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p, c
p, c
1 1
II
1 2
Figure 2.1: Phase diagram redrawn from [10]. The line in the p-T diagram (pressure,
temperature) gives the coexistence line for phases I and II. In multi-component systems
the concentration c is plotted instead of the pressure p. Point ’1’ can be reached by crossing
the coexistence line at point ’2’ by decreasing the temperature at constant pressure p1 (or
concentration c1) which is referred to as ’supercooling’. Or by crossing the coexistence line
at point ’3’ by increasing the pressure or concentration at constant temperature T1 which
is referred to as ’supersaturation’.
systems, e.g. formation of alloys and thirdly transition between different crystal-
lographic orderings.
Fig. 2.1 illustrates the general principle of phase transformation. Here, the ex-
istence of two phases (I and II) in a pressure-temperature diagram (p-T). The
straight line denotes the coexistence of the two phases. Along this line both phases
are in thermodynamic equilibrium and coexist, e.g. ice floats on water without fur-
ther freezing or melting. In multi-component systems the concentration c instead
of the pressure can be plotted. Upon passing the coexistence line the phase change
does not take place immediately but at a certain rate. This rate is determined by
the kinetics of phase change. Reaching ’1’ via ’2’ , i.e. by decreasing the tempera-
ture at constant pressure p1, the process is called supercooling; reaching ’1’ via ’3’,
i.e. by increasing pressure or concentration at constant temperature T1, the pro-
3
T 1
I
T