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%. 

Some metals are soluble as atomic species in molten silicates, the most quantitative studies having been made with Ca0-Si02-Al203(37, 26, 27 mole per cent respectively). The results at 1800 K gave solubilities of 0.055, 0.16, 0.001 and 0.101 for the pure metals Cu, Ag, Au and Pb. When these metal solubilities were compared for metal alloys which produced 1 mm Hg pressure of each of these elements at this temperature, it was found drat the solubility decreases as the atomic radius increases, i.e. when die difference in vapour pressure of die pure metals is removed by alloy formation. If the solution was subjected to a temperature cycle of about 20 K around the control temperamre, the copper solution precipitated copper particles which grew with time. Thus the liquid metal drops, once precipitated, remained stable thereafter. 

The quantity v(c) and the disordered-alloy formation energy (mixing energy)... 

Alloy formation is faster on copper than on steel and tends to be more irregular. With coating thicknesses of 1 /im or less, the thickness of the alloy layer may amount to half that of the complete coating. It consists of a thin layer of CujSn adjacent to the copper and a thicker layer of Cu Snj adjacent to the tin. 

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... 

Sprayed zinc coalings Details of the method and the nature of the coatings are given in Section 12.4. In this method there is no alloy formation and the bond is primarily mechanical. Although porous, the coating is protective partly due to its sacrificial action and partly due to the zinc corrosion products which soon block up the pores, stifling further attack. 

These compounds may reduce the reactivity of lithium and make the lithium deposition morphology smoother as a result of the spontaneous electrochemical alloy formation during the charging of lithium on the anode. The lithium was plated on... 

Scheme 5.1. Alloy formation and segregation in bimetallic systems with one of the metals present as a minority. The scheme qualitatively predicts whether two elements form a surface alloy or a solid solution. The results are valid in vacuum. As soon as an adsorbing gas is...

The basic information is the fact that formation of PC/Cu alloys is rather exothermic (15) so that one would expect more pronounced changes in the electronic structure of Pt and Cu, than with, e.g. Pt/Au, Pt/Ag, (endothermic alloy formation) or Pt/Re, Pd/Ag, Pd/Au (moderately exothermic). 

For a range of simple substitutional solid solutions to form, certain requirements must be met. First, the ions that replace each other must be isovalent. If this were not the case, other structural changes (e.g., vacancies or interstitials) would be required to maintain electroneutrality. Second, the ions that replace each other must be fairly similar in size. From a review of the experimental results on metal alloy formation, it has been suggested that 15% size difference can be tolerated for the formation of a substantial range of substitutional solid solutions. For solid solutions in nomnetal-lic systems, the limiting difference in size appears to be somewhat larger than 15%, although it is very difficult to quantify this. To a certain extent, this is because it is difficult to quantify the sizes of the ions themselves, but also because solid solution formation is very temperature dependent. 

The synergistic elfect seen in Pt-Rn alloys has aronsed great interest, since it opened perspectives for their nse in efficient methanol fnel cells. Many studies were performed to elucidate the origins of this effect. Some workers believe that it is due to changes in the electron strnctnre of platinnm npon alloy formation with ruthenium. A popular interpretation is the bifunctional mechanism, according to which the organic species are preferentially chemisorbed on platinnm sites while the ruthenium sites facilitate the adsorption of the species needed for oxidation of the orgaiuc species. 

Figure 15. Ag/Au alloy formation by digestive ripening of Au and Ag monodispersed particles followed by UV-Vis. (Reprinted with permission from Ref [59], 2006, American Chemical Society.)...

Linear absorption measurements can therefore give the first indication of possible alloy formation. Nevertheless, in systems containing transition metals (Pd-Ag, Co-Ni,. ..) such a simple technique is no longer effective as interband transitions completely mask the SPR peak, resulting in a structurless absorption, which hinders any unambiguous identification of the alloy. In such cases, one has to rely on structural techniques like TEM (selected-area electron diffraction, SAED and energy-dispersive X-ray spectroscopy, EDS) or EXAFS (extended X-ray absorption fine structure) to establish alloy formation. 

To be more confident on alloy formation, we performed EDS compositional analysis with an electron probe size of 1 nm FWHM focused on isolated clusters as expected for bimetallic clusters, Au and Fe signals are together present in the spectrum. The measured Au/Fe atomic ratio on the largest clusters is about 1.6 0.2, indicating an Au-enriched alloy. Moreover also preliminary EXAFS spectroscopy on the sample evidenced a clear Au-Fe correlation, unambiguously confirming alloy... 

The ESCA data does not allow one to distinguish between a Pd°/Sn° mixture and a trae Pd-Sn alloy with these bimetallic catalysts at low Sn loadings, so alloy formation is possible but this possibility caimot be verified with ESCA. 

At the m.p. of aluminium (600°C) an aluminium-sheathed palladium thermocouple formed an alloy with a flash and an exotherm to 2800°C [1]. The use of thin layers of palladium or platinum on aluminium foil or wire as igniters derives from the intense heat of alloy formation, which is sufficient to melt the intermetallic compounds [2],... 

Alloy formation between the powdered metals may be violent Silver fluoride... 

The above work suggests that various factors might determine the extent of alloying and that the result would be specific to the experimental arrangements adopted, e.g., the amount of radiant heat will vary. Therefore, it seems unwise to rely on alloy formation during the deposition of the second layer and, in fact, Cu-Ni films prepared for surface studies by... 

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