Metastable Phases
The situation that, during nucleation of a new phase, the sum of the Gibbs energy terms, ΔG, contains terms with different signs, creates the possibility of the formation of metastable phases. This possibility will be explained based on Fig. 8.1.
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Metastable Phases
8.1
Energetics ofthe Nucleation of Metastable Crystalline Phases
The situation that, during nucleation of a new phase, the sum of the Gibbs energy terms, l1G, contains terms with different signs, creates the possibility of the formation of metastable phases. This possibility will be explained based on Fig. 8.1. In Fig. 8.1 the Gibbs energies of the starting phase x:'(Y).AGYis negative for x < >x:,(y). For the nucleation of the p-phase another driving force is present: AG~ is negative for XB < x:,(~) . Alloys with compositions x:,(~) 10 12 Pa · s, this possibility does not exist any more due to the low mobility. The atomic arrangement existing at TG is frozen. An essential reason for the change in thermodynamic properties at ~ consists in the fact, that below this temperature only one single configuration of the atoms occurs, whereas above ~ the building blocks of the undercooled melt can be arranged in a large number of different ways. Whereas the undercooled liquid is in a metastable equilibrium relative to the stable crystalline situation, in which internal equilibrium can occur, the glass is in a metastable equilibrium which is not in internal equilibrium. The glass forming temperature TG depends on the cooling rate. TG is higher with rapid cooling rates than with slow ones. In each case a different structure is frozen in. Transformations can occur to a limited extent even below TG, to optimize the structural and bonding relationships (relaxation processes). Figure 8.19 represents schematically the heat content of a glass forming substance as a function of temperature. The heat content of the glass is higher than the values obtained by extrapolation of the data of the metastable liquid (above TG).
Fig. 8.19 Schematic presentation of the heat content of a silicate as a function of temperature. a stable melt, b, d supercooled melt, c glass, e crystalline solid, ~HF enthalpy of melting
TG
Temperature
TF
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8 Metastable Phases
The glass forming temperature TG, appearing at a given cooling rate, is fixed by many factors, whose type and importance can be very different from case to case. Thus, glass formation can occur in very different types of substances. It is often limited to a narrow composition range in multi-component solutions. One must mention that glasses, - in addition to the already mentioned silicates - can be formed in borates and phosphates. Also, simple salts such as ZnCI2, can solidify as glasses. For example in the system KNO r Ca(N03h glass formation is found in the region between 40 and 60 wt.-% Ca(N0 3h, where the eutectic composition of this solution is. In addition many organic substances, for example alcohols, with long molecular chains, and many other can form glasses. Glasses are found even in metallic systems, which will be discussed later. Common to all glass forming systems is a strong interaction between dissimilar atoms, resp. a blocking structure of the liquid. Easy nucleation is thus, hindered. A region of the composition range near the eutectic point i
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