A system is at equilibrium when it reaches a condition such that the state of the
system no longer changes unless acted upon by some external phenomenon.
equilibrium condition is seldom reached.
What then is the point in studying equilibrium
Equilibrium phase diagrams give insight to the tendency of the system.
Information for systems that approach equilibrium can be approximated from the phase
For systems removed from equilibrium conditions, phase diagrams show
where the system ‘wants’ to be, and how far from that condition it is.
Read sections 9.1 – 9.17, and 12.7 from your text book (Callister, 7
should be review to you, but if it is not, read it carefully and work through the examples.
The information and exercises will help you in many of your classes.
At first glance, phase diagrams may appear to be theoretical derivations.
have seen how free energy curves may be used to determine phase diagrams, however
this is rarely, if ever, done in practice.
Nearly all phase diagrams are developed
empirically, and those that are not, have to be verified through experimentation.
Several techniques exist to determine phase diagrams.
Thermal analysis is one of
the simplest and most widely used techniques, and it is the focus of this lab, so this
technique will be described last.
Other techniques include direct observation, dip
sampling, fracture at liquation, dilatometry, metallographic examination, x-ray
diffraction, and electrical resistivity.
The direct observation technique involves heating or cooling a sample, and
watching for melting or solidification.
This technique is suitable for classroom
demonstrations, but is plagued with experimental error.
The instant or occurrence of a
transformation is difficult to pinpoint accurately with optical observations, which in turn
will result in erred transformation temperatures.
Dip sampling is one of the oldest techniques.
Liquidus and solidus points are
determined by holding a sample at a fixed temperature in the liquid plus solid range until
equilibrium is established.
Then the phases are separated by canting and/or filtering, and
compositions are analyzed.
This technique is difficult in many systems due to elevated
temperatures and dendritic formation.
Fracture at liquation is useful in determining solidus curves and eutectic points.