Mechanism of amorphous state formation, crystalline structure, and hyperfine interactions in DyMn6−xGe6Fex (≤x≤6) alloys

Abstract

Metallic glass formation is observed in rapidly quenched quaternary DyMn6−xGe6Fex (0≤x≤6) alloys. The easy formation of amorphous states competes with the nucleation of ternary 1:6:6 rare earth-transition metal-metal compounds DyMn6Ge6 and DyFe6Ge6. The ribbon shaped samples were quenched and investigated by x-ray diffraction, differential scanning calorimetry, and F57e Mössbauer spectrometry. Melt-spun alloys from the series of DyMn6−xGe6Fex with x=0, 2≤x≤3, and x=6 do not display an amorphous state but a crystalline chemically disordered structure similar to that of TbCu7- or TbFe6Sn6-type (space group P6/mmm). Amorphous samples exhibit two crystallization steps but there is no clear evidence for a glass transition effect in the calorimetric data. The Kissinger analysis was performed to calculate the effective activation energy Ea, which is equal to 345±20 kJ/mol for amorphous DyMn5.5Ge6Fe0.5 alloy. Mössbauer spectra at 77 and 300 K consist of either magnetic sextets or quadrupolar doublets for high and low Fe content, respectively. The features reflect the dilution of Fe on crystallographic sites and the subsequent increase in topological and chemical disorder when the Mn content increases. The Miedema's semiempirical model was used to calculate the formation enthalpies of amorphous alloys (ΔHform). The calculated values are consistent with experimental results. The present model allows thus to explain the better glass forming ability for the compositions with high Mn content, where ΔHform is the most negative.