Glassy alloys

In this chapter, general aspects and structural properties of crystalline solid phases are described, and a short introduction is given to modulated and quasicrystal structures (quasi-periodic crystals). Elements of structure systematics with the description of a number of structure types are presented in the subsequent Chapter 7. Finally, both in this chapter and in Chapter 6, dedicated to preparation techniques, characteristic features of typical metastable phases are considered with attention to amorphous and glassy alloys. 

General characteristics of alloys such as those presented in Fig. 3.3 have been discussed by Fassler and Hoffmann (1999) in a paper dedicated to valence compounds at the border of intermetallics (alkali and alkaline earth metal stannides and plumbides) . Examples showing gradual transition from valence compounds to intermetallic phases and new possibilities for structural mechanisms and bonding for Sn and Pb have been discussed. Structural relationships with Zintl phases (see Chapter 4) containing discrete and linked polyhedra have been considered. See 3.12 for a few remarks on the relationships between liquid and amorphous glassy alloys. 

Among the different solid phases which may be observed in metallic systems, an interesting group is represented by the amorphous alloys or glassy alloys. 

It is also interesting to notice that the crystallization kinetics are also strongly affected by the presence of impurities, especially oxygen at a level below one percent molar. This contamination can alter the critical cooling rate by two order of magnitude. For instance, the glassy alloy V105, which has... 

Figure 13.09 Dependence of the microhardness (//) and Tg on the phosphorus content in glassy alloys of the P-Se system (After Borisova, 1981).

Fig. 4.3. Amount of molecularly adsorbed CO (100% = maximum observed coverage) as a function of temperature for polycrystalline Ni and various Ni-Zr glassy alloys [4.60]. Each data point was obtained by exposing a clean substrate at a given temperature to 8 Langmuirs CO...

I ig. 4.13. Behavior of glassy Pd,7.r , under CO oxidation conditions ( 4.711. A) Development of CO oxidation aetivily of glassy alloy with time on stream during exposure to reaction conditions. Conditions lixed-bed reactor temperature, 553 K Iced gas mixture, 17% CO, 17% O, 66% N, llow rate 2.5 cm s (STP, 1 bar 0.3 g of alloy. B) X-r iy diffraction pattern of final active catalyst after 6 It on stream (Cli K,)... 

The thermal stability is a severe limitation if the metallic glass is to be used in as-quenched state for catalysis however, that is not necessarily the case if the glassy alloy is used as catalyst precursor. The thermal stability is mainly influenced by the chemical composition of the metallic glass and the medium to which it is exposed. It has been shown that the crystallization temperature can be significantly lowered in the presence of a hydrogen atmosphere [4.23,24,31,50] or an adsorbed organic compound [4.76]. 

The stability problem seems to be receding as more glassy metals are examined under reaction conditions and some are found to be remarkably stable. However, too little information about the factors determining the stability of glassy materials under reaction conditions is still available. There are methods, however, suitable for improving the thermal stability of amorphous materials. Alloying of properly selected components can result in glassy alloys with improved thermal stability or increased activity, which permit low-temperature application. 

Fig. 5.19. Temperature dependence of the Au 5d-state binding energies of Au-Sn a) UPS He I (hv = 21.2 eV) valence band spectra of Sn-rich liquid and glassy alloys b) Au 5d peak positions as a function of temperature [5,70]...

Opportunities for application of new materials as components in electrochemical cells (electrodes, electrolytes, membranes, and separators) are discussed in this section. In addition, electrochemical processing is considered in the sense that it presents opportunities for the synthesis of new materials such as electroepitaxial GaAs, graded alloys, and superlattices. Finally, attention is focused on the evolution of new engineering materials that were developed for reasons other than their electrochemical properties but that in some cases are remarkably inert (glassy alloys). Others that are susceptible to corrosion (some metal-matrix composites) and more traditional materials that are finding service in new applications (structural ceramics in aqueous media, for example) are also considered briefly. 

The icosahedral phase can be prepared from glassy alloys of the composition Pd,Q0 for x = 20 at.% by annealing between 480 and 540°C (Poon et... 

Amorphous alloys were prepared by a melt-spinning technique under argon (Bryan et al. 1988). The inner and outer surface of the glassy alloys were characterized by XRD analysis. The influence of an annealing treatment was also studied by differential scanning calorimetry (DSC). 

The bulk structural changes of the glassy alloy were followed by XRD. Figure 2 depicts the XRD patterns of the precursor alloy before and after oxidation, and after use of the samples in long term CO oxidation tests. 

The XRD pattern of the coprecipitated catalyst (PdZr-c, trace 5) show palladium reflections indicating the presence of palladium crystallites, while the reflections due to zirconia phases are greatly broadened, which suggests amorphous phases. XRD line broadening and electron microscopy indicated that the catalysts prepared by oxidation of the glassy alloy were made up of small poorly crystalline palladium domains of about 5-7 nm lateral dimension. These domains were flilly integrated in predominantly amorphous zirconia. Although the coprecipitated catalyst contained palladium particles of about similar size (8 nm). 

Textural properties of the Pd/Zr02 catalysts prepared by oxidation of the glassy alloy (PdZr-i, PdZr-a) and by coprecipitation (PdZr-c). Pd-p denotes the pure palladium powder reference. 

PdZr-c showed significantly lower carbon incorporation under the same experimental conditions, whereas no CO disproportionation and carbon incorporation could be observed with the palladium powder below 400°C. Thus the intimate interaction of palladium and zirconia seems to be crucial for the high CO disproportionation activity observed with the catalysts derived from the glassy alloy. 

Hartmann, U. (1990). Magnetic force microscopy. Advanced Materials, 2,550-2. Hasegawa, R. (1991). Glassy alloy identification marker. Journal of Applied Physics, 69, 5025-6. 

Kawamura, Y., Shibata, T., Inoue, A., and Matsumoto, T. (1996) Deformation behavior of ZrssAlioNiioCujs glassy alloy with wide supercooled liquid region, Appl. Phys. Lett., 69, 1208-1210. 

Kawashima, A., Zeng, Y., Fukuhara, M., Kurishita, H., Nishiyama, N., Miki, H., and Inoue, A. (2008) Mechanical properties of a Ni6oPd2oPi B3 bulk glassy alloy at cryogenic temperatures, Mater. Sci. Eng., A498, 475-481. 

---