Amorphous alloys metastable state

Amorphous alloys are in a thermodynamically metastable state, and hence essentially they are chemically more reactive than corresponding thermodynamically stable crystalline alloyIf an amorphous alloy crystallises to a single phase having the same composition as the amorphous phase, crystallisation results in a decrease in the activity of the alloy related to the active dissolution rate of the alloy . 

This was the first proven example of the operation of the principle that free energy stored in the metastable amorphous alloy can be used to create a catalytically active species which is still metastable against phase separation and recrystallization, but which is low enough in residual free energy to maintain the catalytically active state for useful lifetimes. 

The noncrystalline sohds described here are amorphous and metastable. This specific thermodynamic property is because they aU originate from the liquid state. The corresponding glassy or vitreous materials are not very common in solid-state chemistry, and only a limited number of molten salts or molten alloys have the characteristics necessary to produce glasses when cooling. 

Glass formation by mechanical alloying of elemental crystalline powders can be considered a special form of solid-state interdiifusion reaction. The basic principles of such a reaction [3.15] are described in Fig. 3.4. As is well known, the thermodynamic stable state of a system is determined by a minimum in the free enthalpy G. In metallic systems, the free enthalpy of the equilibrium crystalline state Gx is always lower than that of the amorphous state Ga below the melting temperature. The amorphous state is a metastable state, i.e., an energy barrier prevents the amorphous phase from spontaneous crystallization. To form an amorphous metal by a solid-state reaction, it is necessary to establish first a crystalline initial state with a high free enthalpy G0 (Fig. 3.4). Depending on the formation process, this initial state can be achieved, for example, by... 

A disadvantage of amorphous alloys is their metastable character which makes them transform into the stable crystalline state as a function of temperature and time. In calorimetric experiments the amorphous-to-crystalline transition is revealed by an exothermic heat effect. Typical traces obtained using a differential scanning calorimeter are shown for amorphous Gd064Co0 36 in fig. 51. The dependence of the crystallization temperature Tx on the heating rate s implies that there is a risk of crystallization taking place even at room temperature after an extended period (s - 0). This is particularly likely when Tx is rather low, and it may have consequences for practical applications. 

Mg and its intermetallic combinations in alloys are known to be efficient hydrogen sorbents [1-3]. To increase the capacity and to decrease the temperature of H sorption and desorption, the metastable state of material may be used. Amorphous alloys are considered to be promising materials for this purpose [4]. 

A review is also given of those areas of solid state research where progress has been made mainly due to the possibility of preparing amorphous alloys. Such areas include, for instance, the determination of the strain-free dilute limit of the Moss-bauer isomer shift of a given isotope in various host metals. In crystalline alloys this line of research is seriously hampered by the presence of the so-called volume misfit contribution, which is difficult to estimate. Also mentioned is the possibility of using amorphous alloys as starting materials for the preparation of metastable inter-metallic compounds. 

The formation enthalpy AHf of amorphous alloys is less negative than that of crystalline materials of similar composition, which means that the former alloys are metastable. As a function of temperature and time the amorphous alloys will therefore transform into the stable crystalline phases. This transformation can conveniently be studied by means of diffraction methods. As will be discussed later on, no sharp diffraction lines occur in the diffraction diagrams of amorphous alloys. The transformation into the crystalline state is generally accompanied by the occurrence of sharp diffraction peaks. In some cases the stable crystalline phases are not reached directly. First one or more metastable crystalline phases may be formed which transform into the stable end products at a later stage of the crystalUzation process. 

Mechanochemical treatment, as already emphasized, produces nanocrystalline/amorphous phases. Such pronounced metastable states readily react at elevated temperatures, thereby related equilibrium phases can be obtained relatively easily. For instance, high-temperature intermetallic compound AlgMos (melting point 2123 K), which, due to significantly different melting points of A1 and Mo is otherwise difficult to prepare from liquid phase, was obtained by continuous heating up to 1400 K of the mechanically alloyed Al-27 at.% Mo powder [46]. 

A metastable phase diagram is different from the stable type. The G(x, T) function of the metastable phases is not located at a minimum position, but at a secondary minimum. Under the condition r(T)>Tp(T) (the critical time), metastable phases transform to stable phases. A method to attain a metastable phase is that the stable phase in the (x T,) state under the condition r(T) < fp(T) changes to the ( 2, T2) state (it is also allowed that only x or T changes). In order to obtain a metastable phase, the corresponding Tp T) must be realized technologically. When long-distance diffusion of atoms is required for crystallization of a liquid, amorphous alloys can be obtained by quenching the liquid under the condition of r(T)polymorphous transformation, only a solid solution will be... 

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. 

An amorphous alloy is an ideal general precursor or reaction intermediate. The amorphous state is metastable with respect to several different crystalline states but the crystalline state which is easiest to nucleate is the one which will form. Thus, the compound which crystallizes is not necessarily the most stable state. Controlling crystallization of amorphous alloys is a general route to both stable and metastable materials (10). 

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. 

Amorphous networks, 4-5 nAmorphous solid synthesis via ultrathin-film multilayer composites analysis of solid-state reaction mechanisms, 357,358/ application to synthesis of metastable ternary compounds, 366 control of crystallization of amorphous aUoy, 360,363,365-367/ control of formation of homogeneous amorphous alloy, 360,361-36 differential scanning calorimetric procedure, 359-360 grazing measurement procedure, 359 lugb-angje XRD procedure, 359 length sddes vs. course of solid-state reactions, 360,361-362/363 quantitative analysis of interdiffiision reaction, 356-357... 

A further method of producing amorphous phases is by a strain-driven solid-state reaction (Blatter and von Allmen 1985, 1988, Blatter et al. 1987, Gfeller et al. 1988). It appears that solid solutions of some transition metal-(Ti,Nb) binary systems, which are only stable at high temperatures, can be made amorphous. This is done by first quenching an alloy to retain the high-temperature solid solution. The alloy is then annealed at low temperatures where the amorphous phase appears transiently during the decomposition of the metastable crystalline phase. The effect was explained by the stabilisation of the liquid phase due to the liquid—>glass... 

Amorphous metals can be prepared in a wide variety of stable and metastable compositions with all catalyti-cally relevant elements. This synthetic flexibility and the isotropic nature of the amorphous state with no defined surface orientations and no defect structure (as no long-range ordering exists) provoked the search for their application in catalysis [21]. The drastic effect of an average statistical mixture of a second metal component to a catalytically active base metal was illustrated in a model experiment of CO chemisorption on polycrystalline Ni which was alloyed by Zr as a crystalline phase and in the amorphous state. As CO... 

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