Preparation of amorphous alloys

Owing to their numerous actual and potential applications, several ternary and complex systems of these metals, especially of aluminium, have been investigated a few examples of the systematics of Al-Me-X alloys are presented in 5.18 and in Fig. 5.41. Recent contributions to this subject have been given with the study of the systems R-Al-Cu (Riani et al. 2005, and references there in). These rare earth alloys, characterized by the formation of several intermediate phases, are interesting also as raw materials for the preparation of amorphous alloys. Regularities in the trends of their properties have been underlined. The experimental and calculated data relevant to the binary systems Al-Fe, Al-Ni and Fe-Ni have been examined and discussed in a paper concerning the assessment of the ternary Al-Fe-Ni system (Eleno et al. 2006). 

The equipment needed in flash evaporation is comparatively modest, and this method is also suitable for the preparation of amorphous alloy films whose constituents have different vapour pressures. It consists of a single heated filament usually made of molybdenum. Powder of the alloy is fed continuously onto the filament, the temperature of which is sufficiently high for evaporation. No shifts in composition of the alloy occur since all the material is evaporated to completion. Devices for monitoring the vapour flux and the source temperature are not needed. The method is restricted to materials that can be obtained in powdered form. 

Fig, 6,18. Preparation of amorphous alloys by the melt-spinning method. 

The oldest preparation of amorphous alloys goes back to the 1930s and was based on the vapour quenching method (Kramer, 1934, 1937). For completeness, however, it should be mentioned here that the preparation of amorphous alloys may actually be much older. In the first part of the 18th century, amorphous alloys had very probably been prepared by Wiirtz, using the electrodeless method in which Ni alloy deposits were obtained by decomposition of nickel hypophosphide. Since scientists in that period still lacked X-ray diffraction facilities, a proper characterization of the deposited material by means of X-ray diffraction was not yet possible. 

Several techniques have been developed to produce metastable amorphous alloys (11). Routes to amorphous alloys include the rapid cooling of molten alloys, referred to as splat cooling or melt spinning, the codeposition of the respective elements, and low temperature solid state amorphization reactions. All of these techniques are based upon limiting the opportunities for the system to nucleate. The important energies in this situation are that required nucleation, and that required for diffusion. Time is also important, as local rearrangements to form nuclei are limited by the diffusion rates. Each of these techniques has drawbacks for the general preparation of amorphous alloys. 

Preparation of amorphous products The preparation of metals (Fe, Co, Ni, Pd, Au), alloys (Au-Pd, Fe-Co, etc.), oxides, chalcogenides, etc. has been reported. The synthesis of sulphides, for instance, has been obtained in solutions (in water, ethanol, etc.) of the metal chloride or acetate using thioacetamide or thiourea as sulphur precursor. 

A common observation in most cases is that the surface of amorphous alloys, especially those containing Ti, Zr and Mo, is largely covered with inactive oxides which impart low electrocatalytic properties to the material as prepared [562, 569, 575], Activation is achieved by removing these oxides either by prepolarization or, more commonly and most efficiently, by leaching in HF [89, 152, 576]. Removal of the passive layer results in a striking enhancement of the electrocatalytic activity [89], but surface analysis has shown [89, 577] that this is due to the formation of a very porous layer of fine particles on the surface (Fig. 32). A Raney type electrode is thus obtained which explains the high electrocatalytic activity. Therefore, it has been suggested [562, 578] that some amorphous alloys are better as catalyst precursors than as catalysts themselves. However, it has been pointed out that the amorphous state appears to favor the formation of such a porous layer which is not effectively formed if the alloy is in the crystalline state [575]. 

C.C. Koch, O.B. Cavin, C.G. McKamey, J.O. Scarbrough, Preparation of amorphous Ni60Nb40 by mechanical alloying, Appl. Phys. Lett. 43 (1983) 1017-1019. 

Supported nickel catalysts, the analogues Raney nickel prepared from amorphous alloys of Ni-Al with a metalloid such as B or P, and supported Rh catalysts are some of the alternatives currently under study. 

In 1979, White [3.2] observed that, by milling elemental Nb and Sn powders, the distinct X-ray diffraction peaks of the elements disappeared and typical diffuse peaks of an amorphous pattern showed up. But these samples did not show the superconducting transition temperature of vapor-quenched amorphous Nb-Sn alloys. In 1983, Koch et al. reported on the Preparation of amorphous Ni60Nb40 by mechanical alloying [3.3]. After the detection of amorphization by solid-state reaction in evaporated multilayer films by Schwarz and Johnson [3.4] (see also Chap. 2), Schwarz et al. [3.5] proposed after investigating glass formation in Ni-Ti alloys, that amorphization by mechanical alloying is also based on the solid-state reaction process. Within the last couple... 

A Japanese patent (126) discloses the preparation of amorphous materials with diverse compositions and their resulting activities for different transformations. One of these specimens, a Ni49Al50Cri alloy, is active in the hydrogenation of thymol to menthols at high pressure (10.1 MPa, 403 K). Treatment with hydrogen (523 K, 1 h) increases the selectivity of the formation of /-menthol (71 versus 54%), whereas a 2-h heat treatment at 873 K decreases markedly the activity (96.7—10.2%). 

In table A5 the superconducting transition temperatures (7 ) of several amorphous alloys are listed together with information on the density of states at the Fermi level N(E ). These latter values are expressed in stages eV atom spin . Included are also values for the electron-phonon coupling constant (X) and the Debye temperature ( d). The method used in the preparation of the alloys is indicated by the symbols Iq (liquid quenched) or vq (vapour quenched). 

The mechanisms and products of crystallization of amorphous alloys are influenced by both inherent (e.g. composition, oxygen) and extraneous (e.g. preparation method, pressure, etc) factors. 

During the searching for strong glass-forming alloys, the effect of composition on the crystallization behavior has been extensively studied in a variety of amorphous alloys, despite of the preparation methods (Suryanarayana Inoue, 2011). Two examples are listed in this section to show how the chemical compositions of amorphous alloys influence the crystallization mechanism and crystallization products. 

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