 0.2 so quite a big difference
Now as we continue to remove energy and form more solid and we move down
along this line for the overall composition we have this liquid
composition
and this solid composition. So both
liquid and solid compositions increase
as the temperature decrease. Eventually course we're gonna have all solid
and then we're back to the same composition so
when we are in between here how much solid and
how much liquid we have is determined by the lever rule which is described in
another videos. So let's look what happens in the phases. We start out the yellow
represent
all liquid. So we get to this point we going to now have
small amounts of solid dispersed in the liquid. it's still mostly liquid
and as we move further down for example to this point
so as we move to this point here we're going to have more solid as we get very close
then the solid pieces are bigger
a much larger fraction a solid a much smaller
fractions liquid and when we get to where we have also solid
we get to the solid-phase now these particles of course
the solid have gotten bigger and we will have grain so we'll have
solid phase with the black lines indicating
the grain boundaries between the solids. This is of course assuming
you know we cool this slow enough that we stay in equilibrium
and then our composition of course the solid is the same as the composition for
our start. So the question might want to ask is why
are these two materials miscible and a lot of it has to do with
they have really similar properties and so if we look up
the crystal structure Rhombohedral crystal structures so they have the same
crystal structure
they also have the same electronegativity
1.9 and they have similar atomic radius so that they can
pack together into this crystal structure not identical
but similar enough that they are miscible as a result of
their properties being similar. So this is the
simplest type of solid-liquid
phase diagram that we'll see where we have one liquid if solid phase we
have no
chemical compounds no one usual behavior.