Diffusion alloy formation

An example where one metal melts before the densihcation process, is the formation of bronze from a 90 10 weight percentage mixture of copper and tin. The tin melts at a temperature of 505 K, and the liquid immediately wets the copper particles, leaving voids in the compact. The tin then diffuses into the copper particles, leaving further voids due to dre Kirkendall effect. The compact is therefore seen to swell before the hnal sintering temperature of 1080 K is reached. After a period of homogenization dictated by tire criterion above, the alloy shrinks on cooling to leave a net dilatation on alloy formation of about 1%. 

Hot Dipped Coatings Major attempts have been made to improve the quality of aluminised steel strip. Requirements on coating thickness and uniformity have been imposed. It is the speed of sheet movement, length of path in the molten bath, temperature and composition of the bath that control the thickness of the intermetallic layer which lies below the aluminium outer surface. The process of intermetallic alloy formation is diffusion controlled, and it is usual that some dissolution of iron into the molten aluminium does occur at a rate, Ac/At, which is given by... 

Several groups have ascribed this irreversible Cd stripping to the formation of an alloy with the Au [136-138], as does this author. If UPD can be thought to result from the energetics of compound formation, it may also result from alloy formation. Evidently, the Cd atoms are able to diffuse into the Au, exposing more surface Au atoms, to react with more Cd. Subsequent stripping then requires the Cd atoms to diffuse to the surface before they can oxidize, resulting in the observed irreversibility in the voltammetry [138]. 

Cathodic deposition of magnesium from various chloride melts on different substrates has been studied by several authors [288-290], In dilute solutions of Mg(II) species the cathode process has been found to be controlled by diffusion of the reactant. Alloy formation has been observed on platinum, as reported by Tunold [288] and Duan et al. [290], The rate constant of the charge transfer process on a Mg/Ni electrode in molten NaCl-CaCl2-MgCl2 was reported by Tunold to have a value of about 0.01 cm s 1. This author also reported underpotential deposition of a monolayer on iron electrodes, at potentials approximately 100 mV positive to the Mg deposition potential. 

In the metal industry, radionuclides may be used to study diffusion and formation of alloys. Corrosion and wear can also be studied with high sensitivity. An example is the investigation of the wear of piston rings in motors after activation of the... 

D Me-S surface alloy and/or 3D Me-S bulk alloy formation and dissolution (eq. (3.83)) is considered as either a heterogeneous chemical reaction (site exchange) or a mass transport process (solid state mutual diffusion of Me and S). In site exchange models, the usual rate equations for the kinetics of heterogeneous reactions of first order (with respect to the species Me in Meads and Me t-S>>) are applied. In solid state diffusion models, Pick s second law and defined boundary conditions must be solved using Laplace transformation. 

It is plausible that the formation of 2D Me-S surface alloy as well as the initial stage of 3D Me-S bulk alloy formation is better described by a site exchange model than by a diffusion model. More advanced stages of 3D Me-S bulk alloy formation are usually characterized by a parabolic rate law and well described by a diffusion model. 

The results strongly depend on the crystallographic orientation of the substrate and on the crystal imperfection density. The time-dependence of 3D Me-S bulk alloy formation obeys a parabolic rate law (Fig. 3.65) as found for many other systems. The results were discussed in terms of a semiinfinite-linear diffusion model assuming mutual diffusion of Me and S and reversible 2D Meads overlayer formation. The following time-dependence of q(,AE,i) was derived... 

Different authors have shown that compositionally modulated 3D metallic alloys can be electrochemically deposited using cyclic polarization conditions and multicomponent electrolytes [6.108, 6.109, 6.113, 6.114]. Alloy formation takes place alternately by diffusion and charge transfer control of the deposition reactions of different metal components. Similar conditions have been used for the deposition of ultrathin metal films and heterostructures (cf. Section 6.4). 

In Fig. 6.21, the solid state diffusion-controlled kinetics of the alloy formation process in the multicomponent system k x hkt)/Pig, Cd " is compared with those in the one-component systems Au( 0/Cd and PigQikt)/Cd. ... 

Section 19.5 deals with the development of novel metal ammine systems and focuses on the design of superior metal ammines by closely integrating experimental and calculational work. From a detailed understanding of structure and stability, porosity and particle size, desorption and diffusion, and alloy formation, it is possible to engineer these materials on the nano-, micro-, and macro-scales. 

Ma, Akis, Ayturk, Guazzone, EngwaU, MardUovich. Characterization of intermetallic diffusion barrier and alloy formation for pd/cu and pd/ag porous stainless steel composite membranes. Ind Eng Chem Res. 2004 43 2936 5. 

SIMS measurements on the precursor confirm intermixing of the layers occurs at room temperature (Figure 1.25). In particular, the Cu signal is clearly seen in both the Sn and the Zn phases. The majority of the Zn is still confined to the top of the precursor, while the Sn is distributed towards the bottom, suggesting it is mainly diffusion of Cu that is responsible for alloy formation. 

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