34
K. Adrjanowicz
2 Results and Discussion
2.1 The Effect of Cooling Rate versus Compression Rate
on the Crystallization Tendency of the Glass-forming
Liquids
When decreasing the temperature at constant pressure a liquid can either crystallize
or form a glass, depending on the applied cooling rate. Generally, crystallization
takes place when the rate of cooling is slow enough so that there is enough time for
the stable nuclei to form and grow into the macroscopic dimensions. On the other
hand, when the cooling rate surpasses the rates of nucleation and crystal growth,
crystallization is avoided. In such a case, a liquid can be supercooled and reach the
glassy state. The critical cooling rate needed to bypass crystallization is an individual
property of each system, meaning that 50 K/min might be far enough for a one glassformer, while not necessarily for the other one. For example, the critical rate needed
to vitrify metallic alloys can reach even 1000 K per 1 ms, while for some silica glasses
1–10 K/h is still relatively “fast” to form a glass, or ironically not slow enough to
form the crystal [68]. As a rule, the critical cooling rate necessary to inhibit the
crystallization depends on the location of the nucleation and growth rates maxima,
their intensity and the extent of overlap, as illustrated in Fig. 2.
For practical reasons, the crystallization/vitrification tendency of various systems
on cooling is often visualized in terms of the time-temperature-transformation (TTT)
diagram (see Fig. 5). TTT demonstrates the location of the crystallization zone with
respect to the temperature and the processing time. From the analysis of the TTT
curve, it is possible to determine the critical cooling rate necessary to avoid crystallization when lowering the temperature of the melt. As demonstrated in Fig. 5,
crossing the ‘nose’ area implies that a particular cooling rate may not be fast enough
to completely suppress the crystallization. In such a case, for the slowest processing
times, one may expect the extent of the crystalline fraction in the final product.
Compression of liquid at a fixed temperature, just like lowering the temperature
under isobaric condition, can be used to promote either crystallization or vitrification. Therefore, in analogy to temperature evolution of the nucleation and growth
rates curves, we can draw a very similar crystallization profile for a pressure case.
When pressurization starts above the melting point, a liquid overpass first via the
crystal growth region located at lower pressures. Then, with increasing the pressure the nucleation process becomes more favorable. Such evolution of the nucleation and crystal growth rates as a function of pressure is schematically presented
in Fig. 6. From the above scheme, one can get an impression that irrespectively
of the chosen thermodynamic variables, T or p, we end up with the same qualitative picture. However, then the question arises: can we use temperature and pressure interchangeably to tune the vitrification/crystallization ability of the molecular
systems?
K. Adrjanowicz
2 Results and Discussion
2.1 The Effect of Cooling Rate versus Compression Rate
on the Crystallization Tendency of the Glass-forming
Liquids
When decreasing the temperature at constant pressure a liquid can either crystallize
or form a glass, depending on the applied cooling rate. Generally, crystallization
takes place when the rate of cooling is slow enough so that there is enough time for
the stable nuclei to form and grow into the macroscopic dimensions. On the other
hand, when the cooling rate surpasses the rates of nucleation and crystal growth,
crystallization is avoided. In such a case, a liquid can be supercooled and reach the
glassy state. The critical cooling rate needed to bypass crystallization is an individual
property of each system, meaning that 50 K/min might be far enough for a one glassformer, while not necessarily for the other one. For example, the critical rate needed
to vitrify metallic alloys can reach even 1000 K per 1 ms, while for some silica glasses
1–10 K/h is still relatively “fast” to form a glass, or ironically not slow enough to
form the crystal [68]. As a rule, the critical cooling rate necessary to inhibit the
crystallization depends on the location of the nucleation and growth rates maxima,
their intensity and the extent of overlap, as illustrated in Fig. 2.
For practical reasons, the crystallization/vitrification tendency of various systems
on cooling is often visualized in terms of the time-temperature-transformation (TTT)
diagram (see Fig. 5). TTT demonstrates the location of the crystallization zone with
respect to the temperature and the processing time. From the analysis of the TTT
curve, it is possible to determine the critical cooling rate necessary to avoid crystallization when lowering the temperature of the melt. As demonstrated in Fig. 5,
crossing the ‘nose’ area implies that a particular cooling rate may not be fast enough
to completely suppress the crystallization. In such a case, for the slowest processing
times, one may expect the extent of the crystalline fraction in the final product.
Compression of liquid at a fixed temperature, just like lowering the temperature
under isobaric condition, can be used to promote either crystallization or vitrification. Therefore, in analogy to temperature evolution of the nucleation and growth
rates curves, we can draw a very similar crystallization profile for a pressure case.
When pressurization starts above the melting point, a liquid overpass first via the
crystal growth region located at lower pressures. Then, with increasing the pressure the nucleation process becomes more favorable. Such evolution of the nucleation and crystal growth rates as a function of pressure is schematically presented
in Fig. 6. From the above scheme, one can get an impression that irrespectively
of the chosen thermodynamic variables, T or p, we end up with the same qualitative picture. However, then the question arises: can we use temperature and pressure interchangeably to tune the vitrification/crystallization ability of the molecular
systems?