Amorphous alloys crystallization

Most systems used in material science are nonequilibrium ones aging (supersaturated) alloys dissociate by initiation and coarsening of decay products. Grains start growing in nano- and polycrystaUine materials, amorphous alloys crystallize, interdiffusion takes place in protective coatings and powder alloys, metals oxidize in the atmosphere irreversibly, and so on. All materials listed above are considered to be either metastable or absolutely unstable ones and it is just a matter of the time period required for relaxation to equilibrium, or, more commonly, to a less nonequilibrium state. The production of those materials following the chemical reactions, thermal treatment or mechanical operation is accompanied thus, by irreversible nonquasistatic processes. 

Diffusion coefficients in amorphous solids such as oxide glasses and glasslike amorphous metals can be measured using any of the methods applicable to crystals. In this way it is possible to obtain the diffusion coefficients of, say, alkah and alkaline earth metals in silicate glasses or the diffusion of metal impurities in amorphous alloys. Unlike diffusion in crystals, diffusion coefficients in amorphous solids tend to alter over time, due to relaxation of the amorphous state at the temperature of the diffusion experiment. 

In amorphous solids there is a considerable disorder and it is impossible to give a description of their structure comparable to that applicable to crystals. In a crystal indeed the identification of all the atoms in the unit cell, at least in principle, is possible with a precise determination of their coordinates. For a glass, only a statistical description may be obtained to this end different experimental techniques are useful and often complementary to each other. Especially important are the methods based on diffraction experiments only these will be briefly mentioned here. The diffraction pattern of an amorphous alloy does not show sharp diffraction peaks as for crystalline materials but only a few broadened peaks. Much more limited information can thus be extracted and only a statistical description of the structure may be obtained. The so-called radial distribution function is defined as ... 

Metal alloys can be amorphous, too. LiquidmetaF alloy is an amorphous alloy of zirconium mixed with nickel, titanium, copper, and beryllium. It is used in the heads of some brands of golf clubs. Traditional metal club heads may have microscopic gaps where planes of metallic crystals meet. These tiny gaps are a potential source of weakness. The amorphous alloy is non-crystalline, so the metal structure does not have potential breakage sites. 

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. 

We used the common principles of quantitative analysis of additive properties of alloys developed in work [3].The following equation that describes the magnetic effect during the crystallization of the amorphous alloy Fes2Si2Bi6 was got ... 

In the same way the equation for the determination of the specific magnetization of boride FesB was obtained. This phase is formed during the crystallization amorphous alloy Fes2Si2Bi6. As we found the specific magnetization of boride FesB is 192 TllO" cmVm. This value is in 1,5 times greater then the specific magnetization of cementite G-FesC and 1,1 times lower then the specific magnetization of a-iron. 

The calculation of measured in our experiments changes of magnetization in the temperature interval of phase transformation permit to determine the specific magnetization of the amorphous alloy Fes2Si2Bi6 at 470°C (743 K, this is the middle temperature of narrow interval of crystallization). 

Nanocrystalline alloys Rapid solidification (amorphization) and crystallization Exchange interaction Composition and proportion of phases... 

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. 

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]. 

First attempts to check this hypothesis [23] revealed a superior catalytic activity of iron in amorphous iron-zirconium alloys in ammonia synthesis compared to the same iron surface exposed in crystalline conventional catalysts. A detailed analysis of the effect subsequently revealed that the alloy, under catalytic conditions, was not amorphous but crystallized into platelets of metastable epsilon-iron supported on Zr-oxide [24, 25]. 

Figure 5. Schematic arrangement of the surface of a partly crystallized E-L TM amorphous alloy such as Pd-Zr. A matrix of zirconia consisting of the two polymorphs holds particles of the L transition metal (Pd) which are structured in a skin of solid solution with oxygen (white) and a nucleus of pure metal (black). The arrows indicate transport pathways for activated oxygen either through bulk diffusion or via the top surface. An intimate contact with a large metal-to-oxide interface volume with ill-defined defective crystal structures (shaded area) is essential for the good catalytic performance. The figure is compiled from the experimental data in the literature [26, 27].

Figure 6. Compilation of TG/DTA responses for the crystallization of the amorphous alloy PdjjZr which was prepared by the melt-spinning technique. The red data were obtained in hydrogen, the blue data in oxygen. The responses in hydrogen are enlarged by a factor of 10, the enlarged weight curve by a factor of 100 relative top the ordinate scales. A SEIKO instrument was used and gas flows of lOOmlmin-1 were adjusted for sample masses of ca. 4 mg.

Most relevant for the oxygen transport should be the defective crystal structure of both catalyst components. The defective structure and the intimate contact of crystallites of the various phases are direct consequences of the fusion of the catalyst precursor and are features which are inaccessible by conventional wet chemical methods of preparation. Possible alternative strategies for the controlled synthesis of such designed interfaces may be provided by modem chemical vapor deposition (CVD) methods with, however, considerably more chemical control than is required for the fusion of an amorphous alloy. 

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