Thermal stability of amorphous Co-based alloys

Amorphous alloys R1−xCox (R = La, Gd, Tb, Er, Th, Zr, and Hf) are prepared by means of the melt spinning technique in the concentration range 0.3 ⪅ x ⪅ 0.6. The electrical resistivity is studied as a function of the temperature (4.2 to 300 K) in the alloys based on La and Tb. The thermal stability of the different amorphous alloys R1−xCox is determined by means of measurements on a differential scanning calorimeter and in some cases also by means of high-temperature resistivity measurements. In Tb69Co31 the crystallization temperature is studied as a function of increasing heating rate. The following trends are observed: The crystallization temperature (Tcr) increases with increasing atomic number of the rare-earth element in R1−xCox (x constant). It also increases in the sequence R = La, Gd, Th, Zr, Hf. For a given R component Tcr tends to higher values if the Co concentration is increased. These trends are explained satisfactorily in terms of a kinetic approach by taking into consideration the differences in the enthalpy of alloying and the enthalpy of vacancy formation. A brief discussion is devoted to the possibility of preparing amorphous alloys by means of H2 take-up in intermetallic compounds.