Metal alloys nucleation temperature

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... 

Perepezko and coworkers have carried out an extensive series of measurements of nucleation in low-melting metal alloys, in most cases obtaining undercooling temperatures over a substantial range of compositions. In this way they have studied the alloy systems Sn-Pb, Cd-Pb, Sb-Pb 48.53-54 Cu-Pb, Pb-Bi,-" Sn-Bi,- 8 Ga-Bi, i Cd-Bi, Cd-Sb,-" and Cu-Te. Earlier work on the nucleation of alloys includes that of Cech and Turnbull on Cu-Ni and of Cech on Fe-Ni. These different studies encompass eutectic, peritectic, and monotectic binary crystallization and provide a rich variety of data for interpretation. 

It is important to realize that corrosion rates may be controlled by any of several thermodynamic or kinetic properties of the alloy-scale-environment system and not just by surface or interface reactions. The three stages of high temperature oxidation of a metal, shown schematically in Fig. 1, serve as an example (7). The first or transient stage includes initial gas adsorption, two-dimensional oxide nucleation, initial three-dimensional oxide formation and finally, formation of the dominant oxide that will control the oxidation rate in Stage II. Various portions of Stage I have been widely studied using surface analytical techniques, but its duration can be very short and it is usually assumed (not always correctly) that Stage I has little impact on ultimate corrosion properties of the material. 

It is possible to create more complex compounds and morphologies, such as alloys or core-shell structures. The nucleation step still occurs with injection of the metal carbonyl. However, when other organometallic precursors are in the solution, you can get the alloy to grow on the nuclei through transmetallation. To prepare core-shell material, the nuclei are allowed to react, then the temperature is reduced and a second precursor is introduced. The initial injection provided the nucleation which the additional materials when decomposed grow on the initial nuclei. 

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