Glass Formation by Mechanical Alloying

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... 

In 1979, White [3.2] observed that, by milling elemental Nb and Sn powders, the distinct X-ray diffraction peaks of the elements disappeared and typical diffuse peaks of an amorphous pattern showed up. But these samples did not show the superconducting transition temperature of vapor-quenched amorphous Nb-Sn alloys. In 1983, Koch et al. reported on the Preparation of amorphous Ni60Nb40 by mechanical alloying [3.3]. After the detection of amorphization by solid-state reaction in evaporated multilayer films by Schwarz and Johnson [3.4] (see also Chap. 2), Schwarz et al. [3.5] proposed after investigating glass formation in Ni-Ti alloys, that amorphization by mechanical alloying is also based on the solid-state reaction process. Within the last couple... 

Schlorke N, Iickert J, Schultz L, Formation and stability of bulk metallic glass forming Mg-Y-Cu alloys produced by mechanical alloying and rapid quenching . Materials Science Forum, 1998 269-272 761-766. 

Figure 2.27 The quenching rate (or difficulty of glass formation) is plotted qualitatively as a function of x in Ge,Sei alloys. Experimental data are indicated by solid horizontal bars for various rates of cooling (water, air and slow quenching). The solid line is drawn through the data to guide the eye the prediction of Phillips s mechanical constraint theory is shown by the dashed line (After Phillips, 1979).

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