Metal alloys nucleation

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... 

Martensitic traasfonnation Master ec[uations Mean field crossover to Ising Mechanical properties Metallic alloys Metallic glasses Metastable alloys Microhardness test Microscopic theory of nucleation... 

One-Step Activation Process. In a one-step activation process, the sensitizing and nucleating solutions are combined into one solution. It is assumed that when this solution is made up, it contains various Sn-Pd chloride complexes (24). These complexes may subsequently transform into colloidal particles of metallic Pd or a metallic alloy (Sn/Pd) to form a colloidal dispersion (19,28). This dispersion is unstable. It may be stabilized by addition of an excess of Sn ions. In this case, Pd particles adsorbed on the nonconductor surface are surrounded by Sn ions. The latter must be removed by solubilizing before electroless plating so that the catalytic Pd on the surface will become exposed, freely available, to subsequent plating. An example of such a solubilizing solution is a mixture of fluoroboric and oxalic acids in a dilute solution, or just plain NaOH or HCl. 

H.I. Aaronson and F.K. LeGoues. An assessment of studies on homogeneous diffusional nucleation kinetics in binary metallic alloys. Metall. Trans. A, 23(7) 1915—1945, 1992. 

There are four types of fundamental subjects involved in the process represented by Eq. (1.1) (1) metal-solution interface as the locus of the deposition process, (2) kinetics and mechanism of the deposition process, (3) nucleation and growth processes of the metal lattice (M a[tice), and (4) structure and properties of the deposits. The material in this book is arranged according to these four fundamental issues. We start by considering the basic components of an electrochemical cell for deposition in the first three chapters. Chapter 2 treats water and ionic solutions Chapter 3, metal and metal surfaces and Chapter 4, the metal-solution interface. In Chapter 5 we discuss the potential difference across an interface. Chapter 6 contains presentation of the kinetics and mechanisms of electrodeposition. Nucleation and growth of thin films and formation of the bulk phase are treated in Chapter 7. Electroless deposition and deposition by displacement are the subject of Chapters 8 and 9, respectively. Chapter 10 contains discussion on the effects of additives in the deposition and nucleation and growth processes. Simultaneous deposition of two or more metals, alloy deposition, is discussed in Chapter 11. The manner in which... 

Nucleation has a number of important practical consequences. In metallurgy, the rate of nucleation of molten metals and metal alloys affects the structural and mechanical properties of the solid metals that are formed upon casting. In the preparation of high-quality crystalline semiconductors through laser melting and resolidification the nucleation step affects the resulting microstructures. In the atmospheric sciences, nucleation of ice in clouds is a widely studied process, while biologists are interested in the ways in which certain plants appear to inhibit nucleation of ice from water and thus show increased resistance to cold. 

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. 

So far we focused primarily on nucleation in metals and metal alloys because that is the class of materials for which the most extensive information on nucleation is available. Another nucleation process that has been closely studied is the crystallization of ice from water and aqueous solutions. As we shall see, some new experimental techniques have been developed in this area that show promise of applicability to certain metal and semimetal systems as well. 

The first steps of alloy deposition differ from the nucleation of a pure metal. If one metal interacts stronger with the substrate, an upd layer formation or even a formation of several layers of a pure phase can precede the formation of alloy nuclei. Special models have been developed for the nucleation process. Mdchev and Lacmann and Milchev have discussed specific questions of alloy nucleation. 

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