Amorphous alloy formation

Ion implantation (qv) has a large (10 K/s) effective quench rate (64). This surface treatment technique allows a wide variety of atomic species to be introduced into the surface. Sputtering and evaporation methods are other very slow approaches to making amorphous films, atom by atom. The processes involve deposition of a vapor onto a cold substrate. The buildup rate (20 p.m/h) is also sensitive to deposition conditions, including the presence of impurity atoms which can faciUtate the formation of an amorphous stmcture. An approach used for metal—metalloid amorphous alloys is chemical deposition and electro deposition. 

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). 

R.B. Schwarz, W.L. Johnson, Formation of amorphous alloy by solid-state reaction of the pure polycrystaUine metals, Phys. Rev. Lett. 51(5) (1983) 415 18. 

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]. 

Similarly, a number of amorphous alloys based on Fe-Zr, Ni-Zr, Co-Zr, Ni-Nb, have not shown any increase in activity over that expected for the mechanical mixture of the crystalline components [571]. For Ni-Nb the overpotential has even increased. Only Cu-Ti alloys have shown apparent synergetic effects, but the results of Machida et al. [89] (cf. Fig. 32) should also be taken into account. Jorge et al. [152] have observed higher activity for the amorphous form of Cu-Ti alloys, but they have attributed it to the preferential dissolution of Ti in the amorphous sample under cathodic load, with formation of a relatively porous Cu layer. The same effect was obtained more rapidly by means of HF etching [89,152]. 

As we discussed in the previous section, Cu addition is quite effective in preventing the formation of compounds in B-rich Fe-Zr-B alloys. This approach is also effective for the Fe93 xM7Bx (M= Ti, Ta and W) alloys. The formation of the tetragonal-Fe3B phase upon primary crystallization in these amorphous alloys is suppressed completely by an addition of 1 at% Cu. As a result, pe > 104 at 1 kHz has been confirmed for nanocrystalline Fe82Ti7B10Cui and Fe82Ta7B10Cui [7]. 

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