Thermal stability of amorphous rare earth-iron alloys

Abstract

Amorphous alloys R 69Fe 31 (R  Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu), Tb 1- x Fe x ( x = 0.31, 0.40, 0.45, 0.50) and Er 1- x Fe x ( x = 0.25, 0.31, 0.33, 0.37, 0.40) were prepared by melt spinning. The electrical resistivity ρ of the alloys based on terbium was studied as a function of temperature (4.2–300 K). Negative values of the temperature coefficient of resistivity were observed in the alloys with iron concentrations corresponding to x = 0.40, x = 0.45 and x = 0.50. The crystallization behaviour of the various alloys was studied using differential scanning calorimetry (DSC) and X-ray diffraction. In general two exothermic heat effects were observed in the DSC traces. The heat effect occurring at the lower temperature T 1 was identified as being caused by precipitation of the rare earth metal. The heat effect occurring at the higher temperature T 2 corresponds to the crystallization of the C15 phase RFe 2. For the heavier rare earth elements (R  Dy — Lu) the temperature t 1 corresponding to the lower crystallization peak increases with increasing atomic number of the rare earth and follows closely the R dependence observed in amorphous R 7Co 3 and R 7Ni 3 alloys. For rare earth elements lighter than dysprosium the temperature T 1 in R 69Fe 31 has a tendency to be much lower than the first crystallization temperature in the corresponding R 7Co 3 and R 7Ni 3 amorphous alloys. Furthermore, T 2 no longer increases with increasing atomic number of R. A discussion is given in terms of the heat of alloying and the heat of vacancy formation.