Structural relaxation and crystallization of Fe 78B 13Si 9 amorphous ribbons studied by dilatometry and differential scanning calorimetry

Abstract

The structural relaxation and crystallization of Fe 78B 13Si 9 amorphous ribbons were studied by dilatometry and differential scanning calorimetry (DSC). An own-built and highly sensitive dilatometer was used allowing samples in the form of thin ribbon to be heated at a slow heating rate (1.66°C min −1) in an ultra-high-vacuum (10 −9 torr). Thermal expansion coefficients were determined on heating and cooling. Isothermal annealings were also performed in situ in the dilatometer at 250, 350 and 450 °C, for 2 h. The mean coefficients of linear thermal expansion (α × 10 6) measured between 60 and 250 °C were respectively < 5 for the as-quenched samples, 6.8 ±0.5 for the relaxed amorphous samples (whatever the relaxation rate), and 11.8 ± 0.5 for the crystallized ones. After the first heating of the as-quenched ribbons to 250, 350 and 450 °C, structural relaxation resulted in a shrinkage of the samples which was measured after cooling to room temperature. The shrinkage of the samples increased with the temperature limit of the previous heating <−2 × 10 −4 at 250°C, \\ ̃ t-5 × 10 −4 at 350 °C, and \\ ̃ t-8 × 10 −4 at 450 °C. No further shrinkage of the samples was detected during the isothermal annealings at 250, 350 and 450 ° C. Crystallization (between 514 and 537 °C) resulted in a more important shrinkage ( \\ ̃ t-50 × 10 −4). DSC was used for determining the relaxation rate of the specimens annealed under the same conditions as used for the the dilatometric measurements. The influence of the heating rate on the relaxation rate was also investigated. Results are interpreted by reference to models previously proposed in the literature for the structural relaxation of amorphous metallic alloys.