Amorphous alloys time-temperature-transformation

A molten metal alloy would normally be expected to crystallize into one or several phases. To form an amorphous, ie, glassy metal alloy from the Hquid state means that the crystallization step must be avoided during solidification. This can be understood by considering a time—temperature—transformation (TTT) diagram (Eig. 2). Nucleating phases require an iacubation time to assemble atoms through a statistical process iato the correct crystal stmcture... 

For a number of applications, particularly those associated with conditions of continuous cooling or heating, equilibrium is clearly never approached and calculations must be modified to take kinetic factors into account. For example, solidification rarely occurs via equilibrium, amorphous phases are formed by a variety of non-equilibrium processing routes and in solid-state transformations in low-alloy steels much work is done to understand time-temperature-transformation diagrams which are non-equilibrium in nature. The next chapter shows how CALPHAD methods can be extended to such cases. 

A disadvantage is the metastable character of the amorphous alloys. As a function of temperature and time amorphous alloys tend to transform into the corresponding crystalline phases. This will be discussed in more detail in section 6. 

The formation enthalpy AHf of amorphous alloys is less negative than that of crystalline materials of similar composition, which means that the former alloys are metastable. As a function of temperature and time the amorphous alloys will therefore transform into the stable crystalline phases. This transformation can conveniently be studied by means of diffraction methods. As will be discussed later on, no sharp diffraction lines occur in the diffraction diagrams of amorphous alloys. The transformation into the crystalline state is generally accompanied by the occurrence of sharp diffraction peaks. In some cases the stable crystalline phases are not reached directly. First one or more metastable crystalline phases may be formed which transform into the stable end products at a later stage of the crystalUzation process. 

A disadvantage of amorphous alloys is their metastable character which makes them transform into the stable crystalline state as a function of temperature and time. In calorimetric experiments the amorphous-to-crystalline transition is revealed by an exothermic heat effect. Typical traces obtained using a differential scanning calorimeter are shown for amorphous Gd064Co0 36 in fig. 51. The dependence of the crystallization temperature Tx on the heating rate s implies that there is a risk of crystallization taking place even at room temperature after an extended period (s - 0). This is particularly likely when Tx is rather low, and it may have consequences for practical applications. 

Phys. Chem. (Engl. Transl), 61(7), 938-941 (1987) (Caleulation, Thermodyn., 20) [1988Cun] Cunat, Ch., A New Concept of Effective Temperature to Correlate Isothermal and Ani-sothermal Continuous Temperature-Time-Transformation Curves-Application to Crystallization of Fe-Ni-B Amorphous Alloys , Mater. Sci. Eng., 97, 297-300 (1988) (Theory, Phase Diagram, 6)... 

In an amorphous material, the alloy, when heated to a constant isothermal temperature and maintained there, shows a dsc trace as in Figure 10 (74). This trace is not a characteristic of microcrystalline growth, but rather can be well described by an isothermal nucleation and growth process based on the Johnson-Mehl-Avrami (JMA) transformation theory (75). The transformed volume fraction at time /can be written as... 

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