Of amorphous alloys

The corrosion behaviour of amorphous alloys has received particular attention since the extraordinarily high corrosion resistance of amorphous iron-chromium-metalloid alloys was reported. The majority of amorphous ferrous alloys contain large amounts of metalloids. The corrosion rate of amorphous iron-metalloid alloys decreases with the addition of most second metallic elements such as titanium, zirconium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, nickel, copper, ruthenium, rhodium, palladium, iridium and platinum . The addition of chromium is particularly effective. For instance amorphous Fe-8Cr-13P-7C alloy passivates spontaneously even in 2 N HCl at ambient temperature ". (The number denoting the concentration of an alloy element in the amorphous alloy formulae is the atomic percent unless otherwise stated.)... 

The high corrosion resistance of amorphous alloys disappears on heat treatment that produces crystallisation . Figure 3.71 shows an example of the... 

As can be seen in Fig. 3.67, the corrosion resistance of amorphous alloys changes with the addition of metalloids, and the beneficial effect of a metaU loid in enhancing corrosion resistance based on passivation decreases in the order phosphorus, carbon, silicon, boron (Fig. 3.72). This is attributed partly to the difference in the speed of accumulation of passivating elements due to active dissolution prior to passivation... 

Stress-corrosion cracking based on active-path corrosion of amorphous alloys has so far only been found when alloys of very low corrosion resistance are corroded under very high applied stresses . However, when the corrosion resistance is sufficiently high, plastic deformation does not affect the passive current density or the pitting potential , and hence amorphous alloys are immune from stress-corrosion cracking. 

The oxidation behaviour of amorphous alloys studied below their crystallisation temperature is not greatly different from that of crystalline metals, although the presence of large amounts of metalloids complicates the situation . ... 

The thickness of amorphous alloys is dependent upon production methods. Rapid quenching from the liquid state, which is the most widely used method, produces generally thin amorphous alloy sheets of 10-30 tm thickness. This has been called melt spinning or the rotating wheel method. Amorphous alloy powder and wire are also produced by modifications of the melt spinning method. The corrosion behaviour of amorphous alloys has been studied mostly using melt-spun specimens. 

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 method of complex study of magnetic effects and the changes of electron diffraction patterns during the heating of amorphous alloys is developed. The study was carried out on the alloys Fe-Si-B, that were the bands in amorphous state. The phase composition that correspond to registrated diffraction patterns and to magnetic effects is established. 

The equations that describe the magnetic effects and the changes of electron diffraction patterns are got in consequence with the data of X-ray investigation of amorphous alloys and the products of crystallization. 

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]. 

In the case of alloys [593, 594], the amorphous alloy of Fe60Co2oB10Silo has been identified as among the most active electrocatalysts, with an activity comparable to polycrystalline Pt. However, the Tafel slope is always close to or higher than 120 mV, and it normally increases with temperature [593] so that the latter has no activating effect on the state of the surface. It has been proposed [594] that the application of amorphous alloys to both electrodes in a water electrolyzer can reduce the expenditure of electrical energy by about 6%. However, the polycrystalline Pt taken as a reference for these studies showed [593] b - 140 mV and jo 10-4 A cm-2. At 1 A cm-2 this polycristalline Pt exhibits an overpotential of 560 mV. If we compare this activity with that claimed [5, 519], for instance, for thermal Ni-Mo alloys, the expectations for amorphous phases cannot be great. 

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]. 

It is instructive to consider the free-energy hierarchy and the metastable phase equilibria when crystallization of an amorphous material is discussed. Koster and Herold [56] discussed these aspects of crystallization and showed that crystallization reactions of amorphous alloys can be classified into the following three types polymorphic, primary and eutectic crystallization reactions. Among these three types, the slowest crystal growth process is expected for primary crystallization and thus, primary crystallization is ideal for tailoring fine microstructures upon decomposition of amorphous alloys. 

The TCR value of an amorphous alloy strongly correlates with its specific resistance [49]. It also corresponds to the thermal effect on the specific resistance, i.e., the TCR increases at the temperature where the specific resistance abruptly decreases. The TCR values of alloys containing refractory metals exceed those of NiP NiReP alloy film annealed at 500°C in particular has an excellent TCR of 18 ppm K [36]. Although the TCR of as-plated crystalline NiMoP is lower than TCR values of amorphous alloys, heat treatment at 500 C slightly decreases the TCR to 134 ppm , which is a relatively high value, while the value measured after... 

The binary amorphous alloy Ni33Zrg7 was investigated by Huot and Brossard (199) for cathodic Hj evolution at 70°C from 30% aqueous KOH. Best performance was achieved after activation in 1 M HF (cf. Refs. 196,197). Initially favorable performance became impaired on cathodic H2 evolution due to H sorption in the alloy, Zr being an H-sorption-promoting metal. This aspect of the behavior of amorphous alloys as H2 evolution cathodes was stressed in more detail in the paper of Schulz et al. (200), where difficulties arising from phase transformations associated with hydride phases and deactivation by H sorption was emphasized, and as in the work of Vracar and Conway (134). 

C. H. Lee and F. A. Kroger, Cathodic deposition of amorphous alloys of silicon, carbon and fluorine,... 

By studying the Mossbauer spectra and the nuclear magnetic resonance of a series of amorphous alloys, it was established [6.44] that their LO differs slightly from that in crystals. 

The analysis of the effect of hydrostatic pressure and electron irradiation on the diffusion-controlled crystallization process of amorphous alloys of the metal-metalloid and metal-metal types in [6.48,49] leads to the following conclusions. 

The most important methods used for the characterization of heterogeneous metal catalysts are applied also to the characterization of amorphous alloy catalysts. 

Differential thermal analysis (DTA) and differential scanning calorimetry (DSC) are also useful methods for structure determination. These methods can detect crystallization of amorphous alloy catalysts as a result of heat treatment (21, 23, 41-44) or as a result of the action of reacting gases, such as in the case of hydrogenation of carbon monoxide (53) or ammonia synthesis (22). 

Beside the beneficial effect of the addition alloying metallic elements that contribute to the increased corrosion resistance, the amorphous structure itself is also responsible for the very low corrosion. For example, crystalline alloys with the same composition exhibit high rates of dissolution. The chemically homogeneous, single-phase nature of amorphous alloys is believed to account for their corrosion resistance (8, 100, 101). This also allows for the formation of a uniform, protective film on the surface of amorphous alloy electrodes. 

The demands of practical applications led to attempts to overcome the high electric resistance of thin ribbons by a new technical solution of laser-induced surface vitrification (105, 106). First an amorphous alloy ribbon was adhered uniformly to a nickel plate by heat treatment. Subsequently, this surface alloy layer was transformed to the amorphous structure by laser surface melting and self-quenching (107). A sample consisting of Pd56Rh23P oSi9 adhered to bulk crystalline nickel exhibited anodic characteristics very similar to those of the melt-spun amorphous ribbon (102). Clearly, similar improvements forced by practical demands will be a part of the future use of amorphous alloys. 

---