Crystallization enthalpy, amorphous alloys

Fig. 8. Crystallization of an amorphous alloy (a) into a stable (cy) or metastable compound (C2). The values given in parentheses represent an order of magnitude estimate of the activation energy for crystallization A act and the crystallization enthalpy AHcr.

Crystallization temperature (T ) for several amorphous alloys and formation enthalpies (AH) calculated by means of Miedema s model for the corresponding (hypothetical) intermetallic compounds. The AH values are given in units of kj per mol of alloy (Miedema et al., 1980). 

Chr] XRD and HREM. Amorphous ribbons Fe7gCr2B22 and Fe7gB22 were obtained by melt spinning, followed by annealing for 1 h at 27 to 627°C with the step of 25°C. For an amorphous alloy Fe76Cr2B22, the crystallization enthalpy was determined as 2.0 eV and were examined. 

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... 

Res] proposed a thermochemical model of the bonds in transition metal borides and tried to calculate the enthalpies of formation of the hypothetical borides M3B and TigB. The increase in the crystallization temperature of an amorphous FcgoTisBis alloy resulting from doping with Ti has been estimated. 

---