Thermal stability of amorphous alloys

From these results it can be concluded that any favourable (2kp= Q ) or unfavourable (2kp Q ) influence on E, even if it were of the order of AH, would have little effect on the magnitude of AE. This means that the Nagel and Tauc criterion is less suited for describing thermal stability of amorphous alloys, i.e. their resistance against crystallization. As briefly indicated above, it is mainly the transformation kinetics that governs the thermal stability of amorphous alloys. A description of the kinetic approach to thermal stability will be presented in the following section. 

In the kinetic approach to thermal stability of amorphous alloys the rate of transformation is assumed to be diffusion-controlled (Uhlman, 1972 Davies, 1976 Takayama, 1976). Atomic motion is expected in all alloys to set in at a temperature where the viscosity ij reaches a critical value (about 10 P). In terms of entropy theory of viscous flow as proposed by Adam and Gibbs (1965) this latter quantity can be given by... 

The particular position taken by alloys for which 2kp = was already mentioned in connection with the Nagel and Tauc criterion discussed in sections 3.1 and 4.2. In the latter section we showed that the Nagel and Tauc criterion is less suited for describing the thermal stability of amorphous alloys. In the former section the possibility was discussed that amorphous alloys for which the criterion 2kp is satisfied may be amenable to easy glass formation owing to the presence of an enhancement of the deepness of the eutectic. 

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. 

It Is well established that the thermal stability of amorphous CuTl hydrides is much less than that of the corresponding non-hydrogenated amorphous alloys or crystalline hydrides example, a-CuTiH 43 ... 

Che] Chen, Jin-Chang., Shen, Bao-Gen., Zhan, Wen-Shan., Zhao, Jian-Gao., Effect of W Addition on Magnetic, Electric Properties and Thermal Stability of Amorphous Fe-B Alloys , Acta Phys.,Sin., 35(8) 979-88 (1986) (Experimental, Magn. Prop., Electr. Prop., 20)... 

Models exist that describe the thermal stability Tx in terms of transformation rates (kinetic approach), where Tx is proportional to AE. In these cases AE is estimated on the basis of a weighted mean sublimation energy of the constituent elements (Davies 1976) or the heat of compound formation (Kiibler et al. 1981) or the formation enthalpy AHh of a hole of size equivalent to the smaller type of atom (Buschow 1982). Of these possibilities the latter appears to be the most promising for predicting the thermal stability of a large variety of amorphous alloys. 

In a first approximation one may assume that the atomic arrangement in all amorphous alloys is similar and corresponds to a statistical distribution of atoms. In that case the values of can be taken to be the same in all alloys. The thermal stability of the glass, when expressed in terms of (or its upper limit T ), is therefore mainly determined by AE. In fact one would derive from eq. (14) that (or Tjj) in all alloys is proportional to AE, i.e. 

Inoue, A. Nishiyama, N. Kimura, H. (1997). Preparation and Thermal Stability of Bulk Amorphous Pd40Cu30Nil0P20 Alloy Cylinder of 72 mm in Diameter. Materials Transactions JIM Vol. 38, No. 2, (February 1997), pp. 179-183, ISSN 0916-1821 Inoue, A. (2000). Stabilization of metallic supercooled liquid and bulk amorphous alloys. 

THERMAL STABILITY OF HYDRIDES OF DISORDERED AND AMORPHOUS ALLOYS... 

Ino] Inoue, A., Kitamura, A., Mastunoto, T., The Effect of Aluminium on Meehanieal Properties and Thermal Stability of (Fe,Ni)-Al-P Ternary Amorphous Alloys , J. Mater. Sci., 18, 753-758 (1983) (Experimental, Phase Relations, Meehan. Prop., Thermodyn.,, 11)... 

---